The Effects of the Bray Climate and Solar Cycle

By Andy May

Javier has posted three new essays on the Bray climate and solar cycle on Judith Curry’s web site. Part A is here, part B is here and Part C is here. In these posts, he lays out the evidence, in some detail, for the climate cycle and the associated solar cycle. Here I will summarize the results of his analysis and explain why it matters. The Bray solar and climate cycle are arguably the most important climate/solar cycle of the Holocene Epoch.

The Bray cycle is about 2450 years from beginning to end and the Bray Lows, which are the coldest portion of the cycle, are the most important events. The Bray lows are easily seen in glacial advances around the world (Bray, 1968), foraminifera fossil records, archaeological records, and lake sediment records. They are less evident in ice core records from Greenland and Antarctica. Figure 1 shows the Bray Lows of the Holocene as blue bars, labeled B5 to B1.

Figure 1

The yellow bars are some of the Eddy cycle lows (~980 years). The black rectangles show the location of major grand solar minima clusters (GSM). We call the warm period from 8000 BC to 3500BC the Holocene Climatic Optimum and the cooling period from 3500 BC to the end of the Little Ice Age (LIA) in the late 19th century the Neoglacial. The blue curve on the plot is my global Holocene temperature reconstruction (see here) and the gray curve is a Holocene Greenland temperature reconstruction described in Kobashi, et al., 2017.

Figure 2 (Sunspot reconstruction source: Solanki, et al., 2004)

Figure 2 is the Holocene sunspot reconstruction published in Nature by Solanki, et al. in 2004. Besides the sunspot records there are other data supporting a 2450-year solar cycle that are in phase with the 2450-year climate cycle. The radiocarbon evidence is discussed in Vasiliev and Dergachev (2001) and O’Brien, et al. (1995) discuss the ice core evidence for the cycle. Other evidence is discussed here. The Bray lows in figure 2 are the same as those in figure 1, but the bars are smaller to allow all the Eddy cycle lows to be displayed. The more severe colder periods with extended periods of minimal solar activity occur when Bray lows and Eddy lows occur at about the same time. However, the sun is not the only natural cause of climate change. Ocean cycles or oscillations, often called “natural variability” can occur on millennial time scales as well. The evidence for these long-term oscillations is discussed in Debret, et al. (2009).

The 20th century had an above average number of sunspots, suggesting an above average level of solar activity, certainly higher than the Little Ice Age. Solanki and Krivova (2003) looked at the influence of TSI (total solar irradiance), total UV solar irradiance, and cosmic ray flux on the Earth’s climate. They concluded that if only these three solar influences affect our climate that the sun could have caused the global warming until 1970, but from 1970 to 2000 the sun probably contributed less than 30% of the warming using a conservative solar reconstruction by Frohlich and Lean (1998). Using a different reconstruction by Willson (1997), the sun may have contributed as much as 50% of the warming. Even though we now have satellite measurements of solar radiation variability, these records are inconsistent with one another leading to multiple reasonable interpretations of long-term and short-term trends in TSI and solar activity. Thus, estimates of solar forcing and the impact of the sun on our climate vary from 30% to nearly 100%, see the discussion on this topic here. All we really know is that solar activity is high today and increased during the 20th century, thus it probably contributed to the recent warming, but the amount of the contribution is still unknown.

As Javier writes:

“Given the strength of the correlation between past cycles of climate change, and cycles in the production and deposition of cosmogenic isotopes, like the Bray cycle, the solar-climate relationship is accepted in paleoclimatology as non-controversial. Sixteen of twenty-eight (57%) of the articles whose climatic evidence has been reviewed here explicitly state that changes in solar forcing are likely to be the cause of the observed climatic changes, and only one explicitly rules them out. Then, why is the solar-climate relationship so controversial outside of the paleoclimatology field?”

There are two main reasons given by Javier’s opponents for why they think solar variability cannot be important in climate change. The first is that the cosmogenic isotopes, 14C and 10Be, used as a proxy for solar activity in pre-instrumental times, are, themselves, affected by climate. Therefore, it is stated, that they are invalid as solar proxies and cannot be compared to a climatic record. But, this criticism does not explain why the proxies compare so well to sunspot records over the past 400 years (see figure 3). The climate of the last 400 years is highly variable and yet 14C and 10Be compare extremely well to the sunspot record. Another way to test the validity of 14C and 10Be is to compare their records to auroral records which are related to solar activity, but not to climate. Here we also see excellent agreement. In figure three, from Javier’s essay, auroral frequency is plotted in 3b and the cosmogenic isotope solar modulation function is plotted in 3a and both are compared to the International Sunspot Record in black and the Hoyt and Schatten sun-spot number in gray in 3c. The pink bars identify de Vries lows (spaced at 210 years) and the gray bars identify grand solar minima. The grand solar minima are labeled with their names. The de Vries 210-year solar cycle is modulated by the Bray cycle such that it is more severe in Bray lows (Hood and Jirikowic, 1990) like the Little Ice Age. If the Bray cycle were of terrestrial origin, then we should not be able to see it modulate the solar de Vries cycle in solar records, but we do see it in figure 3 in the sunspot record and in the auroral frequency record.

Figure 3 (source: Javier, here)

Many researchers have identified a climate cycle or oscillation of about 1500 years (Kern et al., 2012 and Darby, et al. 2012), but it does not show up in cosmogenic records. If the cosmogenic isotopes are reflecting terrestrial climate, as opposed to solar activity, why do they not show the 1500-year climate cycle that most believe is related to ocean oscillations? It is difficult to argue that the climate is contaminating some cosmogenic isotope records, but not others.

The second common objection is that observations of our sun show it to be extraordinarily constant, with the total solar irradiation (TSI) varying over a small and insignificant range every 11 years. This conclusion assumes that any solar climatic effect must be due to changes in TSI and the critics claim there is no evidence for this, thus, they conclude the sun has no impact on climate change. As Javier points out this is a non-sequitur fallacy. There is evidence that the sun is the cause of the Bray cycle, if the mechanism is not a change in TSI, it could be a change in UV radiation, which can vary as much as 100% in one 11-year solar cycle, or some other solar cause such as variability in the Sun’s magnetic field strength.

Solar UV radiation helps create ozone and affects the temperature profile in the stratosphere. Thus, variations in solar UV can change the stratospheric temperature profile and density. The density changes change stratospheric pressure and the geopotential height of the tropopause. Ozone is not distributed equally around the world and the distribution can affect surface weather patterns (Labitzke, 2001). There are also other solar changes that might affect climate, such as the solar magnetic field strength, but we will not discuss these mechanisms here.

Another problem with the assertion that solar activity variability is insignificant is that the long-term sunspot record strongly suggests that there is a secular long-term trend of increasing solar activity as we come out of the Little Ice Age. The Bray cycle modulates the ~210-year de Vries cycle as can be seen in Figure 4. The amplitude of the Gleissberg centennial cycle (~103 years, Tan, 2011) and its half-way point are shown by the orange line. This cycle is also shown by the red curve, which increases from left to right. The ~210-year de Vries cycle lows (also increasing from left to right) are shown in blue. Since solar activity, during the de Vries lows and the centennial lows, is increasing with time, the de Vries lows become less noticeable. The increased activity in the lows makes them harder to detect. Thus, it is expected that the next de Vries low, around 2100, will be barely noticed.

This modulation of the shorter cycles might have created a secular warming trend coming out of the Little Ice Age. This long-term trend is not accounted for in current climate models, thus the impact of the trend, if any, is assigned, erroneously, to human influence.

Figure 4 (source: Javier, here)

I’ve prepared a poster, meant to be printed on 11″x17″ or A3 paper that contains much of the information in the first four figures for the entire Holocene. Click on figure 5 below to download it.

Figure 5 (link to download poster, prints best on 11″x17″ or A3 paper and larger)

The top graph in figure 5 is my Northern Hemisphere temperature reconstruction (see here). The second graph is a plot of a Holocene sunspot reconstruction by Solanki, et al. (2017), overlain with the Bray and Eddy cycle lows and the grand solar minima. The third graph shows my global reconstruction (see here) and the Kobashi, et al. (2017) Holocene Greenland reconstruction. I’ve added some historical notes to provide some temporal perspective. Sea level change, satellite temperatures and some surface temperature records are shown along the bottom of the poster.

