A new technique, if verified, could transform our capacity to understand ancient climates, arming us with far more detailed knowledge of what to expect in a warming world. The method uses carbonate deposits between soil and gravel detritus in arid and semi-arid zones. These are laid down like tree rings, allowing us to learn about the climatic conditions at the time.

Paleoclimatology, the study of climates before we had widespread thermometers, has been hampered by a shortage of suitable proxy measures. Some tree species store indications of the conditions as they grew in their growth rings. Similarly stalactites, stalagmites, and lake sediments can provide an insight into the local environment when they were laid down.

All have their limitations however. The further you go back the less likely it is that there will be tree or coral records, and other measurements tend to be geographically restricted. We have only the vaguest knowledge of the climate over huge areas of the world at crucial points in time.

Professor Ronald Amundson of the University of California, Berkeley has published a new method in the Proceedings of the National Academy of Sciences, and used it to chart the temperature and rainfall of Wyoming’s Wind River Basin over the last 120,000 years.

Amundson’s technique relies on carbonate layers on the bottom of alluvial gravel debris. These deposits, known as pedothems, are far thinner than tree rings, and consequently give far poorer resolution. Instead of being able to tell what a single year was like, they give an average over a thousand years.

"The cool thing that this study reveals is that within soil - an unlikely reservoir given how 'messy' most people think it is - there is a mineral that accumulates steadily and creates some of the most detailed information to date on the Earth's past climates," Amundson said in a statement.

^{During a minor ice age a persistant high pressure system over the Canadian ice sheet reversed the dominant pattern of North American precipitation, bringing stronger rain bearing winds from the Gulf of Mexico during summer, and possibly suppressing winter snows from the Pacific. Erik Oerter}

Amundson and his PhD Student Erik Oerter used laser ablation to collect microscopic samples from pedothems. “It is evident that the carbonate coatings formed in concentric bands around the rocks, much like the annual growth rings in a tree, except that these laminations form over timescales of several hundred years," Oerter said. The ratio of Uranium-234 to Uranium-238 indicates the age of the pedothem, while carbon and oxygen isotopic ratios provide an indication of temperature and rainfall at the time

Moreover, plants preferentially remove carbon-13, so lower levels left in the soil are indicative of higher levels of plant productivity, indicating better growing conditions.

This analysis revealed oxygen-18 levels spiked coinciding with a “previously hypothesized” period between 55,000 and 70,000 years ago. This has been explained as warm winds bringing summer rain from the Gulf of Mexico replacing winter snows from the Pacific. The minor ice age conditions at the time were very different from anything likely to occur soon. However, Oerter said, “The techniques that we developed can now be applied to similar soil deposits to fill in key gaps in the paleoclimate record,” including local effects of past climatic conditions most similar to the greenhouse effect we anticipate.