The two-degree limit has no scientific basis

By Andy May

For decades We have been told that we must not let global warming exceed two degrees Celsius above the “pre-industrial” global average temperature. Recently the IPCC lowered this limit to 1.5°C. In the latest IPCC report, called AR6, pre-industrial is defined as before 1750, but they use global temperatures from 1850-1900 as representative of the period because global average surface temperatures are not available for 1750.^[1] The U.S., Europe, and much of Asia were industrialized by 1900, so their numbers are clearly not representative of the period of interest, unless temperatures remained constant from 1750 to 1900, which is unlikely.

Why the focus on 2°? In a 2014 comment in Nature, David Victor and Charles Kennel tell us that there is little scientific basis for the 2°C figure, but it was a simple focal point and it “sounded bold and perhaps feasible.” (Victor & Kennel, 2014). Then they admit the goal is “effectively unachievable.”

What is the “pre-industrial?” Did it have an ideal climate that we wish to return to? The year 1750 was in the coldest and most miserable part of the Little Ice Age (LIA). The LIA was the coldest period in the Holocene Epoch, or since the last glacial period ended about 12,000 years ago, at least in much of the Northern Hemisphere.

Figure 1. Rosenthal, et al. temperature reconstruction of the 500-meter water in the Makassar Strait, Indonesia. Source: (Rosenthal, Linsley, & Oppo, 2013). For full size, click here or on the figure. The present-day temperature shown is from the Makassar Strait Argo data collected by the University of Hamburg from 2006-2016 at 500m (Gouretski, 2019)

Figure 1 shows a proxy reconstruction of the water temperature at 500 meters in the Makassar Strait of Indonesia. In the strait, the 500-meter water generally flows from the Northern Pacific Ocean to the Indian Ocean and the temperature represents North Pacific temperatures, with a minor contribution from the Banda Sea (Rosenthal, Linsley, & Oppo, 2013). It is similar in shape and amplitude to the various Greenland ice core temperature reconstructions (Figure 2). The reconstructed temperatures are compared to a modern (2006-2016) average 500-meter temperature (the red box) from an analysis of Argo data by the University of Hamburg (Gouretski, 2019).

Figure 2. Various Greenland Ice Core temperature reconstructions. The Vinther reconstruction in black is preferred (Vinther, et al., 2009)

For more details on the Rosenthal reconstruction see here, for more details on the various Greenland ice core reconstructions see here. The Greenland ice sheet has grown and shrunk a lot over the past 12,000 years and the resulting elevation changes have affected the Alley temperature reconstruction shown in light gray in Figure 2. For this reason, the elevation corrected reconstruction by Bo Vinther, shown with a thick black line is preferred (Vinther, et al., 2009). In addition, Figure 2 shows another modern reconstruction from 2000BC to 2000 by Kobashi (Kobashi, et al., 2011).

Both Figure 1 and Figure 2 show a distinct Little Ice Age low temperature, that occurs between 1600 and 1850. All the reconstructions show the Little Ice Age is about three degrees cooler than the Holocene Climatic Optimum, from 8000BC to around 4000BC. Greenland and the North Pacific lie in the Northern Hemisphere, which has a unique temperature history. Figure 3 compares the Northern Hemisphere to other latitude slices, all the slices are 30° of latitude, except the tropics which is from 30°N to 30°S. The Northern and Southern Hemispheres cover 60°N/S to 30°N/S, and the Arctic and Antarctic slices are greater than 60°. Notice that over long periods of time the Antarctic temperatures move in the opposite direction to Northern Hemisphere temperatures.

There are a total of 29 temperature proxy records used to make the curves in Figure 3, and over half of them are in the Northern Hemisphere or in the Arctic. There are seven in the tropics and only three in the Southern Hemisphere and three in the Antarctic. The coverage is very sparse, so sparse it is hard to draw any conclusions about global average temperature from this data. That said, we can look at proxies in a specific location and tell when temperatures at that location were warmer or colder, we can even estimate by how much. Willie Soon and colleagues discuss how to evaluate temperature proxies properly in two 2003 papers.^[2]

It is helpful that Rosenthal’s reconstruction shows about the same temperature profile as the best Greenland reconstruction, that is Vinther’s. They differ in detail, as one might expect and they are thousands of miles apart, but in general they tell the same story.

Figure 3 suggests that most climate change takes place in the Northern Hemisphere between 30°N and 60°N. Temperatures in the tropics don’t change much and Antarctic temperatures tend to move in the opposite direction from the Northern Hemisphere and vary less. Even the Arctic varies less than the Northern Hemisphere, as defined here. Only the tropics, the Arctic, and the Northern Hemisphere show a Little Ice Age, but they bottom at different times. Antarctica warms during the Little Ice Age, and cools after.

Figure 3. Temperature reconstructions by latitudinal slice. The Antarctic is 90°S to 60°S, Southern Hemisphere (SH) is from 60°S to 30°S, and the tropics is from 30°S to 30°N. The Northern Hemisphere follows the same pattern. See here for more details

For this post, I only need to point out that in the North Pacific and Greenland, we have likely seen temperatures three degrees warmer than the pre-industrial period (aka the Little Ice Age) during the Holocene Climatic Optimum from 7500BC to 4400BC. This is the time period that saw the birth of human civilization.

