Negative feedback in the cold: ice retreat produces new carbon sinks in Antarctica

摘要

Feedbacks on climate change so far identified are predominantly positive, enhancing the rate of change. Loss of sea-ice, increase in desert areas, water vapour increase, loss of tropical rain forest and the restriction of significant areas of marine productivity to higher latitude (thus smaller geographical zones) all lead to an enhancement of the rate of change. The other major feedback identified, changes in cloud radiation, will produce either a positive feedback, if high level clouds are produced, or a negative feedback if low level clouds are produced. Few significant negative feedbacks have been identified, let alone quantified. Here, we show that the loss of ice shelves and retreat of coastal glaciers around the Antarctic Peninsula in the last 50 years has exposed at least 2.4 x 104 km2 of new open water. We estimate that these new areas of open water have allowed new phytoplankton blooms containing a total standing stock of similar to 5.0 x 105 tonnes of carbon to be produced. New marine zooplankton and seabed communities have also been produced, which we estimate contain similar to 4.1 x 105 tonnes of carbon. This previously unquantified carbon sink acts as a negative feedback to climate change. New annual productivity, as opposed to standing stock, amounts to 3.5 x 106 tonnes yr-1 of carbon, of which 6.9 x 105 tonnes yr-1 deposits to the seabed. By comparison the total aboveground biomasses of lowland American tropical rainforest is 160-435 tonnes ha-1. Around 50% of this is carbon. On this basis the carbon held in new biomass described here is roughly equivalent to 6000-17 000 ha of tropical rainforest. As ice loss increases in polar regions this feedback will become stronger, and eventually, over thousands to hundreds of thousands of years, over 50 Mtonnes of new carbon could be fixed annually in new coastal phytoplankton blooms and over 10 Mtonnes yr-1 locked in biological standing stock around Antarctica.