The Azimuth Project
paleoclimate (Rev #3, changes)

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The history of the Earth’s climate includes some unique episodes such as

and also some repetitive events or cycles such as:


There are many interesting puzzles or problems in climate history, such as the cause of the glacial cycles, and why the difference between polar and equatorial temperatures is greater when the Earth is cold. On Azimuth, Nathan Urban writes:

By the way, the polar amplification issue brings up an interesting puzzle in paleoclimate, which is the meridional or pole-to-equator temperature gradient problem.

If you go back to some greenhouse periods such as the early Paleogene, the poles were much warmer than today, but the tropics weren’t that much warmer. The difference between polar and tropical surface temperatures, or meridional temperature gradient, was smaller back then &mash; maybe a 15 °C difference instead of the modern ~30 °C difference.

The problem is that we are unable to explain the magnitude of this reduced temperature gradient using our climate models, even though those models do predict polar amplification of warming. This implies that we are missing some positive feedback which is active near the poles, or possibly some negative feedback which is active near the tropics. Maybe clouds, or alterations in meridional ocean heat transport.

This is related to hypothesis about a “tropical thermostat”. This is contentious and some have argued that the gradient wasn’t really as small as some people think. One classic paper is Norris et al., one can work forward from there.

Appy Sluijs discussed this problem with respect to the PETM, as has Huber (and a blog entry); also this paper.

I even found a paper discussing its impact on isotope proxies during the Azolla event.

Abbott and Tziperman have discussed polar cloud feedbacks here and here. Kump and Pollard have also discussed cloud feedbacks, more to get the overall magnitude of the climate right than to explain the temperature gradient, IIRC.

This isn’t really my area and so I don’t have a coherent overview or collection of references. These are just papers I’ve happened across.

Also, there is the faint young sun paradox.


For a good review of Cenozoic climate and CO2, see:

  • Zachos et al., Trends, rhythms, and aberrations in global climate 65 Ma to present, Nature 292 (27 April 2001), 686–693.

Also see:

category: climate