The Equator-to-pole-temperature-gradient

When the world cools at solar lows, the equator-to-pole-temperature-gradient (EPTG) increases. As the solar minimum approaches, the atmosphere reorganizes as the polar cells expand and the Hadley cells contract. The effect is that the polar temperatures extend farther south (in the Northern Hemisphere, north in the southern hemisphere) and as the cold air moves toward the equator the temperature gradient increases. This does not have to happen in both hemispheres at the same time or with the same severity and, probably, most of the time it doesn’t. The effects on the North Atlantic are particularly severe since the reorganization of the atmosphere causes the NAO (North Atlantic Oscillation) to be in nearly a perpetually negative state due to a weakened polar vortex and an expanding polar cell. Because the cooling does not occur across the globe at the same time or with the same severity, many observers have erroneously concluded that Bray lows are regional and not global.

The connection between solar variability and climate change is so obvious, it is hard to explain why so many scientists deny it. In the 20th century solar activity increased and the sun became very active by historical standards and all direct measurements of the sun’s activity are from this period or later. However, during the Maunder minimum virtually all solar activity stopped and the world became very cold as a result. Javier quotes the following from John Eddy, 1976, this is the last sentence in his 1976 paper in Science entitled “The Maunder Minimum:”

“The reality of the Maunder Minimum and its implications of basic solar change may be but one more defeat in our long and losing battle to keep the sun perfect, or, if not perfect, constant, and if inconstant, regular. Why we think the sun should be any of these when other stars are not is more a question for social [science] than for physical science”

The EPTG defines the state of the world climate on a scale between Extreme Hothouse and Severe Icehouse as can be seen in figure 5 from Javier’s essay, originally from Scotese here. The term Icehouse means a time when a thick permanent icecap cover either the North Pole or the South Pole as we see today. The term hothouse refers to a time when both poles are ice free and tropical plants are growing at both poles.

Figure 6 (source: Scotese, here)

The pole-to-pole temperature curves represent the average global temperatures shown on the right of the figure, currently we are in the severe icehouse state because of the very cold temperatures in Antarctica. We lie between curves 6 and 7 on the left side of figure 6, and today´s global mean temperature is ~ 14.5°C.

Figure 7 (source: Scotese here)

In figure 7, Dr. Christopher Scotese has used available geological evidence to determine the climatic state of the Earth over time. The “normal” climate of the Earth is cooling greenhouse to warming hothouse with an average temperature of about 20°C, at this temperature there are no permanent ice caps and life thrives at the poles. Icehouse conditions, such as we live in today, are relatively rare in Earth’s history and have occurred only four times since complex life evolved in the Cambrian. In these periods, the temperature curve in figure 7 moves into the blue area. The three main cold periods are in the late Ordovician, the late Carboniferous and early Permian, and now. There is a fourth at the end of the Cretaceous, caused by a large bolide impact that ended the Cretaceous, but it was short lived. The cool period at the boundary between the Jurassic and the Cretaceous is not thought to have reached icehouse conditions. Dr. Scotese has shown the impact of a 5°C increase in temperature (the IPCC worst case scenario) from todays temperature (labeled 2016) of 14.5°C at the right end of the graph. As you can see 5°C only takes us back to the Earth’s average temperature for the Phanerozoic. This final point is hypothetical only. It is hard to see, but the preindustrial temperature of 13.8°C is identified in figure 7 as well, recent warming to 14.5°C is hardly noticeable at this scale.

The abbreviations in figure 7 are: PETM – the Paleocene-Eocene Thermal Maximum, EEOC – Early Eocene Climatic Optimum, MECO – Mid-Eocene Climatic Optimum, EOT – Eocene-Oligocene Transition, MMCO – Mid-Miocene Climatic Optimum, LGM – Last Glacial Maximum. The PETM was an unusually warm period 50 million years ago when palm trees grew in the Antarctic and Arctic. During the EOT, the permanent Antarctic ice cap began to grow, it was complete by 12 Mya and expanded dramatically 5 Mya. There was no permanent Arctic ice cap 20 Mya and it grew in fits and starts until reaching a maximum around 19,000 years ago during the last glacial maximum.


The 2450-year Bray climate cycle is linked to a solar cycle of the same length. The cycle is related to wind patterns, changes in ocean currents, precipitation and global average temperature. The cycle appears to act on climate through changes in the stratospheric ozone content and pressures. The stratospheric pressure changes affect Tropospheric weather patterns. Evidence suggests that lows in the Bray cycle cause contraction of the Tropospheric Hadley cells and expansion of the polar cells, increasing the Equator-to-Pole temperature gradient. This results in decreasing global temperatures and changing wind and precipitation patterns. The North Atlantic region is affected most by these changes at Bray lows because they cause the North Atlantic oscillation to enter a persistent negative phase. This intensifies winter climatic effects, explaining why the Little Ice Age was more severe in this region than in the rest of the world.

The world is currently within the Quaternary Ice Age and nearly as cold as it has ever been. The normal average temperature of the world is around 20°C, some 5°C warmer than today. To keep recent warming in perspective, it is important to understand that even if the worse predictions of the IPCC were to occur, we would only be returning to the average temperature of the last 560 million years.

My excellent adventure into the March for Science

By Andy May

I love visiting WUWT and Climate, Etc., but most of the visitors to these sites, like me, are skeptical that the current global warming is dangerous. I’ve often visited the notable alarmist websites, such as and, to gain an understanding of why they think the current warming is dangerous and man-made. I’ve even written posts on their views, like here, and I’ve cited their posts where appropriate. But, what about the regular educated population of people who believe global warming is dangerous and carbon dioxide is a pollutant? To be a well-rounded climate science writer shouldn’t I engage with the climate-scientist-on-the-street?

I was recently invited to join the facebook group “March for Science.” At first, I ignored the invitation, after all marching in the streets is a political thing and has nothing to do with science. But, “climate science” is also a political thing these days and has little to do with science, at least the science I knew during my 42-year career as a scientist. So maybe this is a forum I could benefit from, I was fairly sure I’d be bloodied a bit by the experience, but I might learn something. What is the caliber of the forum members? Would it be all ad hominem attacks and appeals to the so-called 97% consensus, or would some of them engage in a real scientific debate on the dangers or lack thereof, of global warming or climate change? Would they debate the evidence that CO2 concentration dominates climate change? I was curious.

Figure 1 is the banner page of the forum, click on the image below to go to the page. I’m not 100% sure this will work for everyone because the page is invitation only.

Figure 1

The photo at the top of the page is not very encouraging. They are not a very scientific looking bunch. The signs are not very scientific either, “speaking TRUTH to power” sounds more like a religious or political slogan than a scientific one. More scientific and more appropriate slogans would be like these:

Science is a perspective. Science is the process that takes us from confusion to understanding in a manner that’s precise, predictive and reliable. Brian Greene, American physicist.

What seems to be missing in common discussions of climate change, whether man-made or not, whether caused by CO2 or not, is that science is a job and a learned skill, it’s a process. It most definitely is not something someone believes in or has faith in, it is not a set of facts anointed by self-appointed “climate scientists” and passed down to the great unwashed masses to be believed without question. Although, unfortunately, you find many who think that. You will see statements from my adventure in the March for Science that sound much like that.

To the best of my knowledge the great Columbia physicist Brian Greene has not spoken out about climate science, but he has stated that:

“… in order to have great breakthroughs in science, you’ve got to go against what the elders are saying.”

Or put another way, go against the consensus. As you might expect, the March for Science members talked a lot about the consensus. Brian Greene does discuss how science became a political prisoner here.

Figure 2 are the rules for posting. I found these encouraging, but quickly found out after my first post that they aren’t followed by many of the members.

Figure 2

But, many visitors to WUWT and don’t follow the rules either, to be fair.

For my first post, I chose to write about Dr. Spencer’s excellent discussion of the American Meteorological Society’s (AMS) criticism of U.S. Energy Secretary Rick Perry. See figure 3 below, click on the figure to read Dr. Spencer’s post or click here. Mr. Perry stated that he did not believe that carbon dioxide was:

“… the primary control knob for the temperature of the Earth and for climate.”

Perry continued that we should not be debating whether man affects climate, humans do affect climate, the debate is over “how much.” This is a very reasonable position to take and Dr. Spencer explains this quite well. The data we have today doesn’t show the amount of current warming attributable to man-made greenhouse gases, the man-made CO2 effect is too small to measure. Dr. Spencer writes about this problem in his book The Great Global Warming Blunder:

“Our satellite instruments still do not have the absolute accuracy to measure the small imbalance from Earth orbit that is believed to exist from more carbon dioxide in the atmosphere, so we cannot even directly measure the mechanism that supposedly causes global warming! As of 2009, it is estimated that humanity’s CO2 emissions …[have] caused an estimated 1.6 watts per square meter of extra energy to be trapped, out of the estimated 235 to 240 watts per square meter that the Earth on average emits to outer space on a continuous basis. We really don’t know the exact magnitude of the average flows of energy in and out of the Earth to better than several watts per square meter. It could be 235, 240, or 245 watts per square meter. I find it amazing that the scientific community’s purported near-certainty that global warming is manmade rests on a forcing mechanism–a radiative imbalance–that is too small to measure.”

