Climate Change and Feedbacks

When the Intergovernmental Panel on Climate Change produced the Third Assessment Report (2001), feedback mechanisms were not considered in predictions of how much temperature would increase this century. It was not known whether positive feedback mechanisms, such as melting ice (reflects 80 – 90% of sunlight) to water (reflects less than 10% of sunlight and heating speeds up), or negative feedback (more clouds reflect more sunlight and heating slows) would dominate. It is now thought that contributions from positive feedback will swamp any negative feedback.

Note: albedo measures reflectivity, so the albedo of ice is above 0.8, and below 0.1 for ocean.

At today’s sessions of the China-US Climate Change Conference at UC, Berkeley, Inez Fung suggested several other mechanisms of positive feedback. The ocean is acidifying as it absorbs carbon dioxide, and so absorbs less. (Eventually the carbon dioxide will be absorbed below the 500 – 1,000 m depth it’s reached so far, but in 200 years since the beginning of the industrial revolution, carbon has remained in the top quarter of the ocean. As carbon dioxide in the top layers moves down, the top layers will be able to absorb more.) Photosynthesis is expected to slow as the Earth warms and dries – there may be more rain, but soil will be dryer. Soil carbon turns over more quickly when warm, that’s why tropical soil contains less carbon.

John Harte cited additional examples. The retreat of ice at the end of the ice age was hurried along by the advance of spruce trees (albedo = 0.1), much more rapid change than if ice had been replaced by rock or silt (albedo = 0.4). Margaret Torn and John Harte studied how carbon dioxide changed in the Vostok ice cores (Antarctica). First the Earth warmed (from orbital changes) and then the warmed Earth added greenhouse gases which warmed the Earth more. We are adding greenhouse gases which warm the Earth which will lead to the release of more carbon dioxide and methane which will warm the Earth more. The more GHG we add to the atmosphere, the stronger will be the positive feedback. Their work will be in the May 26 Geophysical Research Letters.

Part of the Colorado Rockies is Harte’s lab, where heaters mimic climate change. Forbs, or daisies, are declining and sagebrush increasing. The sagebrush has a lower albedo, thus warming the area around a heat loving plant, an advantage to it. One more positive feedback to us.

Other speakers repeated the point made by both Fung and Harte: increases in temperature are going to be more than we thought, and decreases in greenhouse gas emissions need to be faster and more rapid than many now anticipate. Failing to reduce greenhouse gas emissions a lot is likely to be too costly; it is not a serious policy option.

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