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UN Secretary-General Ban Ki-moon says climate change poses a "clear and urgent" challenge the international community must address. Negotiators from nearly 200 countries, including the United States, will meet in Doha, Qatar, for a U.N. climate change summit later this month, November 26 to December 7.

The first decade of this century was the hottest on record, and the vast majority of scientists attribute the changes to greenhouse gases that trap heat in the lower atmosphere. Those gases can be generated naturally or emitted by human activities, such as the burning of fossil fuels.

Extreme weather due to climate change is "the new normal," said UN Secretary-General Ban Ki-moon earlier this month.

"Our challenge remains clear and urgent - to reduce greenhouse gas emissions, to strengthen adaptation to the even larger climate shocks we know are on the way no matter what we do, and to reach a legally-binding climate agreement by 2015 as states agreed to do last year [at the climate change conference] in Durban," said Ban.

The existing agreement to reduce emissions is called the "Kyoto Protocol," and its adoption in 1997 set binding targets for industrialized countries. The first commitment period expires at the end of this year, and negotiators will work on an extension at the climate conference in Doha.

The United States is not a party to the agreement, but President Barack Obama said the U.S. has taken steps to reduce emissions. Obama said the U.S. has doubled the production of clean energy and doubled fuel efficiency standards for cars and trucks in the past four years.

"I am a firm believer that climate change is real, that it is impacted by human behavior and carbon emissions. And as a consequence, I think we've got an obligation to future generations to do something about it," said the president.

Obama said the U.S. will not try to curb climate change at the expense of economic growth.

The Kyoto Protocol does not require developing countries to reduce emissions. That includes China, the world's largest greenhouse gas emitter.

Scientists have long warned about the dangers of ignoring the changing climate, said Ban.

"Our own eyes can see what is happening. There can be no looking away, no persisting with business as usual, no hoping the threat will diminish or disappear," said Ban.

Most climate scientists agree that human activities play a role in climate change.

But they debate how much these activities affect the planet, said Patrick Michaels, a climate scientist with the Cato Institute in Washington.

"If you look at the temperature history, there are two warmings that occur. One is from about 1910 to about 1945. That could not have much to do with carbon dioxide and greenhouse gases because we hadn't put very many in the atmosphere by then. The second one begins around 1977 and ends in the late 1990s. Both warmings are of the same magnitude," said Michaels.

Some industrialized nations say future agreements to limit emissions should apply to all major economies.

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（来源：VOA 编辑：Julie）