Negative feedback

This issue of melting ice caps and global warming continues to worry me. Recent changes to the Artic and ices of Greenland are of particular concern. It seems to me that changes to our planet induced by melting ice might be one of most costly and yet still underappreciated factors in the global warming debate. A recent report in the NY times, emphasizing the role of vicious circles or negative feedback, provides some more food for thought:

Each summer over the past six years, global warming has trimmed this ice’s total area a little more, and each winter the ice’s recovery has been a little less robust. These trends alarmed climate scientists, but most thought that sea ice wouldn’t disappear completely in the Arctic summer before 2040 at the earliest.

But this past summer sent scientists scrambling to redo their estimates. Week by week, the National Snow and Ice Data Center in Boulder, Colo., reported the trend: from 2.23 million square miles of ice remaining on Aug. 8 to 1.6 million square miles on Sept. 16, an astonishing drop from the previous low of 2.05 million square miles, reached in 2005.

The loss of Arctic sea ice won’t be the last abrupt change in earth’s climate, because of feedbacks. One of the climate’s most important destabilizing feedbacks involves Arctic ice. It works like this: our release of carbon dioxide and other greenhouse gases around the planet causes some initial warming that melts some ice. Melting ice leaves behind open ocean water that has a much lower reflectivity (or albedo) than that of ice. Open ocean water absorbs about 80 percent more solar radiation than sea ice does. And so as the sun warms the ocean, even more ice melts, in a vicious circle. This ice-albedo feedback is one of the main reasons warming is happening far faster in the high north, where there are vast stretches of sea ice, than anywhere else on Earth.

There are other destabilizing feedbacks in the carbon cycle that involve the oceans. Each year, the oceans absorb about half the carbon dioxide that humans emit into the atmosphere. But as oceans warm, they will absorb less carbon dioxide, partly because the gas dissolves less readily in warmer water, and partly because warming will reduce the mixing between deep and surface waters that provides nutrients to plankton that absorb carbon dioxide. And when oceans take up less carbon dioxide, warming worsens.

Scientists have done a good job incorporating some feedbacks into their climate models, especially those, like the ice-albedo feedback, that operate directly on the temperature of air or water. But they haven’t incorporated as well feedbacks that operate on the atmosphere’s concentrations of greenhouse gases or that affect the cycle of carbon among air, land, oceans and organisms. Yet these may be the most important feedbacks of all.

Read the rest of the article here.