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Human activity driving unprecedented temperature shift

Average global temperatures are now higher than any point during the past 4,000 years despite being in the midst of what should be a cooling interval, reports a new study published in the journal Science.

The research — based on ice cores and other data from 73 sites sites around the world — constructs a temperature record for the past 11,300 years. It finds that the current global temperature is warmer than 70 to 80 percent of the era since the dawn of human civilization.

“We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years,” said lead author Shaun Marcott of Oregon State University. “Now we know that it is warmer than most of the past 11,300 years.”

Temperature record over the study period. Courtesy of Marcott et al. 2013

The results show that though Earth should be in the midst of a long-term cooling trend due to Earth’s position relative to the sun, temperatures are rising. The authors attribute the aberration to human activities, specifically carbon emissions from fossil fuels use.

“During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more,” Marcott said. “As the Earth’s orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend – but obviously, we are not.”

“The Earth’s climate is complex and responds to multiple forcings, including CO2 and solar insolation,” Marcott continued. “Both of those changed very slowly over the past 11,000 years. But in the last 100 years, the increase in CO2 through increased emissions from human activities has been significant. It is the only variable that can best explain the rapid increase in global temperatures.”

The study notes that warming is expected to further accelerate as carbon emissions continue to rise. Some climate models forecast up to 11.5 degrees Fahrenheit of warming by the end of the century, a rise that would be unprecedented in human history.

“What is most troubling is that this warming will be significantly greater than at any time during the past 11,300 years.” said study co-author Peter Clark.

Marcott et al. A Reconstruction of Regional and Global Temperature for the Past 11,300 Years. SCIENCE VOL 339 8 MARCH 2013

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