By Andy May
The Holocene Thermal Optimum ended at different times in different parts of the world, but it had ended everywhere by 4,000 BP (BP here means the number of years before 2000) and the world began to cool. The timeline shown in Figure 1 shows the GISP2 Central Greenland ice core temperature proxies in blue and the HadCRUT 4.4 surface temperature estimates for the same area in red.
Figure 1 (click on the figure to download in full resolution)
Major events in human civilization are noted on the graph. If you download the pdf it prints well on 8.5×11 inch paper. This timeline provides more detail for the last 4,000 years than I could fit on my previous timeline to 18,000 years. The principle reference for the ice core study can be seen here. At the top of the graph the Blytt and Sernander climate periods are noted, these periods are based on their studies of Danish peat bogs and the names are still used in the literature.
Of necessity, this post covers the climate and human events in the northern hemisphere. Some of the climatic events described, like the Medieval Warm Period, were worldwide events. But, some may have only occurred in the northern hemisphere. Nearly all of the historical notes are from Professor Wolfgang Behringer’s excellent book A Cultural History of Climate. The book was suggested to me by Dr. Ronan Connolly and I highly recommend it. It was originally published in German in 2007, I read the 2010 English translation by Patrick Camiller. The translation is quite good and the book is very readable, even addicting.
The GISP2 temperature record covers the interval from 95 years ago to 50,000 years ago. I pulled a Michael Mann “trick” and spliced measured Greenland (HadCRUT 4.4) surface temperatures onto the ice core record to bring the graph up to the year 2000. That way it shows the modern warm period. The spliced measured temperatures are smoothed with a 20 year moving average. The HadCRUT 4.4 worldwide gridded temperature anomalies were read from a NetCDF dataset. The R script for reading and mapping the data can be seen here. The script is in a Word file due to WordPress restrictions, save it as an “.R” text (ascii) file if you want to run it.
The worldwide HadCRUT temperature anomaly grid is a 5° by 5° grid and many of the points over Greenland are unpopulated (null). The populated points near the GISP2 ice core site were averaged over the 166 year span of the record (since 1850). The points used and the location of the GISP2 ice core are shown in Figure 2 (made with Google Earth) below. The smoothed anomalies were added to the ice core temperature average in the overlapping interval, from 1850 to 1905.
Ice core data from this far north are very useful. It has long been known that ocean and atmospheric temperatures at or near the equator have not changed much for millions of years, but temperature changes at the poles have been large, especially near the North Pole. This has often been called “polar amplification.” As global temperatures rise, evaporation increases in the tropics keeping tropical temperatures steady. As temperatures are “evaporatively buffered” in the tropics, global energy balance constraints force temperatures to increase in the high latitudes during a global warming event. So data from this far north show larger temperature swings than the world at large and they are easier to see.
Newell and Dopplick computed that the maximum temperature of the atmosphere over the ocean is 303K or 30°C (see also Hoffert, et al ). This is very similar to the equatorial sea surface temperature today (see Figure 3) and going back to the Cretaceous Period 65 million years ago. In the Cretaceous the higher sea surface temperatures covered much more of the oceans and there was no polar ice. They conclude that 30°C is the limiting temperature based on the balance between radiative input energy and energy lost due to the evaporation of sea water. Because the world ocean heat capacity is 1,000 times the heat capacity of the atmosphere, this is a very reasonable conclusion. For the details of the heat capacity calculation see the spreadsheet here. One should also consider that while the equatorial ocean surface temperature is about 30°C (Figure 3), the temperature of the world ocean at 2000 meters depth is only about 2°C. For this reason, the oceans are a huge temperature dampener, making the idea of a runaway atmospheric temperature increase unlikely.
Figure 3, Modern Ocean surface temperatures, whole year average, Deg. C (NOAA MIMOC)
Comparing northern hemisphere human history to the Greenland ice core record, it is apparent that large swings of temperatures in Greenland are related to European, Middle Eastern, Asian, and American events. Thus, we can safely assume these temperature swings affected most, if not all of the northern hemisphere. Behringer discusses this relationship in rich detail in his book. Comparing Antarctic ice core temperature proxies to the Greenland ice core records shows that the shorter events are different, or occur at different times in the southern hemisphere. The major events, for example the last glacial maximum and the Holocene Thermal Optimum (when human civilization began, the so called Neolithic Revolution) are apparent in both records.
