Reconstruction of Earth Climate History Shows Significance of Recent Temperature Rise

Note that the Earth's position relative to the sun is such that we should be in the coolest part of this cycle, and instead we are much warmer than average.

http://www.sciencedaily.com/releases/2013/03/130307145303.htm

sciencedaily.com | Mar 7th 2013

Using data from 73 sites around the world, scientists have been able to reconstruct Earth's temperature history back to the end of the last Ice Age, revealing that the planet today is warmer than it has been during 70 to 80 percent of the time over the last 11,300 years.

Of even more concern are projections of global temperature for the year 2100, when virtually every climate model evaluated by the Intergovernmental Panel on Climate Change (IPCC) shows that temperatures will exceed the warmest temperatures during that 11,300-year period known as the Holocene -- under all plausible greenhouse gas emission scenarios.

Results of the study, by researchers at Oregon State University and Harvard University, were published this week in the journal Science. It was funded by the National Science Foundation's Paleoclimate Program.

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We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years," Marcott said. "Now we know that it is warmer than most of the past 11,300 years. This is of particular interest because the Holocene spans the entire period of human civilization."

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What that history shows, the researchers say, is that over the past 5,000 years, Earth on average cooled about 1.3 degrees (Fahrenheit) -- until the past 100 years, when it warmed ̴ 1.3 degrees (F). The largest changes were in the northern hemisphere, where there are more land masses and greater human populations.

Climate models project that global temperature will rise another 2.0 to 11.5 degrees (F) by the end of this century, largely dependent on the magnitude of carbon emissions. "What is most troubling," Clark said, "is that this warming will be significantly greater than at any time during the past 11,300 years."

Marcott said that one of the natural factors affecting global temperatures over the past 11,300 years is gradual change in the distribution of solar insolation associated with Earth's position relative to the sun.

"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."

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"This research shows that we've experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history -- but this change happened a lot more quickly."

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"The Earth's climate is complex and responds to multiple forcings, including CO2 and solar insolation," Marcott said. "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."