A parade of world leaders from heads of state to corporate chiefs urged action on climate change at the largest summit on the issue ever organized. "The time for doubt has passed," said U.N. Secretary General Ban Ki-moon in his opening remarks at the event held Monday at the United Nations. "National action must be at the center of our response to climate change—with industrialized countries taking the lead."

Not all the world's industrialized countries saw it that way, even those who agree global warming is a threat that needs to be stopped. "The core principle of Canada's approach to climate change is balance," Prime Minister Stephen Harper said during his remarks to the conference, stressing that action on climate change should not slow economic activity. "We are balancing environmental protections with economic growth."

Harper is set to join President Bush at a U.S.-led meeting of the 16 largest emitters of greenhouse gases scheduled to be held later this week in Washington, D.C. "The U.S. is fully committed to climate adaptation," U.S. Secretary of State Condoleezza Rice said in an address to the 150 participants on hand at the U.N. event. This represents a public acknowledgement by the Bush administration that global warming is a problem but leaves unclear how exactly the administration hopes to contribute to efforts to solve it other than by helping cope with its impacts.

National representatives discussed such impacts, stressing the urgency for action to slow them in their remarks to attendees during one of four concurrent sessions on curbing greenhouse emissions, adapting to climate change, technological solutions, and how to pay for such changes. Paraguay, for example, is experiencing a crippling drought that has led to the worst forest fires in its recorded history, whereas Kenya's recent drought deprived the country of three quarters of its electricity supply. Diseases like Chikungunya fever, which cripple patients with joint pain, have begun to appear in new areas, such as the Indian Ocean island nation of Mauritius.

And with Arctic ice cover shrinking more than ever this past summer, the Inuit of the northern circumpolar region say their way of life is disappearing. "We are told to adapt," Sheila Watt-Cloutier, chair of the Inuit Circumpolar Council, said during her remarks on adaptation. "I find it deeply frustrating to be speaking on the necessary theme of adaptation and resilience rather than speaking on how to reverse the effects of climate change."

Although there was disagreement on exactly what should be done, there appeared to be a consensus that action should be taken to avert a 2-degree Celsius (3.6-degree Fahrenheit) rise in average global temperatures and to cut emissions of greenhouse gases in half by 2050. "By the middle of this century, we need to at least halve global emissions," German Chancellor Angela Merkel told the conferees.

That translates to stabilizing greenhouse gas levels at roughly 450 parts per million (ppm), according to Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change (IPCC). Already, atmospheric levels of carbon dioxide, the leading greenhouse gas, are approaching 400 ppm, and at least the amount of warming caused by that level is likely by century's end. Therefore, emissions would need to peak then begin to decline in short order—by 2015—in order to reach the 2050 goal.

Small island states, such as Grenada, say that target is not enough to stem the problem and could lead to the disappearance below the waves of some of these nations as well as devastating storm surges and tropical cyclones. "We do not want to have one environmental refugee," Angus Friday, Grenada's ambassador to the U.N., said. "We want to have mitigation efforts that don't take us as high as 2 degrees. If we can do that, we can avoid refugees."

Experts say the key to stemming climate change is changing the way the world uses energy. Renewable energy sources, such as the sun and wind, advances in technology to capture and store the carbon created by burning coal, and even the harvesting of uranium's energy will all likely be required. "We must acknowledge," James Rogers, chief executive of North Carolina–based Duke Energy, said during a conference address, "that if we're not serious about building more nuclear energy [power plants] around the world then we are not serious about addressing climate change."

The event was designed to spur a new global treaty to reduce emissions of greenhouse gases and stem anthropogenic climate change. Negotiations for such a treaty, a successor to the Kyoto Protocol, are scheduled to begin in Bali on December 3. Former action movie star and now governor of California, Arnold Schwarzenegger, urged "action, action, action" from world leaders in his address. Al Gore called for a ban on the construction of new coal-fired power plants, a post-Bali session to review its results, and a high-level meeting every three months afterward "until a treaty is successfully arrived at" by 2010, at the latest, the former U.S. vice president said. "We cannot continue at a slow pace,'' he added. "We must put a price on carbon."

The secretary general was more diplomatic, but no less forceful. "We need to ensure that such an agreement is in force by 2012," he said in closing remarks. But "undoubtedly there is a need for much deeper emission reductions from industrialized countries." He noted: "The cost of inaction will far outweigh the cost of action…. The current level of effort will not suffice."