Max Wilkinson has a go in the Financial Times, complete with an over-the-top headline: Stern's report is based on flawed figures. The issue is Stern's choice of using a discount rate of 2-3% a year to discount the future costs of global warming:
[T]he actual figure used by Sir Nicholas seems to have been between 2 and 3 per cent, less than half the rate that the UK Treasury now uses for assessing large capital projects and much lower than the private sector would expect...
A higher discount rate, predicated on a high economic growth rate and more in line with commercial realities, would discount future costs so that they appear much smaller to today’s consumers and taxpayers. It would make the costs of global warming in 100 years’ time appear small or even negligible in present-day terms.
As far as I can tell, pretty much every structural macroeconomic model makes use of a rate of around 3% to discount future utility. Since there are many reasons to think that an even lower rate is appropriate for an analysis of climate change, it's probably quite sensible to consider slightly lower values as well.
As far as the arithmetic goes, Wilkinson has a point: a higher discount rate would indeed significantly affect the present-value calculations. But there's no earthly reason to think that the proper discount rate is the one used to discount the returns from a capital investment project. The return on those sorts of investments incorporate a risk premium over and above the rate at which the investor discounts future utility. For most investments, that risk premium is positive, since their payoffs are generally positively correlated with movements in consumption. But since the benefits of of addressing climate change are unlikely to have any particular correlation to movements in consumption (i.e., the business cycle), the appropriate risk premium for discounting the benefits of slowing global warming is approximately zero.