BRUSSELS — The bankruptcy this summer of Solyndra — a solar company heavily subsidized by the U.S. government — unleashed a torrent of concern about the risks of wasting taxpayer money on renewable-energy projects.

There have been similar worries in Europe, where bountiful state support led to a boom and bust in the Spanish solar sector and where targets for some biofuels may contribute to greenhouse emissions.

But what are the effects of subsidies that continue to flow to fossil fuels?

Two years ago, in Pittsburgh, the Group of 20 industrialized and developing nations acknowledged that many of these subsidies were wasteful, impeding investment in clean energy and undermining efforts to deal with climate change, and they pledged to step up efforts to get rid of them.

These subsidies are fiendishly difficult to dismantle because of the political risks involved.

In December, Bolivia had to rescind fuel price increases less than a week after announcing them, after violent protests. Early this year, Iran managed to institute sweeping changes, but only after overcoming major obstacles.