Monday, August 26, 2013

Climate change: The long reach

Earth may face far warmer temperatures than previous estimates had indicated

Earth is warming. Sea levels are rising. There’s more carbon in the air, and Arctic ice is melting faster than at any time in recorded history. Scientists who study the environment to better gauge Earth’s future climate now argue that these changes may not reverse for a very long time. Think millennia.
People burn fossil fuels like coal and oil for energy. That burning releases carbon dioxide, a colorless gas. In the air, this gas traps heat at Earth’s surface. And the more carbon dioxide released, the more the planet warms. If current consumption of fossil fuels doesn’t slow, the long-term climate impacts could lastthousands of years — and be more severe than scientists had been expecting. Climatologist Richard Zeebe of the University of Hawaii at Manoa offers this conclusion in a new paper. It appeared August 5 in the Proceedings of the National Academy of Sciences.
Most climate-change studies look at what’s going to happen in the next century or so. During that time, changes in the planet’s environment could nudge global warming even higher. For example: Snow and ice reflect sunlight back into space. But as these melt, sunlight can now reach — and warm — the exposed ground. This extra heat raises the air temperature even more, causing even more snow to melt. This type of rapid exaggeration of impacts is called a “fast feedback.”
Zeebe says it’s important to look at fast feedbacks. However, he adds, they’re limited. From a climate change perspective, “This century is the most important time for the next few generations,” he told Science News. “But the world is not ending in 2100.”
For his new study, Zeebe now focuses on “slow feedbacks.” While fast feedback events unfold over decades or centuries, slow feedbacks can take thousands of years. Melting of continental ice sheets and the migration of plant life — as they relocate to more comfortable areas — are two examples.
Zeebe gathered information from previously published studies investigating how such processes played out over thousands of years during past dramatic changes in climate. Then he came up with a forecast for the future that accounts for both slow and fast feedback processes.
Climate forecasts that use only fast feedbacks predict a 4.5 degree Celsius (8.1 degree Fahrenheit) change by the year 3000. But slow feedbacks added another 1.5° C — for a 6° total increase, Zeebe reports. He also found that slow feedback events will cause global warming to persist for thousands of years after people run out of fossil fuels to burn.
“This study uses our understanding of how the climate works to build an idea of what might happen in the future,” Ana Christina Ravelo told Science News. Ravelo is a climate scientist at the University of California, Santa Cruz. She pointed out that Zeebe’s study also is conservative — which means it might greatly underestimate the true boost in Earth’s temperature.