More than 200 of the world's leading climate scientists signed a petition calling for negotiators in Bali's United Nations Climate Change Conference to set aggressive targets for reducing greenhouse gas emissions. Chad Bouchard reports from Bali.

UNFCCC Executive Secretary Yvo de Boer (R) answers question during press briefing at UN Climate Change Conference 2007, 06 Dec 2007

The declaration released Thursday at the U.N. Climate Change Conference urges governments to cut greenhouse gas emissions in half by 2050.

Most scientists say greenhouse gases such as methane and carbon dioxide, which end up in the earth's atmosphere, are contributing to a rise in global temperatures.

About 215 climate experts signed the document, which was drafted over the last four months. Other groups of scientists such as the Nobel-prize winning Intergovernmental Panel on Climate Change, or IPCC, have released key statements on the science of climate change.

Professor Matthew England from the University of New South Wales says this petition is different because it outlines specific recommendations governments should follow.

"It's delivering a clear message," he said. "It's only four or five paragraphs long, it's got the weight of the scientific community behind it, and hopefully it's going to help the negotiators here in the next few weeks in Bali remember what the climate scientists are saying."

Richard Somerville of the Scripps Institution of Oceanography is careful to point out that the scientists' recommendations are based on predictions using the best available data.

"There isn't a magic number," he noted, "just as there's no magic cholesterol number that your physician can give you below which you're safe and above which you're going to have a heart attack. It's simply a risk factor. And it's our judgment, and that of the signatories that the targets we've outlined are the ones needed to keep that risk within reasonable bounds."

U.N. climate chief Yvo de Boer says it is not clear how much weight the scientists' recommendations will have during the negotiations, but stresses that discussion at the conference is centered on science-based reports such as the one released earlier by the IPCC.

"I think probably this process that's most strongly science driven," he said. "I mean you see the decision to begin negotiating the Kyoto Protocol was based on a report of the IPCC and that's why I hope that we can use the current report of the IPCC as the engine behind the next phase of negotiations."

The petition says if governments are able to ensure greenhouse gas emissions peak and decline within the next 15 years, the world will have a 50 percent chance of keeping temperatures from rising two degrees Celsius above pre-industrial levels.

About 190 countries taking part in the climate change conference this week in Bali.