Dr. Spencer is well qualified to make this judgement call, he is a co-recipient (with Dr. John Christy) of NASA’s Exceptional Scientific Achievement Medal for their pioneering work on global temperature monitoring using satellite microwave data. He is the NASA U.S. Team Leader for the Advanced Microwave Scanning Radiometer, currently flying on NASA’s Aqua satellite. If you are curious about how Spencer and Christy, and the others on their team measure atmospheric temperatures from satellites, I recommend the excellent and easy to understand post by Dr. Spencer (see here).

The estimates that man-made carbon dioxide is causing most of the current warming are based on computer models and not on measurements. More on this calculation from model output can be read on my blog (here) or in Chapter 10 of the AR5 report by the IPCC here. The AMS position is the usual:

“thousands of independent scientists and numerous scientific institutions around the world agree … that [human] emissions of carbon dioxide are the primary cause [of climate change]”

Thus, they simply say trust climate scientists and provide no evidence for their assertion. This is not surprising, since there is no evidence.

Figure 3

Figure 4 has the Like/Dislike breakdown after a few days.

Figure 4

I was a bit surprised that the likes outnumbered the dislikes. Reading the 420+ comments, one would think that the dislikes would be higher, but more likely they were just more vocal. With the basic background covered, I’d like to discuss key parts of the discussion. I participated in the discussion in my spare time over 3 days, but when it became very repetitive on June 25, I stopped. There were numerous threads in the discussion, I will deal with the major topics in turn.

Ad hominem arguments

Most of the comments were ad hominem attacks on either Dr. Spencer or myself, I’ll show a couple of these, then move on to the more interesting stuff. After several attacks on my manhood, mental health and qualifications and seeing nothing about Mr. Perry’s statement or Dr. Spencer’s post, I posted the following which I stole from David Middleton:

Figure 5

This led to 23 replies, a selected few are presented in figure 6.

Figure 6

Most of the 23 replies are along the lines of Tom Stark’s comment. I think Lorcan McGuinness is in a class of his own. Scott McDonald’s comments are quite good. By the way, “OP” stands for original poster (me). So, there are gems to be found in all of this.

Does CO2 dominate climate change?

Besides the ad hominem thread, we had several others of interest. Many presumed to know my motives (sinister, of course) or my interests, like in the following:

Figure 7

So, Ms. Oomen’s professor asserted that we would be insane to deny the existence of radiative forcing. She does not say what radiative forcing she is talking about, but presumably she means CO2. She supplies no evidence or references supporting her assertion, only her MSc in Physics. There were a lot of comments like this, I ignored them.

The effect of solar variability and is the Little Ice Age global?

Then there was this comment by Gilman Ouellette that I thought was quite good, it was the first comment with any substance. I include my reply.

Figure 8

Mr. Ouellette’s first link is to a composite graph of TSI from several satellites from 1975 to June 2010. See figure 9.

Figure 9

The point he is making is that the variability of total solar irradiance (TSI) is too small to matter. This is the same claim that is made by the IPCC in AR5. However, there is vigorous debate on the issue. Soon, Connolly and Connolly have challenged the TSI reconstruction used by the IPCC here and claim the Hoyt and Schatten reconstruction is better and fits the climate record better.

Khider, et al. (2014) compared a very quiet TSI record by Steinhilber et al. (2009) to a western Pacific Ocean temperature reconstruction and determined that the sensitivity to solar variability is 9.3°C to 16.7°C/Wm-2, if Steinhilber is the correct record of TSI. This is very high, much higher than the IPCC estimate of 0.7 to 1°C or the Tung et al.(2008) estimate of 1 to 1.5°C. They offer two possibilities for this:

“There remains the possibility that (1) the Steinhilber et al. [2009] reconstruction underestimate actual TSI variability and (2) the response to small changes in solar irradiance was locally enhanced in western tropical Pacific.”

As discussed in more detail here, Steinhilber et al. (2009) is the TSI reconstruction used by the IPCC to compute human influence on the climate. They believe, as Mr. Ouellette does, that solar output varies very little and, as a result, does not influence climate change. Yet, there are many other, equally likely and peer-reviewed TSI reconstructions that show much more variability. These are shown on the left side of figure 10:

Figure 10 (source: Soon, Connolly and Connolly, 2015)

The top TSI reconstruction by Hoyt and Schatten as updated by Scafetta and Wilson (2014) actually explains most of the warming in the past 150 years, reducing the calculated human influence (Soon, Connolly and Connolly 2015).

Mr. Ouellette’s second reference refers to a paper that proposes that the Medieval Warm Period and the Little Ice Age did not happen around the world at precisely the same time. This is true, but between 1500 AD and 1900 AD every part of the world experienced their respective coolest period in the Holocene, see the world-wide Holocene temperature reconstructions here. Below you can see a blow up of the period 2000 BC to 2000 AD in figure 11:

Figure 11 (source here)

The Northern Hemisphere has a very long and deep Little Ice Age that covers the full period from 1500 to the late 1800’s. Each of the other regions also has its coolest period in this interval, but slightly offset from one another and the cool period is shorter than in the Northern Hemisphere. Thus, the world is at its coolest point globally during the period 1500 AD to 1850 AD, but the severity of the cold is not uniform and the depths of the cold are not synchronous around the world. The tropics and the Southern Hemisphere have their cold period earlier and the Arctic later than the Northern Hemisphere.

The Earth’s emission spectrum

Mr. Ouellette had another very pertinent post that I want to address. See figure 12.

Figure 12 (click on the figure to see a larger version of the graph)

Mr. Ouellette is correct that amplitude changes in the Earth’s infra-red emission spectrum depend, in part, on the temperature of the air emitting the radiation. It also depends upon the greenhouse gas concentration and CO2 is an infra-red (IR) active gas, so changes in CO2 will matter, but water vapor is far more important. Also, water vapor is the primary transporter, through latent heat of evaporation, of heat energy from the Earth’s surface to altitudes high enough that CO2 and other greenhouse gases can emit radiation to space. While it is true, that the NASA AIRS satellite has shown that IR radiation emissions have decreased as CO2 concentration has gone up, this does not prove that CO2 absorption was the only cause of the decrease, although it is likely one of the causes. There are too many unknowns to claim CO2 is the major cause, there is some evidence that global precipitable water in the atmosphere has decreased (see here), solar activity has certainly decreased (see here), ocean heat content has increased (see here) providing more energy storage. So, as I say above, Mr. Ouellette has overstepped his data a bit.

He switches to another topic next and points out that we can tell how much of the CO2 in the atmosphere is from fossil fuels and how much is natural by monitoring how the ratio of carbon-14 and carbon-13 change with time. Quite true, but again he hits the causation wall, is CO2 changing the climate? We still have no evidence for that, there are too many other plausible possibilities. As noted by Mr. Perry in the post, the question is, how much of the warming is due to CO2 and how much is due to other factors?

The so-called 97% Consensus

This is the myth that will not die, and any discussion of climate change will eventually degrade to a discussion of the so-called 97% consensus. It has been very thoroughly de-bunked by Professor Richard Tol here and here. Joseph Bast and Roy Spencer also do a good job of blowing up the myth here. So, I will provide no more commentary, but will show a representative comment to prove that some people still believe that nonsense.

Figure 13

Detection and attribution of man-made climate change

The attribution of climate change to human activities was a widely discussed topic. The posts on this topic are too long to include here, but if you can get to the thread (try here) some are worth reading. I will summarize the discussion. Generally, the idea that CO2 is the control knob for climate was explained using “what else could it be” logic. The proponents, like Chris Colose, acknowledge that solar variability and ocean transport and storage of heat energy could be factors, but claim they cannot account for all the temperature change, so CO2 emissions must account for most of the change. They do not question the low estimates of ocean or solar forcing that the IPCC uses, but others do. There is a discussion of the attribution of climate change to human activity here.

In short, their argument boils down to this: the world is warming, we know precisely how much nature contributed to this warming, so we subtract that from the total and remainder must be due to man’s CO2 emissions because there is nothing else that could have done it. A very weak case indeed.

What should be done about global warming

There was considerable discussion about what to do, if anything, about global warming. Most of the discussion was like the following:

Figure 14

So, one argument is, global warming may lead to a disaster and it may be due to fossil fuel emissions and solving it by reducing or eliminating the use of fossil fuels might be possible, so we should reduce our use of fossil fuels anyway, even if we are not sure. There are a lot of ifs and maybes in there. The opposing argument is, if we stop or reduce the use of fossil fuels, it might ruin our economies and what if warming continues anyway? Wouldn’t we need energy from fossil fuels to help us adapt to warmer temperatures? What if we are wrong about all of this? That was pretty much how it went.


