The link below is to a paper put out by the IMF that attempts to estimate the amount by which fossil fuels are subsidized. The answers are staggeringly large. The good news is that although quite a large fraction of the subsidy is as a result of not making them pay for the adverse effects of climate change, this is outweighed by the subsidy as a result of not making them pay for local damage, such as the costs associated with the effect of pollution on people's health.
This is in principle very good news indeed, because it means that it is in the interests of countries like China to cut down on fossil fuel subsidy even if they act unilaterally. So the seemingly intractable prisoner's-dilemma aspect of the problem may not be so bad after all. In the words of the report itself:
Most energy subsidies arise from the failure to adequately charge for the cost of domestic environmental damage—only about one-quarter of the total is from climate change—so unilateral reform of energy subsidies is mostly in countries’ own interests, although global coordination could strengthen such efforts.
The fiscal, environmental, and welfare impacts of energy subsidy reform are potentially enormous. Eliminating post-tax subsidies in 2015 could raise government revenue by $2.9 trillion (3.6 percent of global GDP), cut global CO2 emissions by more than 20 percent, and cut premature air pollution deaths by more than half. After allowing for the higher energy costs faced by consumers, this action would raise global economic welfare by $1.8 trillion (2.2 percent of global GDP).
It is sometimes said that to persuade climate-change deniers of the need to cut down on fossil fuels, one needs to present them with a positive vision of what the future would be like if we did so, rather than an avoiding-doom picture. Now, amazingly, it looks as though we have the means to do that. Maybe you don't believe in AGW, or believe that money spent combating it would be better spent directly combating poverty. But if the IMF is correct, then ending subsidies on fossil fuels will make us better off, so it will help us to alleviate poverty, whether or not you believe in the other benefits of reducing emissions.