Updated March 7, 2013, 2:03 p.m. ET
Earth Hotter Now Than Most of Past 11,000 Years



By GAUTAM NAIK
New research suggests that average global temperatures were higher in the last decade than over most of the previous 11,300 years, a finding that offers a long-term context for assessing modern-day climate change.
The study, published Thursday in the journal Science, aims to give a global overview of Earth's temperatures over the past 11,300 years—a relatively balmy period known as the Holocene that began after the last major ice age ended and encompasses all of recorded human civilization.
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The research shows that a one-degree temperature variation that took 11 millennia to occur since the end of the last major ice age has now been replicated in 150 years, since the early days of the Industrial Revolution.
Within that framework, the decade 2000-2009 was one of the warmest since modern record-keeping began, but global mean temperatures didn't breach the levels of the early Holocene. Now they are on track to do so, according to the Science paper. If the scientists' forecasts are right, the planet will be warmer in 2100 than it has been for 11,300 years.
The study, conducted by researchers from Oregon State University and Harvard University and funded by the U.S. National Science Foundation, also looks to shed light on a crucial question: Is the spike in global temperature recorded in the past 150 years unusual—the result of greenhouse-gas emissions from human activity—or can it be explained as natural, long-term variations in temperature?
The study points to human activity as the cause, because of the suddenness of the shift in temperature which appears to be out of whack with long-term trends.
"What's different is the rate of change," said Shaun Marcott, a paleoclimatologist at Oregon State University and lead author of the paper. "What we've seen over the past 150 years is much greater than anything we saw in the past 11,000 years."
The task of estimating Earth's ancient climate, a discipline known as paleoclimatology, is a challenging one. It relies on proxy measurements taken from things like marine fossils or ice cores that offer a physical record of past temperature. For example, as part of the process, scientists grow the marine organisms under varying temperatures, and link changes in their shells' chemical signature to different water temperatures. That data can then be used to study marine fossils. To confirm a finding, researchers typically check to see whether temperature records derived from one source—marine fossils, say—match those derived from other, unrelated sources, such as ice cores.
The new data may be the latest flash point in the debate over the reason behind Earth's rising temperatures. Many scientists blame increased emissions of greenhouse gases like carbon dioxide, or CO2. Others disagree, contending that natural factors, such as an increase in cloud cover, are at play.
While past reconstructions of the Holocene tend to provide a regional snapshot, Dr. Marcott's team collated data from 73 sites to build a more global picture from 11,300 years ago. The data was obtained from marine fossils, as well as pollen records from lakes and ice cores from Greenland.
Temperature reconstructions have already indicated that the recent warming trend is atypical in the context of the past 2,000 years. The new study concludes it is unusual over an 11,300-year span as well.
Global mean temperature varied within a range of one degree Celsius during the Holocene. It was mainly driven by a gradual shift in Earth's orbit, which changed the amount of sunlight falling on different parts of the globe.
The first half of the Holocene period was warm. Then there was a cooling trend that lasted 5,000 years or so. About 200 years ago, temperatures began to rise steadily.
Projections indicate that Earth's air temperature could increase anywhere from two degrees to five degrees Celsius by 2100.
"This paper throws down the gauntlet by showing that Earth is on its way to being warmer," said David M. Anderson, who heads the paleoclimatology program at the U.S. National Oceanic and Atmospheric Administration and who wasn't involved in the study. "By 2100, it will be a heck of a lot warmer than it was 11,000 years ago."
The insights that can be gained by studying ancient temperature trends are limited, however.
"It's a weakness to look at the world 11,000 years ago because those were sunlight-driven changes and not CO2-driven changes," Dr. Anderson said. Sunlight-related changes are gradual and vary across different parts of the globe; greenhouse-gas emissions trigger warming everywhere.
Added Dr. Marcott: Under a scenario of increased emissions, "whatever happens will be evenly distributed around the globe."
Write to Gautam Naik at gautam.naik@wsj.com
　
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