The ongoing disruption of the earth’s climate by man-made greenhouse gases is already well beyond dangerous and is careening toward completely unmanageable. Under midrange projections for economic growth and technological change, the planet’s average surface temperature in 2050 will be about two degrees Celsius (3.6 degrees Fahrenheit) higher than its preindustrial value. The last time the earth was that warm was 130,000 years ago, and sea level was four to six meters higher than today. No one knows how long it will take sea level to “catch up” with such an increase; it could be several centuries, or it could be less.

Even with uncertainties, there is reason to believe that tipping points into unmanageable changes will become much more probable for increases larger than two degrees C. To achieve a better-than-even chance of not exceeding that figure, human emissions must start to decline soon, falling to about half of today’s level by 2050 and further thereafter.

Carbon dioxide (CO2) is the most important of civilization’s emissions and the most difficult to reduce. About 80 percent comes from burning coal, oil and natural gas; most of the rest comes from deforestation in the tropics. The largest emitters in 2006 were (in descending order) the U.S., China, Indonesia, Brazil, Russia, India, Japan and Germany. (Numbers are not final, but China appears to have passed the U.S. in 2007.)

There is no way to keep the temperature increase under two degrees C unless these big emitters start taking serious action almost immediately. The U.S. and the other industrial nations on the list have an obligation to lead this transition. They have caused most of the buildup of gases to date, and they have the largest per capita emissions, the greatest wealth and the most technology. And they agreed to their responsibility to lead in the United Nations Framework Convention on Climate Change of 1992, to which the U.S. and 191 other countries are parties.

Unfortunately, the Bush administration has wasted the last eight years. It should have been taking decisive action but engaged instead in systematic understatement of the danger: it has made ridiculous assertions that the U.S. should not do anything that China does not agree to do and has stubbornly insisted that no action should be taken to improve climate change “if it hurts the economy.” This last rationalization translates into “if it costs anybody any money” and is roughly akin to saying that the country should not defend itself against terrorism because that costs money.

There is now reason to hope, however, that this country is about to shift from shameful foot-dragging into the leadership role that the world needs and expects. Such a transition has been made possible by the convergence of several factors: a stream of new science showing an accelerating pace of climate change and its impacts; the everyday experience of people witnessing the change around them (and seeing it on the evening news); the compelling portrayals of what is happening and why, such as Al Gore’s documentary An Inconvenient Truth and the 2007 reports of the Intergovernmental Panel on Climate Change; and the shifting stances of constituencies as diverse as evangelical Christians (who argue for protecting the climate on grounds of stewardship of God’s creation) and military leaders (who argue on grounds of national security).

The impending political tipping point is evident in nationwide opinion polls and in the climate policies already embraced by more than 850 towns and cities and 32 states. It is also manifest in the rapid transition of attitudes among corporations, which have come to see climate-change mitigation and adaptation not only as necessities but as opportunities. When top executives from General Electric, DuPont, Duke Energy and Exelon testified before the Senate Committee on Energy and Natural Resources in favor of federal regulation of greenhouse gas emissions, this surely was the equivalent of the plastic thermometer in the turkey popping up to indicate that it’s done.

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Presidential contenders John McCain and Barack Obama have committed to early enactment of mandatory, economy-wide restrictions on emissions, implemented through tradable permits and designed to reduce emissions by 60 to 80 percent below 1990 levels by 2050. This goal would move the U.S. ahead of the European Union, which since 2005 has had a permit system covering only half its CO2 sources. In my judgment, if the U.S. finally takes the lead, the E.U. will quickly adopt an economy-wide approach. So will Japan, and probably Russia.

Will the Chinese, Brazilians, Indonesians and Indians join such an effort? The conventional wisdom has been “no”—that they care more about economic growth than climate change. But I believe the answer is rapidly changing to “yes.” These developing countries have figured out that the disruption of global climate is already harming them. Changes in monsoon patterns are unleashing floods and droughts and hurting agricultural productivity in China and India. The Tibetan glaciers that control water flows in the great rivers of both those countries are disappearing at an alarming rate. The rain forests in Brazil and Indonesia are drying and, increasingly, burning.

These major emitters also understand that the problem—now their problem—cannot be solved without their participation. China and India have created advisory and policy bodies at the highest levels of government to deal with climate and energy, and they are cooperating with industrial countries to create climate-friendly energy options. Brazil’s leadership has taken a newly positive stance on international measures to reduce deforestation rates and to better enforce its national deforestation laws.

The encouraging attitudes were on full display at the December 2007 meeting of the Conference of the Parties to the U.N. Framework Convention on Climate Change. One spokesperson after another from developing countries averred that once the U.S. and other industrial nations start to lead, their governments will follow. Although some members of the U.S. Congress still seem to doubt this resolve, it is past time for this nation to conduct the experiment and start to lead. That is the best remaining hope for averting global climate catastrophe.

Note: This story was originally published with the title, "One Last Chance to Lead in 2009".