Sept. 24 (Bloomberg) -- A new global commitment to cut greenhouse-gas emissions is urgently needed if the world hopes to avert the most dire affects of human-caused climate change, United Nations Secretary-General Ban Ki-moon said.

``The message is quite simple,'' Ban said today at a special UN session on climate change in New York. ``We know enough to act. If we don't act now, the impact of climate change will be devastating.''

The one-day summit, with about 80 heads of state or government attending, is the largest gathering ever of world leaders focused on the subject. The meeting is intended to set the stage for negotiations on a global climate agreement scheduled to begin in Indonesia in December. A ``real breakthrough'' will be needed to reach a new treaty before the current Kyoto Protocol expires in 2012, Ban said.

A potential major sticking point is President George W. Bush's continued opposition to mandatory limits on carbon dioxide. Scientists say carbon dioxide is one of the main emissions causing temperatures to rise, which may lead to potentially irreversible climate shifts and rising sea levels that would threaten world economies, ecosystems and human health.

World leaders as well as California Governor Arnold Schwarzenegger indirectly ramped up the political pressure on Bush today by stressing the need for the world's major emitters to agree on greenhouse-gas reductions. Bush is set to address the annual UN General Assembly tomorrow.

50 Percent Cut

``All the developed countries and the major emitting countries must commit to the objective of reducing emissions by at least 50 percent between now and 2050,'' French President Nicolas Sarkozy said. ``Collective action is imperative.''

The U.S., the world's biggest greenhouse-gas emitter, blocked an attempt in June to include in a G-8 agreement a pledge to cut global warming pollution in half by mid-century, even as the European Union, Japan and Canada vowed to start their own emissions reductions.

Instead, the U.S. said it would ``consider seriously'' such measures and take part in international talks to craft a new treaty on climate change by 2009, the year Bush leaves office. Secretary of State Condoleezza Rice will host a two-day climate change meeting of about 20 major carbon-emitting nations Sept. 27-28 in Washington.

Bush has proposed a series of such meetings over the next year aimed at influencing the agenda of UN climate talks.

Seeking Alignment

``We'll be working on the process and principles for reaching agreement on a long-term global goal for reducing emissions,'' James Connaughton, Bush's chief environmental adviser, said last week. `` Each country will have different strategies, but we will try to see where we can find alignment of those where appropriate,'' he said.

Ban told UN delegates that ``national action alone is insufficient'' and action on climate change must come ``within a global framework.''

Former Vice President Al Gore, who has focused his work since leaving office on raising awareness of the global warming threat, repeated a call to have a new, stronger climate treaty in place by 2010, two years before the Kyoto treaty expires.

``We can't continue business as usual,'' the Tennessee Democrat, who lost the 2000 presidential election to Bush, said at a UN luncheon. ``We must put a price on carbon.''

Looking Ahead

Schwarzenegger also spoke to the UN climate summit today. California last year passed the first state law to reduce heat- trapping gases, following regulatory efforts by New York and other northeastern states.

``California is moving the U.S. beyond debate and doubt to action,'' he said. ``The time has come to stop looking back at the Kyoto protocol.''

The 1997 Kyoto treaty, which is overseen by the UN, requires participating industrialized countries to curb their carbon emissions from 1990 levels by 5.2 percent by 2012. It was signed but never ratified by the U.S. Developing countries such as China and India aren't required to make mandatory cuts under the accord.

Bush, who will attend a dinner tonight hosted by Ban to discuss climate change, rejected Kyoto targets after taking office in 2001, arguing that a government-mandated carbon limit would hurt the economy. He and lawmakers in Congress at the time also objected because emerging economies, particularly China and India, weren't included in limits.

Emissions and Growth

German Chancellor Angela Merkel, addressing the climate summit, said cutting emissions doesn't mean economic growth has to be curtailed.

``Emerging and developing countries can and should have strong growth, but emissions should increasingly be decoupled from growth,'' she said. Like Gore, Merkel says treaty negotiations should be completed before Kyoto expires.

At least one climate skeptic addressed the UN today, Czech Republic President Vaclav Klaus.

``Contrary to many self-assured and self-serving proclamations, there is no scientific consensus about the causes of recent climate changes,'' Klaus said.

His claim runs counter to the findings of the UN's Intergovernmental Panel on Climate Change, which earlier this year said global warming is ``very likely'' caused by human activities and that efforts to slash greenhouse gases are needed to avert the worst effects of rising temperatures, such as droughts, floods and more intense storms.

The group's study was reviewed by more than 2,500 scientists from 130 countries.

To contact the reporter on this story: Kim Chipman in Washington at kchipman@bloomberg.net.

Last Updated: September 24, 2007 16:14 EDT