The Azimuth Project


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.

Staffan Liljegren recommends this for an integrative overview:

Also see:

A list of references compiled by Nathan Urban:

Oscillators and relaxation phenomena in Pleistocene climate theory

History of the Greenland Ice Sheet: paleoclimatic insights

Back to the future: Greenland’s contribution to sea-level change

New oceanic proxies for paleoclimate

The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future

Methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events: setting appropriate parameters for discussion

Lessons from Earth’s Past

Where are you heading Earth?

Convergent Cenozoic CO2 history

Ice Age Earth Rotation

The snowball Earth hypothesis: testing the limits of global change

What Caused the Younger Dryas Cold Event?

Divergence pitfalls in tree-ring research

The role of Southern Ocean processes in orbital and millennial CO2 variations – A synthesis

Climate of the last million years: new insights from EPICA and other records

Palaeoclimate: Delayed Holocene warming

Progress in Paleoclimate Modeling

Unlocking the mysteries of the ice ages

The Deglaciation of the Northern Hemisphere: A Global Perspective

Pliocene climate, processes and problems

Sea levels: change and variability during warm intervals

Mechanisms of abrupt climate change of the last glacial period

Interglacial diversity

category: climate