Sorry, the Little Ice Age Does Exist

By Andy May

This post has been translated into German by Christian Freuer here.

Renee Hannon (@hannon_renee) pointed out that Raphael Neukom, et al. (2019) compares the modern instrumental temperature record to the Pages2K proxy temperature record and declares that:

“… we find that the coldest epoch of the last millennium—the putative Little Ice Age—is most likely to have experienced the coldest temperatures during the fifteenth century in the central and eastern Pacific Ocean, during the seventeenth century in northwestern Europe and southeastern North America, and during the mid-nineteenth century over most of the remaining regions.”

Neukom, et al., 2019

Then, they compare this to the instrumental record since about 1900AD.

“By contrast, we find that the warmest period of the past two millennia occurred during the twentieth century for more than 98 per cent of the globe. This provides strong evidence that anthropogenic global warming is not only unparalleled in terms of absolute temperatures, but also unprecedented in spatial consistency within the context of the past 2,000 years.”

Neukom, et al., 2019

They are comparing sparse, poorly calibrated, disparate, poorly dated proxies with annual to centennial resolution to a global modern network of calibrated daily thermometer readings and their point relies on timing and accuracy. Global warming in the twentieth century was hardly uniform in any case, nearly all of it took place from 1920-1940 and 1980-2000, and temperatures in Antarctica and much of the Southern Ocean hardly changed at all, probably only 0.2°C since the late 19th century.

The location and type of Pages2K proxies used in the study are shown in their figure 1a, also our figure 1.

Figure 1 identifies some of the various types of proxies used and their locations, but it does not address the temporal resolution of the proxies or how many of them provide temperatures each year between 0AD and 2000AD. These quantities are important if we are to compare the 1,000-year-old Medieval Warming Period or the 400-year-old Little Ice Age to the thousands of calibrated daily thermometer readings we have today and make a judgement about warming or cooling spatial consistency. Figure 2 is figure 1a from Pages 2K, 2019. It provides more detail about the proxies.

Figure 3 shows the temporal resolution of the proxies in the Pages 2K 2017 version, there is no equivalent figure in the Pages 2K 2019 version. For most proxies it varies from annual (beige) to multidecadal (blues). Some of the tropical proxies have sub-annual resolution, they are coded in red in figure 3.

In figure 3, because of the colors, the high-resolution proxies shown in red look impressive, relative to the softer colors used for the lower resolution proxies. Figure 4 shows the availability of each type of proxy, it gives us a clearer picture of the temporal resolution of the Pages 2K proxy temperature reconstruction. We should remember that among the many criticisms of Marcott’s global reconstruction was that as proxies dropped out or entered it as it moved toward the modern period, their entry and exit changed the trends, also see here.

By comparing figures 2 and 3 we can see that, at least in the opinion of Pages 2K, the higher-temporal-resolution proxies are the coral proxies in orange. The coding suggests that these proxies have sub-annual resolution, as discussed in these USGS notes and these NCAR notes. But figure 4 shows that very few of these records extend back to the pre-industrial period, aka The Little Ice Age. By eyeballing figure 4 we see that we lose nearly half of the total proxy records by the middle of the Little Ice Age (~1500 to ~1600) and we lose nearly 80% by the Medieval Warm Period (~1000AD).

In the Methods section of Pages 2K (2019), they say that the sub-annual proxies were reduced to annual values by using the April-March averages in the reconstructions, which reduces the detail. They also admit that none of the reconstruction methods they tried “explicitly consider age uncertainties,” which is a fundamental problem when comparing ancient proxies to modern records.

There are other proxy problems besides their sparseness and poor temporal resolution. They are calibrated to instrumental records from the modern era. Most are organic proxies, tree rings, corals, sponges, etc. which are affected by human CO₂ emissions as well as modern warming. The remaining proxies, such as lake sediments, marine sediments, borehole readings, and ice cores, often lose accuracy and/or resolution with time due to geological (that is preservation) deterioration. In addition, estimating the correct date for a sample becomes less accurate the older the sample, a problem ignored in Pages 2K. In fact, the uncertainties for all reconstruction methods and all measurements increases backward in time (Pages2K, 2019). See figure 3 in the Pages 2k 2019 supplementary materials for a visual feel of the increase in error backward in time.

