Study: Warming fastest since ice age

Thomas Bauska / Oregon State University

Scientist Brian Bencivengo of the National Ice Core Laboratory examines an ice core from the West Antarctic Ice Sheet. Samples from these cores provide information about past air temperatures at the location from which the core was obtained.

By John Roach, Contributing Writer, NBC News

Temperatures are rising faster today than they have at any point since at least the end of the last ice age, about 11,000 years ago, according to a new study.

The finding is based on a global reconstruction of temperature records inferred from ice cores, fossils in ocean sediments and other sources. While previous studies reached similar conclusions, they covered only about 2,000 years. The new reconstruction extends the global record through the Holocene, the most recent geologic epoch.

"Another way to think of it is the period where human civilization was born, created, and developed and then progressed to where we are now," Shuan Marcott, a climate scientist at Oregon State University who led the study, told NBC News.

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In that time humans discovered bread and beer, learned to farm, cobbled together cities, linked them together in a web of global trade, fought wars, and, in the last 100 years or so, burned mountains of fossil fuels that filled the atmosphere with carbon dioxide, a heat trapping gas.

As the fossil fuel-burning ratcheted up, the global temperatures rose 1.3 degrees Fahrenheit. That rise, Marcott said, "Is unprecedented compared to what we are showing in our reconstruction."

The reconstruction paints a picture of Earth gradually warming during the first half of the Holocene, and then, about 5,000 years ago, temperatures steadily dip to the coldest period of the Little Ice Age, about 200 years ago. Over these 5,000 years, the planet cooled 1.3 degrees Fahrenheit.?

Courtesy of Science / AAAS

This graphic of the temperature reconstruction shows the rise and fall global temperatures throughout the Holocene epoch. An earlier reconstruction of the past 2,000 years is shown for comparison.

This gradual rise and fall of global temperatures are governed by Earth?s orbital position relative to the sun, Marcott explained. Other studies attribute the warming since 1950 to human activity.

Overall, Marcott and colleagues note Thursday in the journal Science, the planet today is warmer than it has been for about 75 percent of the Holocene. Given the rate of warming projected by climate models, the planet will be warmer by 2100 than at any point since at least the last ice age.

Nick Pisias / Oregon State University

Researchers prepare to deploy a core barrel off the side of the Reseach Vessel Melville to collect sediments from the ocean floor. Samples from these cores provide information about past sea surface temperatures at the location from which the core was obtained.

Michael Mann is a climate scientist at Pennsylvania State University whose 1,000-year global temperature reconstruction published in 1999 resulted in the iconic "hockey stick" graph that shows the unprecedented warmth of the past century. He said the new reconstruction is "important."

"The real issue, from a climate change impacts point of view, is the rate of change ? because that?s what challenges our adaptive capacity," he said in a statement to NBC News. "And this paper suggests that the current rate has no precedent as far back as we can go with any confidence."

The resolution of the reconstruction is averaged into 100-year segments, which means yearly or every-decade variability fails to show up in the new study. There could have been a period sometime in the past 11,000 years that was warmer than today, but if so it wasn?t sustained for at least 100 years.

"That is a drawback," Marcott acknowledged, though he said the resolution is sufficient to show the rapid change between 1900 and 2000 and the change projected by climate models between 2000 and 2100.

Mann?s only concern with the study is that the temperature records are biased toward higher latitudes of the Northern Hemisphere, where a known long-term cooling trend due to Earth?s orbital position was pronounced.

"It suggests that the true conclusions might even be stronger than their already quite strong conclusions regarding the unprecedented nature of recent warming," he said. "It may be that you have to go even further back in time to find warmth comparable ? at the global scale ? to what we are seeing today."

John Roach is a contributing writer to NBC News. To learn more about him, check out his website.?

Source: http://science.nbcnews.com/_news/2013/03/07/17225440-warming-fastest-since-dawn-of-civilization-study-shows?lite

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