In the international debate over climate change, the crucial sticking point for nations both rich and poor is the economic challenge of switching from cheap fossil fuels that emit heat-trapping gases to cleaner but presumably more expensive forms of energy.

Less often discussed, but of critical importance nonetheless, is the high price in death and dollars already exacted by our reliance on carbon-based fuels that emit the heat-trapping gases linked to global warming. For the United States, the bill for health problems associated with burning coal and oil is about $120 billion a year.

The extent of the largely hidden toll on human health from air pollution emitted by power plants and vehicles was quantified in a congressionally mandated study released last week by the National Academy of Sciences. The study's authors concluded that almost 20,000 people die prematurely each year from airborne pollutants, including tiny particles of soot, sulfur dioxide and nitrogen oxides.

Emissions from coal and oil-based fuels were found to affect human health in roughly equal measure. But the study is noteworthy as well for what it did not measure, such as damage caused by emissions from planes, ships and trains; the environmental impact of coal mining; runoff from chemicals used to filter smokestack emissions; and the effects of climate change wrought by our coal- and oil-dependent economy.

The study found that corn-based ethanol fuel is a poor

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antidote for the country's oil addiction. The environmental damage caused by this dirty renewable exceeds that of gasoline, but pales in consequence to the external costs of burning coal. Those amount to 3.2 cents per kilowatt-hour, compared with 1.2 to 1.7 cents per vehicle mile traveled.

Two important conclusions about the debate over global warming can be drawn from the study.

The first is that the economic cost of switching to alternative power sources such as wind and solar is not nearly as great as claimed by the extractive industries, and by those businesses -- and motorists -- wedded to cheaper fossil fuels. That is because the indirect costs of carbon fuels far exceed those of clean-energy sources, which mass production and new markets would, over time, make more affordable.

The second point is this: Many who disbelieve the scientific consensus that burning carbon is changing global climate patterns argue that developing cleaner energy is either too expensive or downright unnecessary. Perhaps if they read the study they would see that, in terms of death and illness, fossil fuels would not be cheap at half the price.