The world is in the midst of a great political transfor- mation in which climate change has moved to the center of nation- al and global politics. For politicians in persistent denial about the need to act -- including President Bush, Australian Prime Minister John Howard and Canadian Prime Minister Stephen Harper -- there is no longer any place to hide.

These political leaders have pretended that climate change is a mere hypothesis. For several years, the Bush administration tried to hide the facts from the public, deleting references to man-made climate from government documents and even trying to suppress statements by leading government scientists. Until recently, ExxonMobil and other companies paid lobbyists to try to distort the public debate.

Yet truth has triumphed over political maneuvers. The climate itself is sending a powerful and often devastating message. Hurricane Katrina made the U.S. public aware that global warming would likely raise the intensity of destructive storms. Australia's great drought this past year has similarly made a mockery of Howard's dismissive attitude toward climate change.

Scientists themselves have operated with great seriousness of purpose in educating the public. We can thank the United Nations for that. This year, the U.N.-sponsored Intergovernmental Panel on Climate Change, a worldwide body of hundreds of climate scientists, is releasing its fourth round of reports, starting with the one issued early this month.

That report was unequivocal: There is a powerful scientific consensus that human activity -- mainly the burning of fossil fuels (coal, oil, gas), as well as deforestation and other land uses (such as growing paddy rice) -- leads to massive emissions of carbon dioxide into the air. This is causing climate change, which is accelerating and poses serious risks to the planet.

By shifting to alternative energy sources, economizing on energy use and capturing and safely storing the carbon dioxide produced by fossil fuels, global society can limit its emissions of carbon dioxide to prudent levels at an estimated cost of under 1 percent of global income. The changeover to a sustainable energy system will take decades and will require carbon taxes and emission permits to create market-based incentives for companies and individuals to switch to new kinds of electrical power plants, new kinds of automobiles and ``green buildings.''

But a reasonable timetable is possible.

• By the end of 2007, the world's governments should begin negotiations on a climate-change system for the years after 2012, when the current Kyoto Protocol expires.

• During 2008, basic principles should be established.

• By 2009, the world community -- including the two largest emitters of carbon dioxide, the United States and China -- should be ready to make a serious deal.

• By 2010, the deal should be concluded and ratified, in time to replace the Kyoto Protocol.

Individuals and companies need to make their own voices heard. The Earth Institute at Columbia University hosted a Global Roundtable of leading businesses, environmental groups and other international organizations to reach a consensus to help inform the upcoming negotiations.

The roundtable produced an important Statement of Principles and a longer overall statement that has been signed by many of the world's largest businesses, including those based in the United States, Europe, Canada, China and India. Many of the world's leading scientists signed it, too.

Global climate change requires global decisions, and initiatives like the Roundtable Statement show that we can find areas of agreement for powerful action. It's time for the world's political holdouts to join that effort.

Jeffrey Sachs is professor of economics and director of the Earth Institute at Columbia University.

©2007 Project Syndicate

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