Research released on Thursday in the journal Science uses fossils of tiny marine organisms to reconstruct global temperatures back to the end of the last ice age. It shows how the globe for several thousands of years was cooling until an unprecedented reversal in the 20th century.
Scientists say it is further evidence that modern-day global warming isn't natural, but the result of rising carbon dioxide emissions that have rapidly grown since the Industrial Revolution began roughly 250 years ago.
The decade of 1900 to 1910 was one of the coolest in the past 11,300 years — cooler than 95 percent of the other years, the marine fossil data suggest. Yet 100 years later, the decade of 2000 to 2010 was one of the warmest, said study lead author Shaun Marcott of Oregon State University. Global thermometer records only go back to 1880, and those show the last decade was the hottest for this more recent time period.
"In 100 years, we've gone from the cold end of the spectrum to the warm end of the spectrum," Marcott said. "We've never seen something this rapid. Even in the ice age the global temperature never changed this quickly."
Using fossils from all over the world, Marcott presents the longest continuous record of Earth's average temperature. One of his co-authors last year used the same method to look even farther back. This study fills in the crucial post-ice age time during early human civilization.
Marcott's data indicates that it took 4,000 years for the world to warm about 1.25 degrees from the end of the ice age to about 7,000 years ago. The same fossil-based data suggest a similar level of warming occurring in just one generation: from the 1920s to the 1940s. Actual thermometer records don't show the rise from the 1920s to the 1940s was quite that big and Marcott said for such recent time periods it is better to use actual thermometer readings than his proxies.
Before this study, continuous temperature record reconstruction only went back about 2,000 years. The temperature trend produces a line shaped like a "hockey stick" with a sudden spike after what had been a fairly steady line. That data came from tree rings, ice cores and lake sediments.
Marcott wanted to go farther back, to the end of the last ice age in more detail by using the same marine fossil method his colleague used. That period also coincides with a "really important time for the history of our planet," said Smithsonian Institution research anthropologist Torben Rick. That's the time when people started to first domesticate animals and start agriculture, which is connected to the end of the ice age.
Marcott's research finds the climate had been gently warming out of the ice age with a slow cooling that started about 6,000 years ago.
Then the cooling reversed with a vengeance.
The study shows the recent heat spike "has no precedent as far back as we can go with any confidence, 11,000 years arguably," said Pennsylvania State University professor Michael Mann, who wrote the original hockey stick study but wasn't part of this research. He said scientists may have to go back 125,000 years to find warmer temperatures potentially rivaling today's.
However, another outside scientist, Jeff Severinghaus of the Scripps Institution of Oceanography thinks temperatures may have been notably warmer just 12,000 years ago, at least in Greenland based on research by some of his colleagues.
Several outside scientists praised the methods Marcott used, but said it might be a bit too oriented toward the Northern Hemisphere.
Marcott said the general downward trend of temperatures that reversed 100 years ago seemed to indicate the Earth was heading either toward another ice age or little ice age from about 1550 to 1850. Or it was continuing to cool naturally until greenhouse gases from the burning of fossil fuels changed everything.
The reason the globe warmed after the ice age and then started cooling about 6,000 years ago has to do with the tilt of the Earth and its distance from the sun, said Marcott and Severinghaus. Distance and angle in the summer matter because of heat absorption and reflection and ground cover.
"We have, through human emissions of carbon dioxide and other heat-trapping gases, indefinitely delayed the onset of the next ice age and are now heading into an unknown future where humans control the thermostat of the planet," said Katharine Hayhoe, an atmospheric scientist at Texas Tech University, responding in an email.
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