Most of the 420+ comments were vacuous ad hominem attacks or assertions made without any support or references. But, a few were interesting and thoughtful by people who had obviously studied the subject and knew what they were talking about. I tried to touch on them above to give the reader a flavor of the more knowledgeable alarmist positions. It was a bit difficult at times to wade through the ad hominem attacks on Dr. Spencer and myself, but I still found the experience worthwhile. It gave me more of a perspective on the other side of the debate. There were also more people supporting my position than I would have expected. You can see some of their comments in the figures above. These are just a few of the favorable comments I received. So, even at web sites like this one, skeptical thinking can intrude. Maybe the climate science community isn’t quite as polarized as we often assume. Something to think about.

Global versus Greenland Holocene Temperatures

By Andy May

Last week, I posted a global temperature reconstruction based mostly on Marcott, et al. 2013 proxies. The post can be found here. In the comments on the Wattsupwiththat post there was considerable discussion about the difference between my Northern Hemisphere mid-latitude (30°N to 60°N) and the GISP2 Richard Alley central Greenland temperature reconstruction (see here for the reference and data). See the comments by Dr. Don Easterbrook and Joachim Seifert (weltklima) here and here, as well as their earlier comments.

Richard Alley’s (Richard Alley, 2000) central Greenland reconstruction has become the de facto standard reconstruction and is displayed often in papers and posts. And, truth be told, I’ve often used it. See here for an example. But, it is a central Greenland reconstruction, uncorrected for elevation differences over time, and all of Greenland is north of 60°N. A better comparison is with my Arctic reconstruction that goes from 60°N to the North Pole. This comparison is shown in figure 1.

Figure 1

Alley’s reconstruction is based upon trapped air in ice cores taken from central Greenland and his proxies are calibrated to air temperatures on land. My Arctic reconstruction is based upon nine proxies, five are marine proxies and 3 are land proxies. Only one of the land proxies is a Greenland ice core and I used a composite of two Greenland area ice cores, Agassiz and Renland, by Vinther, et al. (2009) and not the better-known Alley reconstruction. The Vinther reconstruction and the Alley reconstruction are compared, using actual temperature, in figure 2.

Figure 2

As can be seen in figure 2, the Vinther Agassiz and Renland reconstruction is less erratic and has a more prominent Holocene Climatic Optimum (HCO) than the Alley reconstruction. In addition, the Vinther Medieval Warm Period is older and the Roman and Minoan Warm periods are far less prominent and offset in time. Notice the reconstructions match in the Little Ice Age (LIA) and that the Vinther Holocene Climatic Optimum (HCO) from 8000 BC to 4500 BC is more prominent. The HCO doesn’t really show up in the Alley record. Below we compare our Arctic reconstruction to the Vinther record in Figure 3.

Figure 3

Vinther’s record shows a more prominent HCO than ours, more detail and a deeper LIA. Finally, let’s compare both Vinther and Alley to our Northern Hemisphere mid-latitude reconstruction in figures 4 and 5.

Figure 4

It is interesting that Vinther agrees with the mid-latitude Northern Hemisphere reconstruction in the Neoglacial period (roughly 5700 BP or 4300 BCto the present), but agrees better with the Arctic reconstruction during the HCO. I’m not completely sure why that is.

Figure 5

Comparing figure 4 to figure 5, we can see that Alley has a very flat trend and is more active than Vinther. Vinther is a better match to our Northern Hemisphere mid-latitude reconstruction. Alley’s reconstruction starts to show the HCO and then fizzles at about 1,000 years in to it. Figures 4 and 5 are anomalies from the mean temperature from 9000 BP to 500 BP, however, which distorts the picture a bit given the two reconstructions differ on the temperatures of the HCO and the LIA. I refer you to figure 2, where we compare Vinther to Alley in actual temperature and not in an anomaly form. Here the two reconstructions agree on the temperature of LIA, but the Alley reconstruction does not see the HCO. We see that the key difference between the two is the degree of warming during the HCO.

Why are Alley and Vinther different?

The short answer is that Vinther, et al. (2009) corrected their ice core records, including GISP II and GRIP, for elevation differences and Alley did not. In Vinther’s words:

“The previous interpretation of evidence from stable isotopes (δ18O) in water from GIS [Greenland Ice Sheet] ice cores was that Holocene climate variability on the GIS differed spatially and that a consistent Holocene climate optimum—the unusually warm period from about 9,000 to 6,000 years ago found in many northern latitude palaeoclimate records—did not exist. Here we extract both the Greenland Holocene temperature history and the evolution of GIS surface elevation at four GIS locations. We achieve this by comparing δ18O from GIS ice cores with δ18O from ice cores from small marginal icecaps [Agassiz and Renland]. Contrary to the earlier interpretation of δ18O evidence from ice cores, our new temperature history reveals a pronounced Holocene climatic optimum in Greenland coinciding with maximum thinning near the GIS margins. Our δ18O -based results are corroborated by the air content of ice cores, a proxy for surface elevation.”

In figure 6 we see a summary of the Vinther, et al. (2009) data, it is their figure 1.

Figure 6 (Source: Vinther, et al. 2009)

The six cores are well distributed across Greenland, with Agassiz on Ellesmere Island very close to Greenland. Agassiz and Renland are both coastal cores and have similar profiles. It is possible to reconstruct the elevation histories for these two locations with confidence, so they are used to develop corrections for the remaining 4 ice cores. All six core records shown were included in the Vinther, et al. (2009) reconstruction after adjustment for elevation and ice thickness changes, but the Agassiz and Renland cores are the key cores. The corrections to these cores are shown in 6D. The δ18O profiles for these cores, after the uplift (or elevation) correction has been applied, is shown in 6c. Considering that Agassiz and Renland are on opposite sides of the GIS and 1,500 km apart, the agreement between the two corrected records is astounding, as Vinther, et al. (2009) described it in their paper.

Alley’s reconstruction focused on the GRIP and GISP II cores, these two cores are 30 km apart in central Greenland, they are combined into one point called GRIP in figure 6.

Below is a better location map for the Greenland ice cores, shown as figure 7.

Figure 7 (Source CDIAC)

Temperature determination in these ice cores is done with a function of δ18O and it has been shown by Johnsen and White (1989) that the average δ18O level over and around the Greenland Ice Sheet (GIS) is almost completely described by altitude (-0.6‰/100m) and latitude (-0.54‰/degree N). The altitude effect is due to the moist-adiabatic cooling of an air mass rising above the GIS. As it cools, precipitation and fractionation take place. There are more details on this in the Vinther, et al., 2009 supplementary materials. Thus, there is a sound basis for building a good δ18O temperature record if the altitude of the ice surface is known throughout the Holocene. Elevation differences must be taken into account. As Vinther, et al. (2009) write:

“… the differences in the long-term δ18O trends seem to be related to changing GIS elevation …”

The Holocene Climatic Optimum was a warm period and it caused melting of the GIS. Thinning at the Camp Century and DYE-3 sites started very soon after the HCO began over 9,000 years ago. The thinning progressed from there to the GISP II/GRIP location in a few thousand years, certainly by 6,000 BP. This affected the GISP δ18O temperature record and all but eliminated the HCO response that we see in other Northern Hemisphere records. The elevation corrections applied to the four sites, including GRIP, NGRIP and GISP II are shown in figure 8, from Vinther, et al. (2009).

Figure 8

The Camp Century and DYE-3 locations are on the coast and they are affected most. In the interior GRIP and NGRIP locations (remember GISP II is next to GRIP, see figure 7) the effect is less, but still significant.


If we accept the work that Vinther, et al. (2009) have done as being correct, and I see no problems with it, then the Aggasiz-Renland δ18O records, after correction for elevation records are correct. These records are 1,500 km apart and on opposite sides of the GIS, thus the temperature record of Greenland for this period must be fairly uniform for this period of time. Because of the geological conditions at the Aggasiz and Renland sites, their elevation histories can be reconstruction with some confidence as explained in Vinther, et al.’s paper and supplementary materials. Given that we also know the controls on the average δ18O with confidence, then we can provide a reliable temperature record for these sites. This is the record used in my Arctic reconstruction and the other 8 records used in the reconstruction agree fairly well.

Vinther, et al.’s reconstruction also agrees well with my Northern Hemisphere reconstruction from 4,000 BC to the present. It reaches a lower temperature extreme in the HCO, but matches the HCO of my Arctic reconstruction. Generally, I prefer the Vinther et al. reconstruction to Alley’s earlier GISP II reconstruction for the purpose of detecting the major climatic events of the Holocene and estimating the difference between HCO temperatures and LIA temperatures.

