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Emissions could fuel global warming for millennia

Climate simulation projects effects of greenhouse gases farther into the future than ever before

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If human fossil fuel consumption continues as projected the environmental effects will be felt for millennia, according to a new analysis that considers long-term feedback processes not usually included in climate projections. Taking those processes into account increases the estimated likelihood of eventual ice sheet disintegration and major sea level rise.

As atmospheric carbon dioxide rises and the planet heats up, the resulting changes in atmospheric water vapor, cloud cover, snow cover and sea ice can amplify the effects of the warming. For example, as snow cover decreases, the amount of exposed earth increases and absorbs more heat. This increased heat absorption in turn melts more snow. Researchers call these processes “fast feedbacks” because they exert their effects on the climate within decades.

Climate scientists use these fast feedbacks to make predictions about future temperatures. But fast feedbacks alone don’t allow researchers to predict changes occurring on millennial timescales, and most projections of human-caused climate change go only to 2100.

“This century is the most important time for the next few generations,” says climatologist Richard Zeebe of the University of Hawaii at Manoa. “But the world is not ending in 2100.”

To look farther into the future, Zeebe dove millions of years into the past, using published data to map out feedback relationships between climate and planetary processes that take place over millennia rather than decades. These processes include the melting of continental ice sheets, changes in vegetation and decreases in the ocean’s ability to remove carbon dioxide from the atmosphere. All of these processes increase the pace of global warming, which in turn accelerates the processes themselves, though on very long timescales.

Zeebe incorporated these slow effects into a climate scenario in which humans emit 2.5 trillion metric tons of carbon into the atmosphere over 500 years, a conservative estimate of how much carbon humans will burn through before fossil fuels run out or become too costly to extract. Considering the slow feedbacks bumped the projected temperature change up by 1.5 degrees Celsius compared with standard projections, to nearly 6 degrees by the year 3000. Zeebe’s calculations, published August 5 in the Proceedings of the National Academy of Sciences, predict that in 10000, long after the planet's fossil fuels have run out, it will still be 4 degrees hotter than today, versus about 2 degrees warmer in scenarios that consider only fast feedbacks.

While any climate prediction carries uncertainties, Zeebe’s calculations take a very conservative approach, says paleoclimatologist Ana Christina Ravelo of the University of California, Santa Cruz. “This study uses our understanding of how the climate works to build an idea of what might happen in the future.”

The predicted sustained warming could even cause the disintegration of continental ice sheets hundreds or thousands of years from now. With slow feedbacks factored in, Zeebe calculates that 1 trillion to 1.7 trillion metric tons of carbon emissions could cause the Greenland ice sheet to disintegrate. With fast feedbacks alone, far more carbon — 2.2 trillion metric tons — would be needed to seal the ice sheet’s fate. Humans have released more than 365 billion metric tons of carbon over the last 250 years, and currently do so at a rate of 10 billion metric tons per year.

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