The still-controversial Kyoto Protocol, a pact among 35 countries, aims to curb industrial pollution.
More than a century ago, Swedish scientist Svante Arrhenius warned that burning oil and coal could lead to an atmospheric buildup of carbon dioxide that would eventually warm the planet. It was a radical idea back in those horse-and-buggy days.
Most experts now believe that to be true, and this week a group of nations took an important if modest step in addressing the concern that motor vehicles, factories, and other development are changing Earth's climate in ways that could be dangerous.
The Kyoto Protocol, which goes into effect Wednesday, commits 35 industrialized countries to reducing by 2012 their emissions of six "greenhouse gases" that trap heat in the earth's atmosphere - principally carbon dioxide - by 5 percent less than 1990 levels.
Seven years in the making and ratified by 141 countries, the agreement - formulated in Kyoto, Japan, in 1997 - has been controversial from the start. Advocates see it as a baby step along the necessary road to reducing human impact on climate before the oceans rise and prairie songbirds emigrate to the Arctic. Of opponents who accept the notion of global warming at all, some say the accord is based on sketchy science and in any case would damage the economic growth most people demand.
Since the beginning of the Industrial Revolution, scientists have established, the concentration of atmospheric CO2 has increased 35 percent. While debates have raged over the extent and causes, and even the reality, of global warming, the evidence of climate change keeps mounting. This has included record annual temperature levels, retreating glaciers, shrinking Arctic ice, and shifting global rainfall patterns.
Last week, NASA said a combination of weather conditions and greenhouse gases could make this year the warmest on record. Earlier this month, experts at a government-sponsored conference in England reported "greater clarity and reduced uncertainty about the impacts of climate change."
Countries will use a variety of means to comply with Kyoto. The European Union, for example, just launched a program in which some 12,000 power plants and factories can buy and sell carbon dioxide allowances on the way to meeting their carbon-reduction goals by 2012. They can also earn credits against their targets by financing clean-energy technology in developing countries.