The climate models agreed upon by the Intergovernmental Panel on Climate Change (IPCC) deal with some, but by no means all, of the warming effects of emissions that can occur as a result of positive feedback loops. This is because climatologists, as scientists, are given to producing testable hypotheses and there are often not enough data to satisfy that requirement for a number of the feedback loop issues. But a number of climatologists have nevertheless assessed the data and offered judgments about the importance of possible feedback effects, even in this century. NASA's James Hansen puts it succinctly: "I'm a modeler, too, but I rate data higher than models."2 Positive feedback loops can relatively quickly accelerate climate change to the tipping point, at which it becomes impossible to reverse destructive trends, even with future reductions of greenhouse gas emissions from human activities. Several such positive feedback loops are conceivable in this century, such as the risk that freshwater from melting Greenland glaciers would slow the North Atlantic meridional overturning circulation, changing ocean currents and attenuating the Gulf Stream's ability to warm Europe.
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