Discussion

By comparing minimum and maximum proxy temperatures over the Little Ice Age (~1300 to ~1850) and comparing them to minimum and maximum modern daily instrumental measurements since 1900, Neukom, et al. would have us believe that the modern warm period is unique. We have thousands of weather stations, ocean buoys, and ARGO floats around the world today. It is well known that daily temperature extremes often exceed 30°C, which is more than 70 times the differences being estimated by Neukom, et al. as shown in figure 5, which is from their paper. These daily extremes are captured in instrumental data, but only seen occasionally, by pure chance, inaccurately, and in a very naturally smoothed way in the proxies. The proxies undersample both the high and low temperatures. Further, accuracy decreases backward in time.

The Pages 2K 2019 paper emphasizes that tree ring records are detrended which causes a loss of “multicentennial-scale temperature variability.” Variability is exactly what Neukom, et al. are trying to measure! Then Pages 2K conclude that low-resolution marine records seem to overestimate the true variance—now just how do they know the true variance? They should just say they don’t know, which is the truth of the matter.

Claims that modern warming is unusual cannot be supported with the Pages 2K dataset. This is best seen in Neukom, et al.’s figure 2 that Renee Hannon found. It is shown in figure 5.

Except for the portion of figure 5 that is after 1900AD, the globe is very sparsely sampled, but at least temperature is measured with instruments in the 20th century. Before 1700AD, the record is not only poorly sampled, but is comprised of only the low temporal resolution, poorly dated, inaccurate proxy data described above.

How do we know that the global warming observed since 1950 or so is unusual in any respect? As seen in figure 6, modern warming has not happened uniformly around the planet and some areas have cooled according to AR6 and HadCRUT5.

In warming and cooling trends, changes do not occur at the same time all over the world. Convection and atmospheric circulation assure us of that. The precision and global coverage of our measurements determines how well we see the evolution of warming or cooling. What Neukom and Pages 2K observe may simply be due to the change in accuracy, coverage, and resolution of their proxy measurements of the Little Ice Age and the Medieval Warm Period, relative to today. I would be willing to bet that the extremes seen in figure 5 are a better representation of reality than their reconstruction, and I seriously doubt they can refute that statement.

Comparisons of measurements in the modern instrumental period to ancient proxies were invalid in Mann’s notorious hockey stick, and they are still invalid. Besides, whether a change in climate is global or regional is not evidence that it is due to changing CO₂ concentrations or natural. It is hard to show that global average temperature is even a valid climate metric. Air circulation patterns change all the time, especially in winter.

It is well known that the coldest period of the Little Ice Age occurred at different times in different places. It was around 1810 in the Makassar Strait in Indonesia and around 1650AD in Greenland, as shown here in figures 1 and 2.

However, the general temperature trends at both locations, over 10,000 miles apart, are similar. The effects of climate forcing, whether from the Sun or greenhouse gases, are not instantaneous everywhere at the same time. Solar energy penetrates deeply into the ocean which causes a significant delay in its climate effects, it also increases the power of solar radiation changes because solar changes accumulate in the oceans, which act as a battery. Greenhouse gas radiation does not penetrate the ocean surface, this both reduces its impact on climate relative to the Sun and reduces the delay in its effect, providing some support for Neukom’s hypothesis that more global synchronicity of climate change points toward greenhouse gas warming.

But solar-driven changes in air circulation patterns caused by ENSO, the PDO, NAO, AMO, etc. (see here) still cause regional and, sometimes global, delays in the relatively modest climatic impact of greenhouse gases.

Creating global reconstructions from proxies is foolish due to the sparseness of the proxies, their inaccuracy, and their different temporal resolutions. You can reduce resolution, but you cannot increase it. It is more meaningful to compare modern temperatures at a specific proxy location to that proxy, than to try and combine proxies into a global temperature, as discussed here.

Download the bibliography here.

Renee Hannon kindly reviewed this post.