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New research may resolve a climate ‘conundrum’ across the history of human civilization

Posted on 14 June 2017 by dana1981

Earth’s last ice age ended about 12,000 years ago. The warmer and more stable climate the followed allowed for the development of agriculture and the rise of human civilization. This important period encompassing the past 12,000 years is referred to as the Holocene geological epoch. It also created a “conundrum” for climate scientists, because global temperatures simulated by climate models didn’t match reconstructions from proxy data.

To be specific, the overall temperature change during the Holocene matched pretty well in reconstructions and models, but the pattern didn’t. The best proxy reconstruction from a 2013 paper led by Shaun Marcott estimated more warming than models from 12,000 to 7,000 years ago. Then over the past 7,000 years, Marcott’s reconstruction estimated about 0.5°C cooling while model simulations showed the planet warming by about the same amount.

A new paper led by Jonathan Baker may help to resolve that discrepancy. The scientists examined stalagmites from a cave in the southern Ural Mountains of Russia. The ratio of oxygen isotopes in the stalagmites can be used to estimate past winter temperatures. The Marcott study had one known shortcoming – the proxy temperature data they used mostly represented the summer season. And as Baker explained, changes in the Earth’s orbital cycles have caused summer cooling and winter warming during the Holocene:

Because our orbit is elliptical, we’re not always the same distance from the sun. About 10,000 years ago, Earth was closest to the sun during summer and farthest during winter. Today it is the opposite. Based on this variable alone, we would expect winter warming and summer cooling in the northern hemisphere (and vice versa in the southern hemisphere) over the last 10,000 years.

During the period from 15,000 to 7,000 years ago, temperatures were rising because large ice sheets were disappearing. That was especially true in the summer because back then, the Earth was closest to the sun during that season. So the Marcott temperature reconstruction, which was predominantly based on summer temperature proxies, estimated a lot of warming from 15,000 to 7,000 years ago (more than in model simulations), then a small cooling thereafter, while models simulate a slight warming over the past 7,000 years due to a slow rise in greenhouse gases.

The stalagmite data in the Baker study show that winter temperatures behaved differently and can reconcile the discrepancies between the Marcott reconstruction and model simulations. This suggests that the climate models are right – Earth’s surface temperature warmed rapidly at the end of the last ice age, from about 17,000 to 7,000 years ago, then the rate of warming slowed as the climate stabilized. However, it didn’t reverse into a cooling trend, because atmospheric greenhouse gas levels were rising.

Then of course came the Industrial Revolution 200 years ago, and carbon dioxide levels consequently shot up due to humans burning fossil fuels. As a result, temperatures have spiked as well. Over the past 130 years, global surface temperatures have risen about 20 times faster than when the Earth transitioned out of the last ice age. Over the past 40 years, the rate of global warming has been 3 times faster yet.

And that’s in comparison to Earth’s fastest natural climate change, when it’s transitioning from an ice age to a warm period. Over the past 7,000 years, when human civilization was able to develop and thrive, Earth’s temperatures and climate were quite stable. The temperature change during the past 7,000 years was about 0.5°C. Humans have caused that much warming in just the past 25 years. If we follow through with the Paris agreement and manage to limit global warming to 2°C over a 200-year period, in that best-case scenario the Earth would still warm 20 times faster than a natural ice age transition. If we fail to cut carbon pollution, that rate could speed to more than 50 times faster than Earth’s fastest natural climate change.

There are several important points we can take from the Baker study.

Click here to read the rest

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Comments 1 to 6:

  1. Good  article, that resolves a long standing mystery. It shows the considerable power of climate models, but this message will be lost on the sceptics.

    The following graph may be useful, and was posted on RC some time ago. It covers the last 12,000 years and includes the temperature data from Marcott, plus various model temperature estimates, plus CO2 levels, all on the same graph, so the divergence is clear. Be warned, it appears to be from a sceptics blog, but does appear accurate.

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  2. Based on many of the posted comments, one might conclude that the word "Skeptical" in the website title doesn't imply skepticism about scientific proclamations. So, one may wonder what the adjective is meant to imply.

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    Moderator Response:

    [JH] Sloganeering snipped. 

    The explanatory statement appearing directly under Skeptical Science in the header of each page of this website reads as follows:

    Getting skeptical about global warming skepticism

  3. I apologize but i need help. I'm confused about the temperature changes over the Holocene since the end of the last ice age (or glaciation period). It appears that this piece and the plot provided by nigelj depict a warming of 1.5°C or so. Plots I've seen from ice core data over the last 400k years appears to me to show about an 8-10°C warming. (which actually does seem like an awful lot). Using that range,  I've been trying to impress my denialist friends that the earth had warmed at about 1°C per 1000 years, whereas now it's warming that much in just a century or so due to human driven climate change, 10 times as fast. 

    What am I missing here? This climate scientist wannabe would appreciate your help.

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  4. Johnboy @3, the graph in my link  only goes back about 12,000 years, so only shows some of the warming since the last ice age ended.

    The last ice age ended about 20, 000 years ago and there is about 5 degrees of warming coming out of the ice age. Refer in the following article from RC. Scroll down to figure 4.  This also shows 20th century temperatures spliced on.

    I'm no expert, and stand to be corrected if my interpretations are wrong, but the link in my post appears correct, and RC is a very reliable website run by climate scientists.

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  5. Nigelj@1,

    It's always better to look at the actual published source rather than dodgy 'skeptical' blog.

    The 'conundrum' Dana is talking about likely comes from (Liu et al 2014) where they state:

    A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [M13]. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.

    So L14 has already pointed out the possible M13 sesonal and hemispheric bias. They, hower, looked at SST reconstructon biases only. Look at Figure3 in L14: it has been known that N hemisphere models (3B) do match Marcott (3A) at least in sign.

    However, note that the total cooling shown by M13 from the peak of Holocene (ca 7ka BP) to the LIA dip, is some 0.5-0.6 degC only. Not 1.4C as the 'skeptical' blog clearly exaggerated. BTW, that latter graph is hardly readable with 4 plots superposed. The obliquity plot has nothing to do with the rest of the plots because Milankovic forcings do not have direct effect on global temperature, they only produce variations in Arctic temperatures. However 'skeptics' have superimposed and scaled the obliquity plot only to suggest to uninformed that obliquity is in direct correlation with temperature shown by M13, maybe to justify the bogus 'neo-glacial' label there. My 'uninformed' question would be then: why the obliquity is so different than T in the very first section of it labeled 'pre-boreal' (whatever that mysterious term means)? Logical answer: because the obliquity has nothing to do with this picture and does not belong there.

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  6. Chriskoz, yes fair criticism of the composite graph. However its very, very difficult to understand the article without some sort of graph, and it was something I had come across, and at least it showed broadly what was going on. 

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