Some 300 environmental activists yell their support for stricter pollution rules proposed by the Environmental Protection Agency. (Gene J. Puskar/AP)

FOR MORE than a century, scientists have understood the basic physics of the greenhouse effect. For decades, they’ve realized humans can affect the climate by burning coal, oil and gas. But the country’s leaders remain divided on the need to curb greenhouse emissions, let alone how to do it.

Among mainstream scientists, this paralysis is mind-boggling.

There is now no doubt that the world is warming. In 2010, the National Oceanic and Atmospheric Administration deemed this conclusion “unequivocal,” pointing to multiple, independent lines of evidence, including decades of direct temperature readings. In 2011, Richard Muller, a University of California at Berkeley scientist and former climate-change skeptic, verified this conclusion after a two-year review of the data. The complaint that scientists did not predict a slowdown in warming lately does not contradict this finding: Climate change is a long-term phenomenon; the line will go up and down here and there, but the general direction will be up. As the most authoritative source on climate science, the Intergovernmental Panel on Climate Change, explained in its Fifth Assessment Report last year, “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.”

Further, the panel found, “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” Among many pieces of evidence is the breakneck rate of carbon dioxide accumulation in the atmosphere, coinciding with measured temperature rise. Other human “fingerprints” are becoming visible: Scientists, for example, are seeing a pattern of warming in the troposphere and cooling in the stratosphere that suggests greenhouse gases — not, say, variations in solar activity — are the cause.

The most reasonable climate skeptics accept these findings but point to two areas of continuing uncertainty. First, scientists are unsure precisely how much the Earth will heat up in response to a given increase in atmospheric carbon dioxide concentrations — the climate’s sensitivity to carbon. The experts have offered a range of possibilities. Second, it is hard to predict exactly what a given increase in global temperature will do to human society.

Neither is a good excuse for inaction. Recent papers argue for adjusting the sensitivity range up and down, but the overall picture is stable: Scientists have reason to warn against betting human welfare on the proposition that low-end estimates will pan out. Even if the experts are too pessimistic, recent research indicates, that would not eliminate the need to slash carbon dioxide emissions over the next several decades.

Scientists can also reasonably anticipate myriad negative effects in certain emissions scenarios, including sea-level rise, higher storm surges in coastal areas, more flooding elsewhere, increasingly frequent and severe heat and wildfires, extreme precipitation events, widespread changes in habitats and agricultural resources, ocean acidification with dire consequences for coral and other species, drought and the spread of disease. Between $66 billion and $106 billion worth of U.S. property will likely be below sea level by mid-century. The federal government will probably have to spend billions more in disaster relief.

Waiting to deal with carbon emissions until the effects are clearer or technology improves is not a wise strategy. The emissions humans put into the atmosphere now will affect the climate in the middle of the century and onward. Technological change, meanwhile, could make a future transition away from fossil fuels cheap — or it might not, leaving the world with a terrible choice between sharply reducing emissions at huge cost or suffering through the effects of unabated warming.

Businesses that do not hedge against the threat of uncertain outcomes fail. The world cannot afford such recklessness on climate change.