The study, five years in the making, drew on the work of 87 scientists in 24 countries as part of the International Geosphere-Biosphere Program. One goal of the 27-year-old program is to gain a deeper understanding of Earth's climate history and the factors that contribute to climate variability.
The study used nature's proxies for thermometers – tree rings, pollen, and other natural temperature indicators – to build continent by continent a coordinated record of temperature changes during the past two millenniums.
Scientists use this proxy approach to reach further into the climate's temperature history than the relatively short thermometer record allows. Such efforts aim to put today's climate into a deeper historical context as well as to identify the duration and possible triggers for natural swings that the climate undergoes over a variety of time scales.
Last March, for instance, a team led by Shaun Marcott at Oregon State University used climate proxies to build a global temperature record reaching back 1,200 years – one that also noted the pre-1900 cooling trend.
Until now, however, the proxy approach has been used to reconstruct changes in global-average and hemisphere-wide temperatures, Dr. Kaufman explains.
"There was very little information about past climate variability at the regional scale," he says. Yet the team notes that no one lives in a global-average world. People live in specific regions where geography plays a vital role in shaping the climate patterns they experience.