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Earth's cooling came to sudden halt in 1900, study shows

An international study used tree rings and pollen to build the first record of global climate change, continent by continent, over 2,000 years.

By Staff writer / April 23, 2013

Emperor penguins walk across sea ice near Ross Island, Antarctica, in this 2012 photo released by Thomas Beer. The continent's pristine habitat provides a laboratory for scientists studying the effects of climate change.

Courtesy Thomas Beer/AP/File

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A reconstruction of 2,000 years of global temperatures shows that a long-term decline in Earth's temperatures ended abruptly about 1900, replaced by a warming trend that has continued despite the persistence into the 20th century of the factors driving the cooling, according to a new study.

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Indeed, for several continents, the increase in global average temperatures from the 19th century to the 20th was the highest century-to-century increase during the 2,000-year span, the study indicates. It's the first study to attempt building a millennial-scale climate history, continent by continent.

The research wasn't designed to identify the cause of the warming trend, which climate researchers say has been triggered by a buildup of greenhouse gases – mainly carbon dioxide – as humans burned increasing amounts of fossil fuel and altered the landscape in ways that released CO2.

Still, it's hard to explain 20th-century warming without including the influence of rising CO2 levels, because the factors driving the cooling were still present, notes Darrell Kaufman, a researcher at Northern Arizona University and one of the lead authors on the paper formally reporting the results in the journal Nature Geoscience.

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.

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