We also saw high temperatures during the so-called Minoan Warm Period about 1600BC, prior to the collapse of the Bronze Age civilization around 1200BC, due to cold and drought. The cooling after the Minoan Warm Period, lasted from around 1200BC until about 700BC. During this miserable period, called the Greek Dark Age, Mycenean civilization collapses, the Hittite Empire falls, and there were severe droughts in the U.S., India, Europe, Africa, and China.^[3]

Little Ice Age
The Greek Dark Age was cold and miserable, but nothing like the bitterly cold, dry, and stormy Little Ice Age, also known as the “pre-industrial” period that the IPCC measures climate change from.

The cold spells that frequently occurred during the pre-industrial period caused humanity many problems. Paul Homewood as well as Wolfgang Behringer’s excellent book, A Cultural History of Climate, and an article by Geoffrey Parker offer us a lot of historical examples. All over the world most glaciers reached their maximum Holocene extent during the Little Ice Age. In Chamonix, France advancing glaciers swallowed entire villages.

There was no summer during 1675, and it was the second coldest summer in the past 600 years in North America according to proxy evidence. The winter of 1657-1658 was particularly brutal. Massachusetts Bay and the Delaware River both froze over, allowing people and deer to cross on the ice. The Baltic Sea froze so hard that horses and loaded wagons could cross from Gdansk, Poland to the Hel Peninsula over 10 miles north of the city. Yet, the following summer was excessively hot in Italy and Greece. In India the monsoon failed that year, resulting in a devastating famine.

Between 1660 and 1680, more typhoons struck southern China at Guangdong Province, than at any other time in recorded history. In 1666, a hailstorm hit England and some of the hailstones were a foot in circumference, the size of softballs.

An enormously destructive hurricane hit the Caribbean Islands of Guadeloupe and Martinique in 1666 that resulted in 2,000 deaths and the destruction of a shore battery with walls 6 feet thick, as well as numerous ships.

Egypt in the 1670s had many very severe winters and people began to wear fur coats, something that had never happened in Egypt before. In the 1680s, the Sahel in Africa suffered a severe drought and Lake Chad reached the lowest level ever recorded.

The winter of 1691-1692 was very severe, starving wolves entered Vienna, Austria and attacked men and women on the streets. All the canals in Venice froze over and the mouth of the Nile River was choked with ice for a week. The cold of the 1690s caused a major famine in northern Europe and half the population of Finland died, as well as 15% of the population in Scotland. The Scottish famine was an important factor in its forced union with England. Mixed in with the cold years were occasional summers of intense heat and drought, such as the summers of 1693 and 1694 when the heat was unbearable in both England and Italy.

In 1715 a devastating hurricane struck the Bahamas and Florida killing between 1,000 and 2,000 people. That winter it was -20°C in Paris. There was a frost fair in London on the frozen Thames that year, with bonfires roasting oxen, carriages driven on the ice, and ice skating. The Little Ice Age was one of, if not the stormiest period in the Holocene in Europe, according to a study by Susana Costas and her colleagues.^[4] So much for the argument that extreme weather is increasing in modern times, it is not.

Is the two-degree limit meaningful?
According to Der Spiegel, Germany’s past Environment Minister, Norbert Röttgen, said in 2009 that if the two-degree limit were exceeded, “life on our planet, as we know it today, would no longer be possible.”

Der Spiegel calls this scientific nonsense. They claim that the father of the two-degree limit was Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research (PIK). Schellnhuber tells them that he estimated average global temperatures since the rise of Homo sapiens and decided from proxy data that over the past 130,000 years global average surface temperatures were never more than two degrees higher than before the beginning of the industrial revolution. This became the two-degree limit proposed by the European Council of environment ministers in 1996. The limit is arbitrary and speculative, and contradicted, at least in the Northern Hemisphere, by the data we show above. Currently Schellnhuber admits that keeping global warming below two degrees is not feasible. He shows no data that suggests 2° of warming is dangerous, he just attempts to show it is unusual.

Prior to 1900, data from the Southern Hemisphere and oceans is extremely sparse, and prior to 1850, there are only six proxy temperature records south of 30°S and the whole Earth only has 29. How does he know that the global average surface temperature did not exceed two degrees Celsius? The short answer is, he doesn’t. Estimating global average surface temperature before 1900 is speculative, and before 1850, virtually impossible, there just isn’t enough good data.

Ice cores and other temperature proxies are not very accurate, dating the observations is problematic, but as a general approximation of annual to multidecadal temperature at a specific location they are OK and likely in the ballpark. We can be comfortable that temperatures in Greenland and in the North Pacific, near the Makassar Strait, were three degrees warmer than during the Little Ice Age (pardon me, I mean the “pre-industrial”) between 6,000 and 10,000 years ago, at least most of the time.

