A day before Barack Obama clinched the Democratic presidential nomination, his colleagues in the Senate began preparing for the biggest global warming vote in Congressional history. America’s Climate Security Act would for the first time impose large mandatory cuts on greenhouse gas emissions. The bill is not expected to become law, if only because of George Bush’s promised veto. But the Senate debate could reveal a lot about how the next Congress and the next President, whether Obama or John McCain, will address the most urgent issue facing humanity.

In contrast to Bush, McCain and Obama recognize climate change as a top-priority threat that requires action now. Environmentally, Obama’s proposals are stronger. The Democrat favors what science says is necessary: an 80 percent cut in emissions, from 1990 levels, by 2050. Obama would achieve this through a “cap and trade” system that sells corporations permits to emit greenhouse gases and then invests the revenue in green energy development and rebates to Americans hit with higher energy prices. McCain, who co-sponsored the last important climate bill, in 2005, supports a 60 percent emissions cut by 2050. But it is doubtful that McCain’s approach would actually deliver such large cuts, since his cap-and-trade system would give most permits away free, a provision environmentalists attack as a corporate giveaway. Obama, by contrast, proposes to sell all emissions permits at auction. Obama is also much less enthusiastic than McCain about nuclear power as a response to climate change.

The Climate Security Act–whose cap-and-trade system aims to reduce emissions by 19 percent by 2020 and 71 percent by 2050–goes further than McCain’s proposal but falls well short of Obama’s. It also confronts both candidates with a political minefield. With gas hovering near $4 a gallon, politicians are wary of any measure that could raise prices even higher. Further complicating matters is an explosive new study that says that reversing climate change will require a swift end to burning coal. Neither candidate seems likely to endorse that idea (though Obama’s website says he’ll consider it) since it would all but doom his chances in Appalachia and other coal regions in November.

The coal ban recommendation comes from James Hansen of NASA, the dean of America’s climate scientists. In April Hansen co-wrote a study that found that global greenhouse gas emissions must be cut much more sharply than anyone previously assumed if humanity wishes to avoid the worst scenarios of climate change. The concentration of carbon dioxide in the atmosphere is now 385 parts per million and climbing 2 ppm a year. Alarmingly, Hansen’s study concluded that 350 ppm is the maximum compatible with a livable planet. In other words, humanity is already in the danger zone and must reverse course rapidly.

“We need a moratorium on the construction of traditional coal-fired power plants by 2010 and a phaseout by 2030,” Hansen told me. This farewell to coal “has to be global,” he added, and include China and India, which insist that burning coal is essential to lifting their people out of poverty. Yet eliminating coal burning is not unthinkable. Already about sixty of the 150 US coal plants planned a year ago have been canceled and another fifty are being contested. Moreover, a recent article in Scientific American suggested that solar thermal power could supply all of America’s electricity. A self-described conservative, Hansen blames “special interests” for blocking these and other green energy solutions. “There’s no reason we can’t make the changes necessary except that the fossil fuel industries are determining governments’ policies,” he said.

The Climate Security Act is a case in point, argue Friends of the Earth and Greenpeace, which have urged defeat of the bill if it is not strengthened. Contrary to the bill’s stated goal of 71 percent emissions cuts by 2050, the Environmental Protection Agency estimates that it would deliver cuts of just 25 percent. Why? Largely because the bill gives away 49 percent of the emissions permits, thus reducing the incentive for corporations and consumers to switch to greener energy sources. Nevertheless, most environmental groups and former Vice President Al Gore support the bill while urging that it be improved.

At press time, it remained unclear what role Obama and McCain would play in the Senate debate. But whatever the outcome, the fight for a new policy on climate change is just beginning. The real showdown comes next year, when a new Congress and President tackle the issue afresh. Despite its weaknesses, the Climate Security Act marks a decisive shift; its rhetorical commitment to 71 percent emissions reductions goes well beyond what was considered politically realistic even a year ago. But the earth does not compromise. If Hansen is right, the government will have to take much larger steps, and soon, if we are to salvage a livable planet.