However, for locating climate events in time and whether the event is a warming event or a cooling event, using a single ice core proxy, that is well dated is fine. And the dating error in ice cores is very low, less than 1% (Alley, 2000). It is just that the magnitude of the temperature swings are probably incorrect in the GISP II and GRIP cores due to elevation changes as Vinther, et al. have shown. These changes (or errors in temperature) are the most severe in the HCO. This problem affects the magnitude of the estimated temperature but not the timing of the events.

Using multiple proxies, as I have, helps measure a more accurate and robust temperature anomaly for a region or the whole globe, but adversely affects the timing of events due to averaging multiple proxies with possibly inaccurate dates. Dating errors of 100 to 150 years are probably common and when averaging records with this sort of error, there will be loss of short term amplitude and problems estimating the timing of major events. This always needs to be considered in this sort of work. Amplitude reduction or excessive smoothing of the temperature reconstruction can be minimized by using fewer proxies, higher resolution proxies (shorter sample intervals), minimizing the proxy drop out at both ends of the reconstruction by avoiding short term proxies, and selecting proxies that are not overly affected by local events or local geology. Careful proxy selection is critical for a robust record, for more details on proxies to be avoided and proxies to include see my posts on the reconstructions I made. The final post, which will lead you to the others is here.

So, what is the purpose? Do you want to know, as accurately as possible, when a Northern Hemisphere warming event or cooling event occurred? Then using GISP II or GRIP will work best. Do you want to estimate the average temperature change during the event? Then I would recommend my reconstructions, but realize that the estimate may be conservative and the date of the event may be incorrect by 100 to 150 years. Our knowledge and data about Holocene temperatures are limited, but by using what we have wisely we can begin to get our arms around it.

A Holocene Temperature Reconstruction Part 4: The global reconstruction

By Andy May

In previous posts (here, here and here), we have shown reconstructions for the Antarctic, Southern Hemisphere mid-latitudes, the tropics, the Northern Hemisphere mid-latitudes, and the Arctic. Here we combine them into a simple global temperature reconstruction. The five regional reconstructions are shown in figure 1. The R code to map the proxy locations, the references and metadata for the proxies, and the global reconstruction spreadsheet can be downloaded here. For a description of the proxies and methods used, see part 1, here.

Figure 1A, all proxies except TN057-17 on the Antarctic Polar Front

Figure 1B, the proxies used for the reconstructions

It is interesting that the Northern Hemisphere is the odd reconstruction. This was also true for the Marcott et al. (2013) Northern Hemisphere reconstruction from 30°N to 60°N, see figure S10f, in their supplementary materials. The Northern Hemisphere has the greatest temperature variation of the five regions and a clearly different trend. Is this because it contains most of the land? Perhaps so. It may be, in part, the impact of the melting continental glaciers from the last glacial advance. Certainly, the high Northern Hemisphere insolation, early in the Holocene due to orbital precession and obliquity played a significant role (see figure 2 in part 1, also shown for convenience as figure 2 below). In the figure, the colored curves are the seasonal changes due to precession and the background color is insolation by latitude due to obliquity changes. The black curve is the Greenland NGRIP temperature reconstruction, note that the end of the last glacial period is when both orbital obliquity and precession hit their peak insolation in the Northern Hemisphere. The labels on the curves indicate Northern Hemisphere as “N” and Southern Hemisphere as “S.” The letters after that are the first letters of the months of the year. At the beginning of the Holocene, the Northern Hemisphere summer had maximal insolation due to precession and the higher latitudes (poles) had greater insolation, due to obliquity, at the expense of the tropics. Thus, both the precession cycle and the obliquity cycle were in their warmest phases for the Northern Hemisphere mid and high latitudes. This changed a few thousand years later and the climatic equator (the Intertropical Convergence Zone) shifted and the long Neoglacial cooling period began (see figure 12, in part 2).

Figure 2 (Source: Javier, see his post for a detailed explanation of the figure.)

The Southern Hemisphere is also a bit anomalous, with a dip in the period of the HCO, corresponding with a dip in winter insolation in the Southern Hemisphere. The other interesting thing about the reconstructions is that the Northern Hemisphere has a higher and longer Holocene Climatic Optimum. The Northern Hemisphere was affected much more by the last glacial advance due to the large continental ice masses there. The Southern Hemisphere ice was mostly sea ice which, presumably, melts at a steadier rate with less dramatic effect.

The Arctic and Antarctic each cover 6.7% of the globe, the southern and northern mid-latitudes cover 18.3% each and the tropics covers 50%. If we weight each reconstruction by the area of its region we get the reconstruction in figure 3. Figure 3A uses all proxies, except for TN057-17, which was removed in part 2. Figure 3B also eliminates ODP-658C, KY07-04-01 and OCE326-GGC26. The removal of the latter three proxies are discussed in part 2 and part 3. The two reconstructions only differ in detail.

Figure 3A, all proxies

Figure 3B, three additional proxies removed

We will discuss the reconstruction in figure 3B since we prefer it. In this reconstruction, the depth of the Little Ice Age (LIA) occurs in 1610 AD. The apparent Medieval Warm Period (MWP) is smeared over several hundred years and occurs from around 510 AD to 1050 AD which does not fit the historical record. Oddly, only the Southern Hemisphere and the tropics show a distinct Medieval Warm Period (MWP) in its historical time. This is despite abundant historical evidence of a Northern Hemisphere MWP from around 900 AD to 1200 AD. The Antarctic reconstruction shows several warm spikes during the period, but nothing very distinct. The reason for the lack of a distinct MWP signature in the northern reconstructions is not known. In part 3 we looked at the individual proxies for the Northern Hemisphere and saw that they disagree on the presence and timing of the MWP.

The Roman Warm Period (RWP) shows up well in the reconstruction, at about the right time. The “collapse of civilization” at the end of the Bronze Age is clearly seen. The 4.2 kiloyear event that led to the collapse of the Akkadian empire in 4170 BP can be seen (deMenocal, 2001). The 5.9 kyr event that occurred as the Sahara was turning into a desert, causing a great migration to the Nile valley that ultimately resulted in the Egyptian Old Kingdom is clearly seen. The LIA is the most significant climatic event of the Holocene without question, but the second most severe climatic event may well be the 8.2 kyr event. This event ended the Pre-Pottery Neolithic B culture and was when the Black Sea was catastrophically connected to the Mediterranean in an event that may be remembered as Noah’s great flood (Ryan and Pittman). The 10.3 kiloyear event takes place about the time the Pre-Pottery Neolithic period began. For more details on human history and climate change see “Climate and Human Civilization over the last 18,000 years” here. The historical climatic events match this reconstruction well, except for the MWP.

The details of the regional areas are in Table 1. This table is different from the one presented in part 1 of this series because after part 1 was put up we dropped ODP-658C from the tropics reconstruction and KY07-04-01 and OCE326-GGC26 from the Northern Hemisphere reconstruction. Marcott, et al. (2013) used 73 proxies for their reconstruction and our first pass retained 31 of these and added the Rosenthal et al. (2013) Indonesian proxy for a total of 32. As the study progressed we dropped three more proxies and ended with 29. Fifty-five percent of the proxies are north of 30°N and only 21% are south of 30°S.

Table 1

If we simply average the 5 reconstructions with no weighting, we get the reconstruction in figure 4.

Figure 4, Straight average, no weighting, final proxy set

The two reconstructions are not very different. In this reconstruction, the depth of the Little Ice Age (LIA) occurs between 1530 AD and 1670 AD and the temperature anomaly is -0.84°C. The Holocene Climatic Optimum (HCO) runs from 10500 BP to 4500 BP and has numerous peaks between 0.35°C and 0.48°C. Figure 3B is similar, with a slightly larger temperature range. The average temperature difference then, in these reconstructions, is between 1.2°C and 1.4°C. This compares well to the geological and biological evidence presented in Javier, 2017.

A word about error

There are many sources of potential error in these reconstructions. In this series of posts, we have emphasized those sources we thought were most important and significant. Specifically, we focused on the geographic distribution of the proxies, proxy selection, the choice of the mean used to generate the temperature anomalies, the effects of proxy dropout, proxy resolution, and the impact of local conditions on the proxies. The latter problem relates to how applicable the proxy is to regional climate as opposed to local climate. Examples of inappropriate proxies due to local conditions are TN057-17 and ODP-658C which are discussed in part 2.

Dating the proxy samples can be problematic. Marcott, et al. (2013) emphasize potential dating errors in their paper and supplementary materials. They consider dating errors to be the largest source of error. Marcott, et al. (2013) also provide a very detailed discussion of proxy-to-temperature calibration uncertainty in their supplementary materials. Generally, they assume one standard deviation (normally distributed) to be the error inherent in the proxy-to-temperature conversion, otherwise they follow the proxy author’s recommendations.

