That means that between Inauguration Day in January 2009 and 2015, either John McCain or Barack Obama will face the most momentous political challenge of all time.
Reflecting a consensus of hundreds of scientists around the world, the Intergovernmental Panel on Climate Change has affirmed that greenhouse gas emissions are raising the Earth's temperature. The Earth is on a trajectory to warm more than 4.5 degrees Fahrenheit by around mid-century.
Exceeding that threshold could trigger a series of phenomena: Arable land will turn into desert, higher sea levels will flood coastal areas, and changes in the convection of the oceans will alter currents, such as the Gulf Stream, that determine regional weather patterns.
Manhattan and Florida would be under water, while Nevada would have no water at all. Some Russians quip that they would welcome a more temperate climate, but they would probably be sorry to lose St. Petersburg. Countries such as Bangladesh and Mali do not have the resources to mitigate or even to adapt to the impact of climate change; millions would flee coastal flooding and the desertification of farmlands, creating instant ''climate refugees.''
The head of the Nobel Prize-winning IPCC, R.K. Pachauri, recently told us: ''The cities, power plants and factories we build in the next seven years will shape our climate in mid-century. We have to act now to price carbon and create incentives to change the way we use energy and spread technology - and thereby avert nothing less than an existential threat to civilization.''
Urgent and drastic action by the international community is required, and the United States must take the lead.
Americans produce more than four times as much carbon per capita as the Chinese; 12 times as much as Indians; and more than twice as much as citizens of Germany, France, Britain and Japan. Unless the United States acts first, it will have no credibility in persuading other countries to do their share.
To their credit, McCain and Obama support the creation of a cap-and-trade system that would limit national emissions. Trading among firms would put a price on carbon. That is an essential step toward changing industry behavior, encouraging energy conservation and providing an incentive for new technologies.
As the most powerful national economy, the United States can set an example for the world in harnessing wind and solar power; ''sequestering'' (or capturing) carbon from coal plants; and developing cellulosic ethanol and safe civilian nuclear power as alternatives to fossil fuels.
But the domestic obstacles to these and other measures are daunting. While some industries will prosper, other sectors of the economy, especially those that produce or rely on coal, steel and cement, will contract. Electricity prices will increase in the near and middle terms. Many workers and households will need help with the costs of transition.
Coping with the resulting economic and political hardships would be onerous even if the next president inherited forward-looking climate-change policies. But George W. Bush has pursued an ''anti-policy,'' based on a combination of denial, procrastination and backsliding. His successor will have to make up for lost time while also dealing with a half-trillion-dollar federal deficit, a recession and a national housing crunch, a looming health-care crisis, wars in Iraq and Afghanistan, and diplomatic showdowns with North Korea and Iran.
The winner in November will need all the help he can get - including from his opponent, who will go back to the Senate as a major voice on this and other issues. The next president will also need support from the private sector, nongovernmental organizations, academia and - crucially - citizens who recognize the consequences if they do not consent to sacrifices and changes in lifestyle.
Many Americans will accept that logic, and make real changes, only if they believe greenhouse gas emissions will affect them personally. Today's adults, even if they will not be around at mid-century, must think about the fate of their children and grandchildren.
Obama can look to his two daughters, and McCain to his four grandchildren. They are among nearly 75 million Americans - and 2.2 billion people worldwide - younger than 18. That generation will be in its 40s or 50s when one of two things happens: Either the temperature of the planet warms more than 4.5 degrees and vast regions slide toward being uninhabitable, or the wisdom of the next president and his fellow leaders around the world pays off in the ultimate reward - survival.
* CARLOS PASCUAL and STROBE TALBOTT are, respectively, vice president for foreign policy studies and president of the Brookings Institution. They are involved in a joint project with Stanford University and New York University on global governance, including on the issue of climate change.