Potsdam, 23 April 2014 – SIR – You described the estimated costs of mitigation in a report by the Intergovernmental Panel on Climate Change as “preposterous” for being too low (“Another week, another report”, April 19th). But the cost of 0.06 percentage points of economic growth that you quoted is a reduction in the annual average consumption growth rate over the 21st century in stringent mitigation scenarios. This should not be compared with absolute reductions in economic output or consumption in a particular year. Indeed, the reductions in the rate of consumption growth correspond to median consumption losses of 1.7% by 2030 and 3.4% by 2050 relative to what would otherwise happen.