Yale Professor and Nobel Prize winner William Nordhaus probably first suggested the two-degree limit in 1977 (Nordhaus, 1977). Nordhaus uses the same logic that Schellnhuber used. He speculates that the maximum global surface temperature over the past 100,000 years was two degrees higher than in the late 19^th century, so we should not exceed that. The problem is neither Nordhaus nor Schellnhuber know how much global surface temperatures have varied before 1850, and no one else does either. No one has any data that suggests two degrees of warming is dangerous for humanity or the environment.

The world is historically very cold and has been cold for millions of years, as can be seen by the Smithsonian Institute proxy reconstruction of the past 500 million years shown in Figure 4. This is still a proxy temperature reconstruction and has the same caveats mentioned above and the temporal resolution is very poor, since each point represents over five million years, but it is the best we can do. It shows the global average surface temperature for the past 500 million years is about 20-21 degrees Celsius, thus, currently, we are over five degrees below average.

Figure 4. Smithsonian estimate of global average surface temperature for the Phanerozoic, the past 500 million years. Source: (Scott & Lindsey, 2020)

Figure 4 was created using ideas developed by Chris Scotese, who also participated in the construction of the figure.^[5] Scotese has shown that the global average temperature is closely related to the equator-to-pole temperature gradient because equatorial temperatures don’t vary much over time, but polar temperatures do. Thus, warming of the entire Earth, is mainly a function of warming at the poles.

Temperature proxies that can be traced into the distant past only exist in a few places around the world. Proxies are not thermometers, and their relationship to past temperatures, especially thousands of years ago is a bit speculative. These proxies are sometimes sensitive to only one season, usually summer. Currently temperatures are rising much more quickly in the winter and at night. Winters are warming at twice the rate of summers and, as a result, the climate is milder now than in the late 19th century. In the same way, nights are warming faster than days, again making the climate milder. Finally, temperatures in the Arctic are rising much more quickly than in the rest of the world, including the Antarctic. This is arguably a very good thing for Siberia, Canada, Scandinavia, and Alaska. Figure 5 shows Earth’s warming from 1979 to 2019. The equatorial region shows minimal warming, the Southern Hemisphere shows almost no warming, with some areas actually cooling, and the Arctic and Eastern Europe have warmed quite a lot.

Figure 5. Warming from 1979 to 2019. Source: NASA

The impact of “global” warming depends upon where you live and what the past temperatures were.

Conclusions
In short, the two-degree limit has no scientific basis, it is an artificial political goal, meant to frighten the public into doing something. In a long and confusing paper, Jeroen van der Sluijs, et al. argue that artificially constructed political goals, like climate sensitivity or the two-degree limit, are political “anchors.” These are oversimplifications of a complex scientific topic that can help politicians energize a confused public into doing something, even if the scientific literature does not support it, or is too difficult to understand.

David Victor and Charles Kennel (Victor & Kennel, 2014) call the two-degree limit “wrong-headed.” As they point out, and Der Spiegel agrees, whether climate change is dangerous or not, depends upon where the observer resides. Any climate change is bound to be beneficial in some places and a problem in others.

Reto Knutti and colleagues write that the two-degree limit was a political decision, and that no scientific assessment ever recommended a particular target. Policymakers like to hide behind scientific evidence, demand actionable science, and claim to make science-based decisions, but Knutti, et al. argue that their approach is like a salad bar — they simply pick and choose (and fund) politically convenient studies, rather than a proper, balanced search for the truth. They continue:

“This 2°C warming target is perceived by the public as a universally accepted goal, identified by scientists as a safe limit that avoids dangerous climate change. This perception is incorrect: no scientific assessment has clearly justified or defended the 2°C target as a safe level of warming, and indeed, this is not a problem that science alone can address.”^[6]

We evolved and currently live in an ice age that began millions of years ago. The Holocene Epoch, which began 11,700 years ago, is only the latest of many warm periods (Interglacials) in this ongoing ice age. For a complete list of the interglacials of the past 2,000,000 years, see Figure 4 here. Ultimately, the planet will return to a glacial state. At that time, as well as now, humans must adapt to climate changes, and what we must do to adapt depends upon where we live.

Global mitigation of climate change is not possible, there is no well-defined solution for everyone. Local adaptation is the best solution, and this is regardless of the cause of the changes. As for the hysterical politicians, like AOC, declaring we are all going to die from climate change, forget it. We will not,^[7] as long as the oceans exist, the overall average surface temperature of the Earth is capped somewhere below a comfortable 30°C (86°F). Nothing to worry about here folks.

Download the bibliography here.

1. (IPCC, 2021, pp. SPM-5, TS-11, TS-28) ↑
2. (Soon & Baliunas, Proxy climatic and environmental changes of the past 1000 years, 2003) and (Soon, Baliunas, Idso, Idso, & Legates, 2003b), also see (May, 2020c, pp. 49-86) ↑
3. (Cline, 2014, p. Kindle location 3237) ↑
4. (Costas, Naughton, Goble, & Renssen, 2016) ↑
5. (Scotese, Song, Mills, & Meer, 2021) ↑
6. (Knutti, Rogelj, & Sedláček, 2016) ↑
7. (Newell & Dopplick, 1979) and (Sud, Walker, & Lau, 1999), as well as many others (see Richard Willoughby’s posts here and here) ↑

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