All this sounds biblical.

　　If warming is so dreadful, would a proto-Stern of 12,000 years ago have warned his contemporaries of the dire results of the looming end of the ice age? The answer to this query is presumably no. Warming can be – and, in that case was – highly beneficial. But this does not mean rapid warming would now be so. We – and the rest of life – are well adapted to today’s world. While human beings cope superbly with change, the speed and scale of the potential disruption would test that ability to the limit.

　　Modelling work done for the review concluded the costs of climate change over the next two centuries could be equivalent to a reduction of 5 per cent in average consumption per head. This is itself equal only to the loss of two years of economic growth. But?the costs?of failure to act?might?be?as?much as 20 per cent of gross world product. The report compares such costs with those associated with “the great wars and the economic depression of the first half of the twentieth century”. Worse, in this case, “it will be difficult or impossible to reverse these changes”. Moreover, these costs would fall heavily on the poorest.

　　It seems simple common sense, therefore, to reduce the dangers, provided the costs of doing so are modest. Houses rarely catch fire, but few would question the wisdom of buying cheap smoke alarms.

　　The question is how expensive the recommended actions would be. On this the review is encouraging. The economic costs of mitigating the rise in greenhouse gas seem modest. The review estimates them at as little as

　　1 per cent of global gross product, though, here too, there is a range of uncertainty. One per cent is itself just a few months of economic growth.

　　The report argues for setting a target of 450-550ppm of carbon dioxide equivalent: anything higher, it argues, would be too risky; anything lower would be too costly. Given the trends, even achieving that target would require a massive shift (see chart). The longer change is postponed, however, the bigger the dangers and the higher the costs of forestalling them: “Delay in taking action would lead both to more climate change and, ultimately, higher mitigation costs.” Given the costs and benefits estimated in the report, early action seems sensible. But that conclusion depends in part on the discount rate used. The review argues, sensibly, that there is no reason why the welfare of our generations should be intrinsically more important than those of our grandchildren. The only other reason for a high discount rate is high economic growth. But if climate change slows economic growth significantly, as seems likely, a low discount rate makes sense. Under that assumption, the case for early action becomes even stronger.