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Ice age study delivers blow to global-warming skeptics

A new study finds that rising levels of carbon dioxide drove rising temperatures at the end of the last ice age. The findings contrast with previous studies, which skeptics of human-triggered global warming said showed that CO2 levels weren't an important factor. 

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“It will take many centuries and beyond to fully feel the effects,” Shakun says.

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The international team, led by Shakun and Oregon State University paleoclimatologist Peter Clark, based its work on the chemical makeup of air samples contained in bubbles trapped in ice cores. Ice records from Antarctica go back some 800,000 years.

But the researchers also drew temperature information from 80 locations around the globe, spanning northern and southern hemispheres. Sources to track temperature changes over time ranged from microfossils in deep-ocean sediments to pollen trapped in sediments in freshwater lakes.

The team's results show that the initial trigger for warming to end the last ice age was a periodic change in the angle of Earth's tilt and in the orientation of its axis. This brought more sunlight to warm northern latitudes. As mile-thick ice sheets covering vast areas of the northern hemisphere's continents began to melt, fresh water poured into the oceans, particularly into the North Atlantic, changing mechanisms that governed the climate.

Sea levels rose five to 10 meters within a few hundred years, and the Atlantic's deep-ocean “conveyor belt” slowed. Typically, the conveyor pulls warm surface water north from the tropics to cool, sink, and move south along the bottom as colder water. But the added fresh water from melting ice sheets slowed the conveyor, cooling the north and warming the southern ocean, which reaches Antarctica.

The warmer waters in the southern ocean reduced the extent of sea ice around the continent, leaving more surface water exposed to exchange gases with the atmosphere. Changing wind patterns from the warming increased the pace at which CO2-rich water deep in the ocean welled up and vented CO2 into the atmosphere.

In essence, where today's CO2 comes from vast reservoirs of carbon stored underground as coal, oil, and natural gas, or as methane trapped in polar permafrost, the reservoir of carbon CO2 introduced during the end of the ice age initially came from stores deep in the ocean.

Climate scientists' historical attempts to understand the processes that ended past ice ages have laid the groundwork for the current understanding of how CO2 influences the climate.

As far back as 1896, Swedish physicist Svante Arrhenius published a paper in the Philosophical Magazine and Journal of Science that he hoped would help solve the climate riddle ice ages presented. The 38-page paper included painstakingly handwritten calculations – arguably the first global climate model – that estimated the climate's sensitivity to changes in CO2 levels. Remarkably, he reached a figure comparable to the one scientists today see as the most-likely value. 

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