The timeline begins at 4,000 BP and it shows steady cooling to the Little Ice Age (roughly 1250 to 1850 AD). It is easy to see that the Roman Warm Period was cooler (at least in Greenland) than the Minoan Warm Period and the Medieval and Modern Warm Periods are cooler than the Roman Warm Period. The Little Ice Age was a horrible time for mankind according to Behringer. Glaciers advanced in the Alps and destroyed homes, it was a time of perpetual war, famines and plagues. Horrible persecutions of Jews and “witches” were common. Society was suffering from the cold and lack of food and they needed to blame someone. They chose Jews and old unmarried women unfortunately. Over 50,000 witches were burned alive. Tens of thousands of Jews were massacred. Not because there was any proof, just because someone had to suffer for the bad climate. Some people, the masses mainly, seem to need to blame someone or humanity’s sins for natural disasters. Behringer notes that in The Little Ice Age: “In a society with no concept of the accidental, there was a tendency to personalize misfortune.”
Today environmental zealots assume that climate change is not only man-made but dangerous. They have no proof of either assertion, of course, but they “believe” with religious fervor in both ideas. Then we see, in Behringer’s book, the following quote from Archbishop Agobard of Lyons (769-840 AD) in his sermon “On Hail and Thunder:”
“In these parts nearly everyone – nobles and common folk, town and country, young and old – believe that human beings can bring about hail and thunder…We have seen and heard how most people are gripped by such nonsense, indeed possessed by such stupidity…”
Behringer offers up a 1486 woodcut of a sorceress conjuring up a hailstorm, I show this below in Figure 4. Behringer labels this woodcut “Anthropogenic Climate Change.” This is on page 129.
Figure 4: Anthropogenic Climate Change
One needs to ask “Have we, as a society, learned anything since 1486?” Why do some need to assume that certain people must be to blame for natural disasters?
There are several lessons to be learned from Behringer’s book and the timeline. First, there is no perfect temperature. Man, even in pre-industrial times, adapted to a variety of temperatures and he has always done better in warm times and worse in cold times. Second, why would anyone want to go back to the pre-industrial climate? The Washington Post says the goal of the Paris Climate Conference was get the world to agree to limit global warming to less than two degrees above pre-industrial temperatures. Pre-industrial times? That’s the Little Ice Age, when it snowed in July, a time of endless war, famine and plague. According to the Greenland ice core proxy data, temperatures 180 years ago were the nearly the coldest seen since the end of the last glacial period 10,000 years ago! Why measure our success in combating anthropogenic warming, if there is any such thing, from such an unusually cold time? It makes no sense, those times were awful.
As Behringer explains, the Neolithic (new stone age) revolution and the rise of ancient civilization became possible when temperatures were one to two degrees warmer than today. At the current rate of warming, even in polar amplified Greenland, this will take at least 200 years to reach. Why not set the limit at two degrees above the temperatures in the Holocene Thermal Optimum? That was the temperature that allowed civilization to begin after all. I, for one, would like to go back to that period much more than the Little Ice Age. Why have a limit? Evaporation limits the maximum temperature to 29-30 degrees anyway, we go to places with that temperature for a winter vacation all the time.
Currently temperatures are evaporatively buffered in the tropics at 29°-30°C. The rest of the planet could rise to nearly this temperature over many thousands of years, but it isn’t likely and it isn’t a catastrophe. Further, the Earth has already been there in the Paleocene-Eocene Thermal Maximum 55 million years ago and in the Cretaceous Period 65 million years ago when the Earth was ice free and palm trees grew in both Alaska and Antarctica. If warming continues, man-made or not, the ice at the poles will melt over time; but we can move to higher ground, we can adapt. It will not happen quickly but over many generations, most will not even notice it is happening. We are much more advanced today can adapt more easily than our ancestors and they adapted to even more extreme climatic events.
The last part of Behringer’s book is a good summary of the climate change debate in 2007. We have to admire the way he presents both sides of the debate in a “just the facts” manner. The following quote is from the penultimate page of the book:
“…cooling has always resulted in major social upheavals, whereas warming has sometimes led to a blossoming of culture. If we can learn anything from the history of culture, it is that, even if humans were ‘children of the Ice Age’, civilization was a product of climatic warming.”
“The future is hard to foresee. Serious scientists should refrain from slipping into the role of Nostradamus. Computer simulations cannot be better than the premises that guided the input of data: they show what is expected to happen, not the actual future. The history of the sciences is also a history of false theories and wrong predictions.”
Not much to add to that.