The climate system is replete with feedbacks -- some of them accelerate global warming and some slow it down. A new modeling study suggests that the melting of the Antarctic Ice Sheet may end up slowing global warming. Good news? Yes and no.
Complex systems such as the climate have many interacting parts, including the ocean, atmosphere, ice sheets, and forests. The interplay of these components results in what we call feedbacks. Positive feedbacks act to enhance or accelerate any change in the system. Negative feedbacks do the opposite.
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Some Examples of Feedbacks in the Climate System
In the case of global warming, we hope to find negative feedbacks -- processes that would counteract the effect of the warming from greenhouse gases. An example of such a feedback is the so-called carbon dioxide (CO) fertilization effect whereby increases in atmospheric CO from burning fossil fuels enhance photosynthesis and growth of trees which tends to draw down the concentration of CO, thus counteracting the effect of the fossil fuel burning.
Unfortunately this negative feedback tends to be canceled out by a positive feedback involving forests -- increases in CO raise temperatures, which in turn increase the rate at which bacteria decompose leaves and litter on the forest floor, leading to more CO emissions.
Other positive feedbacks in the climate system include the:
- water vapor feedback, whereby increases in temperature lead to more evaporation of water, which, because water is a greenhouse gas, leads to more warming and
- ice-albedo feedback, whereby warmer temperatures melt ice allowing the darker ground or water underneath to absorb more of the sunlight, leading to more warming.
The possible shutdown of the "thermohaline circulation" is an example of a negative feedback that is too strong -- it takes us out of the frying pan and into the freezer. In this feedback, global warming causes the shutdown of the Gulf Stream and the onset of ice-age conditions in the Northern Hemisphere. Despite the dire depictions in the movie The Day After Tomorrow, most scientists believe this to be a very remote possibility.
All of the feedbacks described above act on timescales of decades and so are most relevant to the fate of the climate over our own lifetime as well as our children's and grandchildren's.
There are also feedbacks, mostly negative, that act on very long timescales -- as in hundreds of millions of years. They are important since they help to stabilize the Earth's climate over geologic time but are hardly relevant to society as it confronts global warming today.
Antarctic Thaw Reduces Amount of Warming but Adds to Sea-Level Rise
Now, D. Swingedouw and colleagues from the Institute of Astronomy and Geophysics at the Catholic University of Louvain and University of Brussels in Belgium have added a new twist to the study of climate feedbacks. Their study published in Geophysical Research Letters proposes the existence of a negative feedback involving the Antarctic Ice Sheet.
In their model calculations, they find that as the Antarctic Ice Sheet melts, large amounts of freshwater are released into the Southern Ocean. This decreases the salinity of the Southern Ocean's surface waters, making them less dense. The presence of less dense water on the surface inhibits vertical mixing and this in turn encourages the formation of sea ice. The more sea ice, the more sunlight is reflected, and this leads to cooling -- and thus a counteracting of the initial warming that started the whole chain. A negative feedback like this doesn't mean there is no net warming, just that the amount of warming is reduced.
This is good news -- a negative feedback helps to reduce the consequences of our emissions of greenhouse gases. Well, it's qualified good news. For one, melting of the Antarctic Ice Sheet comes with significant consequences -- like a 180-foot increase in sea level. By comparison the melting of the Greenland Ice Sheet would lead to a (plant tongue in cheek) mere 20-foot increase in sea level. Moreover, this feedback would play out over hundreds, perhaps thousands of years.
The climate is a complex system and scientists will no doubt be studying and discovering new things about it for many decades to come, just like this fascinating Swingedouw study. But the basic facts of global warming are unlikely to change significantly -- the globe is warming due to human activities and serious consequences are in store unless we slow our emissions of greenhouse gases. Indeed it is becoming more and more certain that those serious consequences have already begun unfolding.
Dr. Bill Chameides is a member of the National Academy of Sciences and the dean of Duke University's Nicholas School of the Environment. He blogs regularly at www.thegreengrok.com.