Marcott, et al. assumed a fundamental dating error of 120 to 150 years for most cases and accounted for it using a Monte Carlo procedure (1,000 realizations) which is detailed in their supplementary materials. For the layer counted Antarctic ice-core records they assumed a ±2% uncertainty and for Greenland cores they assumed a ±1% error. All radiocarbon dates were recalibrated using IntCal09. Our reconstructions use the original published dates and not the recalibrated dates.

Dating errors and proxy-to-temperature errors are undoubtedly important and Marcott et al. (2013) provide a good discussion of these problems and their supplementary database contains estimates for these sources of uncertainty. They also considered that some of the proxies may have a seasonal bias and attempted to account for this source of error in their Monte Carlo procedure. They do not believe that seasonal bias is an important source of error. We have nothing to add to their work on these uncertainties and the interested reader is referred to their paper. They do present an interesting figure in their supplementary materials displaying the 1,000 Monte Carlo realizations that result from their study of error due to dating and proxy-to-temperature conversion. It suggests that error due to these factors is roughly ±0.5°C. We show their figure as our figure 5:

Figure 5 (Source: Marcott, et al., 2013 supplementary material)

Marcott, et al. (2013) also provide their own latitudinal temperature reconstructions and display them in their supplementary figure S10, not reproduced here. Their regional reconstructions are different in detail than ours because they use more proxies, but their 30°N to 60°N reconstruction for the Holocene is the same big outlier we see in our figures 1A and 1B. They also note, as others have, that computer simulations of Holocene climate do not agree with the proxy reconstructions, the so-called Holocene temperature conundrum. The largest difference between the simulation results and the proxy reconstructions occurs in the mid-high latitude Northern Hemisphere, which suggests that the models are missing some key component of Northern Hemisphere climate. They suggest that the models may not be modeling north Atlantic Ocean circulation properly, we agree. The global climate models also have other problems, for a discussion see here.

We believe the greater source of error in these reconstructions is in the proxy selection. As documented in this series, some of the original 73 proxies are affected by resolution issues that hide significant climatic events and some are affected by local conditions that have no regional or global significance. Others cover short time spans that do not cover the two most important climatic features of the Holocene, the Little Ice Age and the Holocene Climatic Optimum.


We’ve tried to address the criticism of the Marcott et al. (2013) global temperature reconstruction. Steve McIntyre, Grant Foster and others contested their adjustments of the published proxy dates, their inclusion of some inconsistent proxies, and not compensating very well for proxy drop out. Javier has pointed out that their proxy reconstruction does not reflect abundant geological and biological evidence that the average sea surface temperatures were at least one degree Celsius warmer during the Holocene Climatic Optimum than during the Little Ice Age. In addition, the use of proxies that do not cover the interval from the LIA to the HCO is problematic since these are the two best defined temperature extremes in the period. Further, we are using temperature anomalies from the mean to build these reconstructions and prefer to get the mean from the period 9000 BP to 500BP so that the mean represents both the high temperatures of HCO and low temperatures of the LIA. This is not possible if the proxy does not cover this interval.

We also avoided proxies with long sample intervals (greater than 130 years) because they tend to reduce the resolution of the reconstruction and they dampen (“average out”) important details. The smallest climate cycle is roughly 61 to 64 years, the so-called “stadium wave,” and we want to try and get close to seeing its influence. In this simple reconstruction, we have tried to address these issues.

The reconstructions show a difference of 1.2°C to 1.4°C between the LIA and the HCO. This suggests that the underlying data support this temperature difference. These reconstructions also show more detail. The additional detail appears to correspond to known climatic events. While the LIA, HCO, Roman Warm Period, Minoan Warm Period and other historical events show up well in the reconstructions, the Medieval Warm Period does not, it appears dampened and offset in time from historical records. The reasons for this are unclear. As discussed in part 3, some Northern Hemisphere proxies show an MWP and some do not. The proxies may be wrong or perhaps the MWP occurred in different times or in different intensity in different places, smearing it on a global reconstruction. Either way proxy choice determines the MWP intensity and timing, which is disappointing. More work and better proxies are needed to improve our Holocene temperature record.

An accurate Holocene temperature reconstruction is not possible, even measuring the potential error in a reconstruction this long is incredibly difficult. Marcott, et al. (2013) did a good job of estimating dating error and proxy-to-temperature error, in our opinion. But, they do not address the other issues, such as proxy selection, that may be more important. But, even without a viable error calculation, a generally accepted estimate of Holocene temperature trends is greatly desired. To understand the present, we must know the past. This is a very simple reconstruction and it is not meant to be definitive, but we present it as a starting point for future work. It is a presentation of the data and some useful tools needed to work the data.

To improve the reconstruction, I think we need to compare it and the component proxies to other data. In particular, historical records, archeological records, glacial advance histories, biological and geological data. This “outside data” can be used to select proxies and guide the reconstruction.

The R code to map the proxy locations, the references and metadata for the proxies, and the global reconstruction spreadsheet can be downloaded here.

I am very grateful to Javier who has read this post and made many very helpful suggestions. Any errors are the author’s alone.

A Holocene Temperature Reconstruction Part 3: The NH and Arctic

By Andy May

In the last post (see here) we reexamined the Marcott, et al. (2013) proxies for the Southern Hemisphere mid-latitudes and the tropics. In this post, we will present two more reconstructions using their proxies, these are for the Northern Hemisphere mid-latitudes (30°N to 60°N) and for the Arctic region (60°N to 90°N). These two regions contain over half of the proxies used in this study. The next post will present a global area-weighted composite temperature reconstruction. As we did in the previous two posts, we will examine each proxy and reject any that have an average time step greater than 130 years or if it does not cover at least part of the Little Ice Age (LIA) and the Holocene Climatic Optimum (HCO). We are looking for coverage from 9000 BP to 500 BP or very close to these values. Only simple statistical techniques that are easy to explain will be used.

Northern hemisphere mid-latitudes

There are 10 proxies that meet our basic criteria for the Northern Hemisphere reconstruction, although two of them are combined into one record. The final reconstruction is shown in figure 1. Figure 1A includes all proxies that meet our basic criteria, figure 1B excludes two anomalous proxies and trims the early data from two more to avoid spikes caused by proxy drop out. The R code, input and output datasets can be downloaded here.

Figure 1A, all proxies that meet the basic criteria (resolution and span)

Figure 1B, excludes KY07-04-01 and OCE326-GGC26

If all proxies are included, as in figure 1A, this reconstruction shows a very flat Holocene Climatic Optimum (HCO) from 10000 BP to 6800 BP and then a steady decline in temperatures to the Little Ice Age (LIA) around 240 years ago (180 BP or about 1770 AD). The range of Holocene temperatures in both reconstructions is 4°C, this is the largest range of any region, including the Arctic. We generally prefer the reconstruction in figure 1B and will discuss the features of this reconstruction here. Since the temperature change in this reconstruction exceeds that seen in the Antarctic and Arctic reconstructions, it calls into question the concept of “Polar Amplification.” We cannot say polar amplification does not exist, but we do not see evidence of it in these proxies. Excluding the two anomalous proxies the coldest portion of the LIA was around 1610 AD.

The 17th and 18th centuries were a time of intense cold weather in Europe, Asia and North America, these centuries were the worst part of the LIA. The early 18th century saw lakes freeze solid in Italy and ice skating took place in Venice. Ships were frozen into ice in New England in 1740. More stories of the severe cold in the Northern Hemisphere in the 18th century can be seen here. The 17th century, if anything, was worse. The 17th century revolutions, droughts, famines, wars and other calamities are detailed in Geoffrey Parker’s book Global Crisis.

In Parker’s book, we see historical records of unusually cold and devastating winters that occurred in Europe and the Middle East in 1620, the United States between 1640 and 1644, China in 1640, Hungary between 1638 and 1641. 1641 remains the coldest year ever in Scandinavia. In the Balkans, in 1654, wine and olive oil froze in jars. In Egypt, in the 1670’s, a country where furs were unknown, was so cold that the citizens started wearing fur coats. Crop yields plunged in Guangxi and Guangdong (Hong Kong area) in southern China due to very cold weather in 1633 and 1634. Icebergs floated down the Thames River in January of 1649 as Charles Stuart was beheaded. In 1698 it was reported, in London, by John Evelyn that the weather was colder than anyone could remember. Harvests failed in Scotland every year between 1688 and 1698 mainly due to cold. And the stories go on and on.

