";Either governments are not serious about climate change or fossil-fuel firms are overvalued";

The Economist

MARKETS can misprice risk, as investors in subprime mortgages discovered in 2008. Several recent reports suggest that markets are now overlooking the risk of "unburnable carbon". The share prices of oil, gas and coal companies depend in part on their reserves. The more fossil fuels a firm has underground, the more valuable its shares. But what if some of those reserves can never be dug up and burned?

If governments were determined to implement their climate policies, a lot of that carbon would have to be left in the ground, says Carbon Tracker, a non-profit organisation, and the Grantham Research Institute on Climate Change, part of the London School of Economics. Their analysis starts by estimating the amount of carbon dioxide that could be put into the atmosphere if global temperatures are not to rise by more than 2°C, the most that climate scientists deem prudent. The maximum, says the report, is about 1,000 gigatons (GTCO2) between now and 2050. The report calls this the world's "carbon budget".

Existing fossil-fuel reserves already contain far more carbon than that. According to the International Energy Agency (IEA), in its "World Energy Outlook", total proven international reserves contain 2,860GTCO2 -- almost three times the carbon budget. The report refers to the excess as "unburnable carbon".

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