The highest Medieval Warm Period (MWP) peak is at 890 AD. The Medieval Warm Period is very tepid in this reconstruction. Some of the proxies show a bump near the historical MWP and some do not. Below are plots of each set, figure 2 is the set with a visible MWP and figure 3 is the set without.

The proxies with an apparent MWP in figure 2, reach their peaks at different times and they do not line up well, this spreads out the MWP in a reconstruction and dampens the amplitude. The only two that line up are Flarken Lake (Sweden) and D13822 (Portugal). The MWP peak in the MD01-2421 composite from Japan occurs a little later it does in the Newfoundland proxy OCE326-GGC26. MD95-2015 (southwest of Iceland) is a very anomalous proxy with peaks at 1110 AD and 760 AD. In short, in this reconstruction, while it appears the LIA is well defined, the MWP is not. The historical warming from around 760 AD to 1200 AD shows up in these proxies, but not as a single well-defined event.

Figure 2

The Northern Hemisphere proxies in figure 3 do not have a noticeable temperature anomaly in the MWP. KY07-04-01 is in the East China Sea, south of Japan. CH07-98-GGC19 is off the US east coast near Washington, DC; it shows a minor bump around 1060 AD to 900 AD. OCE326-GGC30 is near Nova Scotia, Canada; it shows no response at all. The IOW merged dataset is from the Baltic Sea near Sweden and it also shows no MWP response. These proxies run counter to historical records for this time period.

Figure 3

The Roman Warm Period peak is at 90 BC (figures 1A and 1B) and very noticeable in the reconstruction. So, we see the LIA and the Roman Warm Period here, but the MWP not so clearly. This could be because the proxies are erroneous or because the MWP occurred at different times in different areas and was dampened by averaging. The MWP exists, it is a matter of historical record, but it does not show up well in these proxies.

All nine proxy records are shown in figure 4A.

Figure 4A, all proxies

Figure 4B, proxies used

The anomalous records in figure 4A are OCE326-GGC26 (Sachs 2007, near Newfoundland), KY07-04-01 (Kubota et al., 2010, just south of Japan) and Flarken Lake (Seppa et al., 2005, in Sweden). Flarken Lake is probably being affected by meltwater from glaciers that remained in the area long after the last glacial maximum. The retreating ice delayed the Holocene Climatic Optimum in many northern areas (Bender, 2013). We do not think Flarken Lake was a problem and retained the proxy.


This proxy is just south of Newfoundland and near the Grand Banks. See the location map in figure 5. This proxy record is plotted alongside its neighbor, OCE326-30GGC, in figure 6. Both proxies agree well from 8000 BP to 0 BP, then OCE326-GGC30 flattens out like most of the Northern Hemisphere proxies and OCE326-GGC26 makes a large jump in temperature ending with a 7°C anomaly 11410 BP. This proxy is problematic and was excluded from the reconstruction.

Figure 5 (source: Sachs, 2007)

Figure 6


This proxy was also excluded from the reconstruction for being anomalous. The core is from the East China Sea near the southern tip of Japan. See the map in figure 7.

Figure 7 (Source: Kubota, et al., 2010)

Figure 8

The KY07-04-01 proxy is plotted in figure 8. The proxy is very flat from the present day to 10000 BP, with minor fluctuations up and down. This is a Mg/Ca proxy and the core is located near the mouth of the Changjiang (or Yangtze) River. This river is the largest in China, and has its mouth north of Shanghai. The water here is a varying mixture of fresh water from the river and sea water from the East China Sea. The composition of the water varies with monsoon intensity. The river discharge has also varied during the Holocene as inland glaciers melted. Finally, as noted in Kubota, et al. (2010), this temperature proxy does not compare well with other proxies in the area. The other proxies show normal cooling during the Holocene, see the lower portion of figure 9, which is taken from Kubota, et al., 2010. We chose to exclude this proxy from the reconstruction.

Figure 9 (source: Kubota, et al., 2010)

A map of all the Northern Hemisphere proxy locations can be seen in figure 10. For this region, we have a widespread set of proxies.

Figure 10

The Northern Hemisphere proxies represent a larger range of temperatures and a larger range of temperature anomalies than the other regions. For this reason, proxy drop out at the beginning of the proxy records, about 12000 BP, causes larger than normal temperature fluctuations. This is easily seen in figure 4A between 12000 BP and 10000 BP. Even after excluding KY07-04-01 and OCE326-GGC26, unrealistic fluctuations appeared as proxies ended in the early time and dropped out. To avoid this, we deleted the earliest 3 samples of OCE326-GGC30 and the earliest 20 samples of CH07-98-GGC19. By comparing the right side of figures 4A and 4B you can see what was eliminated.

Proxy drop out at both ends of the reconstructions is a problem. We are using anomalies from the mean for these reconstructions which helps, since the anomalies tend to have similar ranges. But, in the case of the Northern Hemisphere, even the anomalies have widely different values and depending upon the order in which they drop out, they can cause strange spikes at the beginning and the end of each reconstruction. The earliest few values and the last few values for the Northern Hemisphere proxies are shown in figure 11 as an example.

Figure 11

At 0 BP (1950 AD, the upper panel) we have no values for four of the proxies, “NA” means no value. The three remaining proxies have values of -1.698, -3.422, and -2.323, that average to -2.481. At -20 BP (1970) we only have two values as GGC30A has dropped out, so the average is a very different -1.86. Compare this to the situation at very early time where the MD012421 proxy is about -5.5 and the GGC30 proxy is 1.84 and you can see the problem with drop out in the Northern Hemisphere. This problem is much less pronounced in the other regions which show less variability. We only trimmed excessive values like this in the Northern Hemisphere.

Arctic reconstruction

The Arctic reconstruction is shown in figure 11. The R code and input and output datasets can be downloaded here.

Figure 11

The lowest point in the LIA in this reconstruction, occurs at 1850 AD. There is no well-defined Medieval Warm Period, but there are peaks at 850 AD and 1070 AD. The Roman Warm Period is seen from 270 BC to 50 BC. Figure 12 plots the nine component proxies.

Figure 12

A map showing the proxy locations is presented in figure 13. The proxies are all in the north Atlantic area, but widespread.

Figure 13

Only JR51GC-35 and GIK23258-2 look a little anomalous, but not severely so. GIK23258-2 (Sarnthein, et al., 2003) is the most northerly proxy with a latitude of 75°N. This may explain the slightly anomalous warm anomalies at about 9000 BP and between 2500 BP and 1000 BP. The Iceland proxy JR51GC-35 is quite spiky. The location of the JR51GC-35 is shown in figure 14. It is in an area where multiple currents can influence the temperature quite dramatically, which probably explains the spiky nature of the curve.

Figure 14


These regions have the most data and are probably well represented by the proxies. The Northern Hemisphere mid-latitude reconstruction is quite different from the other regions. Most of the regions show a Holocene temperature variability of ±1°C whereas the Northern hemisphere reconstruction shows a temperature variability of ±4°C.

Considering the abundant historical evidence from the Northern Hemisphere for the Medieval Warm Period, it is odd that this climatic event does not show up well in the Northern Hemisphere reconstruction. It is possible that this warming event took place in different areas at different times and this smeared and dampened the record. The variation in the Northern Hemisphere proxies suggests that the climatic history of the Northern Hemisphere was very complex during the Holocene, relative to the other regions.

We see no evidence of polar amplification in these reconstructions. The Northern Hemisphere mid-latitudes shows a larger range of temperatures than either the Arctic or the Antarctic.

The Northern Hemisphere reconstruction illustrates the problem with proxy selection and with temperature proxies in general. Proxies are not thermometers, they do not measure temperature directly. They react to temperature in the present day in a certain way and we assume they react in the same way in the distant past. How accurate are the temperature estimates? Further, we assume that burial and time have had no effect, or a predictable effect on the quantities measured. We assume that we have measured the age and depth or height of each sample accurately. Finally, we assume that each proxy represents the surface temperature of a very large area with no local distortions. So, how do we choose which proxies to include and which to reject? Our basic requirements of a span of 9000 BP to 500 BP and a resolution better than 130 years are reasonable. Was it reasonable to reject KY07-04-01 and OCE326-GGC26? Perhaps, but it is hard to tell, the decision was mostly subjective.

In the next post, we will present a global reconstruction. We will also discuss the various sources of error in the proxies.

The R code and input and output datasets for the Arctic reconstruction can be downloaded here.

I am very grateful to Javier who has read this post and made many very helpful suggestions. Any errors are the author’s alone.

A Holocene Temperature Reconstruction Part 2: More reconstructions

By Andy May

In the last post (see here) we introduced a new Holocene temperature reconstruction for Antarctica using some of the Marcott, et al. (2013) proxies. In this post, we will present two more reconstructions, one for the Southern Hemisphere mid-latitudes (60°S to 30°S) and another for the tropics (30°S to 30°N). The next post will present the Northern Hemisphere mid-latitudes (30°N to 60°N) and the Arctic (60°N to the North Pole). As we did for the Antarctic, we will examine each proxy and reject any that have an average time step greater than 130 years or if it does not cover at least part of the Little Ice Age (LIA) and the Holocene Climatic Optimum (HCO). We are looking for coverage from 9000 BP to 500 BP or very close to these values. Only simple statistical techniques that are easy to explain will be used.

Southern Hemisphere mid-latitudes

Our reconstruction for this region is shown in figure 1. The R code and the input and output datasets for the Southern Hemisphere mid-latitudes can be downloaded here.

Figure 1

This reconstruction has a more defined HCO than we saw in the Antarctic and it is placed between 8000 BP and 5000 BP. The HCO occurs at different times in different places as discussed by Renssen, et al. (2012). Following this, the temperature generally drops to a low in the LIA. In this reconstruction, we see two LIA lows, one at 1690 AD and one at 1550 AD. The Medieval Warm Period (peak 1030 AD) and the Roman Warm Period (peak 90 BC) are very distinct in this reconstruction. The Minoan Warm Period (peak 1890 BC) can also be seen.

There are only four proxies in this reconstruction, three in New Zealand and one off the coast of Chile. The locations are shown in figure 2.

Figure 2

Two of the proxies have been combined into one record, the three proxies used are plotted in figure 3.

Figure 3

Three proxies were rejected due to large sample intervals, TN057-17 (Nielsen, et al., 2004) was rejected because it was very anomalous. See the plot in figure 4.

Figure 4

TN057-17 is a sea surface temperature proxy located in the Southern Ocean, right on the Antarctic Polar Front (APF), see Figure 5. The APF has a very abrupt sea surface temperature change. It is the southern limit of synchrony with the Northern Hemisphere climate system. This location, currently, has sea ice cover about two weeks per year (Nielsen, et al., 2004) but the time of ice cover has changed a lot during the Holocene and this has probably had a dramatic effect on the proxy. The maximum sea ice cover was 4300 BP, which is also the time of the lowest TN057-17 temperature.

Figure 5, source (Nielsen, et al., 2004)

Sea ice presence (SIP) at the TN057-17 location is shown in figure 6.

Figure 6, Sea ice presence (SIP) at TN057-17 (Source: Nielsen, et al., 2004)

There is a risk that the TN057-17 proxy has been affected by local conditions that are only vaguely connected to climate change and for this reason the proxy was rejected.

The Chilean GeoB 3313-1 proxy (Lamy et al., 2002) only went back to 7000 BP and for this reason would be rejected on its own. But, the New Zealand proxy MD97-2121 (Pahnke and Sachs, et al., 2006) has nearly the same latitude and is continuous from 12464 BP to 3316 BP. So, these two proxies were merged by adjusting them to the mean of the overlapping interval 6900 BP to 4000 BP. See figure 7.

Figure 7

The logic in combining these two proxies is it gives us one more proxy in a region that has few available and, at least part of this composite is outside the New Zealand area. The most recent portion of MD97-2121 (4000 BP to 3300 BP) was not used in the composite as it looked suspicious. Pahnke and Sacks (2006) report that the MD97-2121 core top (the most recent sediments in the core) may be problematic due to lack of recent sediments at the cored location. In any case, a 3000-year-old core top, presumably close to the sea floor, has probably been churned quite a bit and should be greeted with suspicion. Older than 4000 BP, the results are consistent with GeoB-3313-1.


The reconstruction for the tropics (30°S to 30°N) is shown in figure 8. The ODP-658C proxy is problematic, so we present a reconstruction including it in figure 8A and one without it in figure 8B. The R code and the input and output datasets for the tropics can be downloaded here.

Figure 8A, with the ODP-658C proxy

Figure 8B, without the ODP-658C proxy

There is a very distinct LIA at 1630 AD. The two peaks around the classical MWP are 1090AD and 930 AD. The Roman Warm Period (90 BC) and the Minoan Warm Period are apparent. In this reconstruction, the HCO is from 9600 BP to 7700 BP.

There are eight usable proxies in the tropics using our criteria, they are plotted in figure 9.

Figure 9

The location of the proxies is shown in figure 10, there are quite a few in Indonesia so we did not put arrows on the figure for each one. One of the Indonesian proxies, Ros_BJ813GGC, is not a Marcott et al. (2013) proxy. It is from Rosenthal, et al. 2013.

Figure 10

The rejected proxies, except for ODP-658C, were all because of resolution or because they did not cover the period from the Holocene Climatic Optimum to the Little Ice Age. ODP-658C (deMenocal et al., 2000) is the brown line in figure 9. It is located off West Africa, the northern most arrow off West Africa in figure 10. The proxy is plotted in figure 11. This proxy was left out of the final reconstruction, but figure 8A shows a reconstruction that includes it.

Figure 11

The sharp break in the proxy at 5700 BP appears to be a data problem until we consider that this is the end of the African humid period when the Sahara turned into a desert due to the abrupt movement of the Intertropical Convergence Zone or “ITCZ” (deMenocal et al. 2000 and Javier, 2017). Figure 12 shows the worldwide change that took place around 5700 BP, it is from Javier’s essay here. This change in the location of the climatic equator (ITCZ) is often called the Mid-Holocene Transition when the world goes from the Holocene Climatic Optimum (HCO) period to the Neoglacial period. This data suggests that the Mid-Holocene Transition, in this area, occurred in less than 120 years between 5808 BP and 5683 BP using the dates given by deMenocal, et al. This is how long it took for sea surface temperatures at the ODP-658C location to increase over 2°C. The core location is shown in figure 12 with a red star. Around 5700 BP the ITCZ migrated from north of this location to south of it.

Figure 12 (Source: Javier, here)

The proxy temperature record labeled 17940 (Pelejero, et al., 1999) from the South China Sea could also be considered slightly anomalous since in the Neoglacial period it trends warmer, rather than cooler. It also shows no Holocene Climatic Optimum. The proxy is displayed below in figure 13.

Figure 13

The South China Sea is a very large marginal basin in the western Pacific. It is bounded by broad shallow shelves in the northwest and southwest that emerge during periods of low sea level. Sea level was low enough during the early Holocene for these shelves to be emergent. The position of the shelves can be seen by following the 100-meter isobath (water depth) contour in the map in figure 14. In addition, core 17940 is only 400 km from the mouth of the Pearl River, the second largest river in China. Due to the large sediment discharge from the river the location has a very high sedimentation rate, further it was larger in the past as the glaciers retreated to their present position and sea level was lower. We kept the proxy in the reconstruction, but recognize that it is sensitive to the changes in sea level experienced during the Holocene and to changes in the discharge rate from the Pearl River. The sea surface temperatures of the other cores in the South China Sea are similar.

Figure 14 (Source: Pelejero, et al. 1999)

In the next post, we will discuss the Northern Hemisphere Holocene proxies and the Arctic proxies. The supplementary materials for the tropics reconstruction can be downloaded here.


The Southern Hemisphere mid-latitude reconstruction is built from very little data. The data is mostly in the New Zealand area and cannot be considered representative of the whole region. But, we work with what we have.

The tropics reconstruction is built from widely dispersed proxies that sample all major ocean basins. It is probably representative of the region. The reconstruction without the anomalous ODP-658C proxy is probably the best to use. The Mid-Holocene Transition is a real event, but probably had no global temperature effect. It is merely a shift in the climatic equator or the Intertropical Convergence Zone or “ITCZ.” The ODP-658C location warmed dramatically 5700 BP, but presumably another location, that is not sampled, cooled as dramatically. Including the warming at the ODP-658C site, without the cooling elsewhere distorts the regional and global picture. Therefore, we excluded the proxy.

I am very grateful to Javier who has read this post and made many very helpful suggestions. Any errors are the author’s alone.

Thermodynamics and the greenhouse effect

There is an exciting new post on here, that discusses a new paper on thermodynamics and the greenhouse effect.  In addition to Gerlich and Tscheuschner and the new paper Hertzberg, et al. (2017), the recent paper Kramm and Dlugi (2011) is interesting.

Yes, indeed, all objects radiate energy if their temperature is above absolute zero.  No question about it.  But, if you place an object that is radiating at 101 degrees C next to an object radiating at 100 degrees C, they will both soon be radiating at 101 degrees C, not 201 degrees C.  A cooler object cannot warm a warmer object, it does not happen, sorry.  The second law of thermodynamics does apply.

“Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don’t understand it, but by that time you are so used to it, it doesn’t bother you any more. (Physicist Arnold Sommerfeld (1868-1951))”