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updated 11/30/2010 8:18:47 AM ET 2010-11-30T13:18:47

World leaders have gathered once again in an attempt to hammer out steps to reduce greenhouse gas emissions and limit global warming over the 21st century. This effort's venue: Cancun, Mexico.

A year after contentious talks in Copenhagen exposed a stark division between developed nations and those with industrializing economies, negotiators have plenty of work ahead of them starting Monday (Nov. 29).

Even if nations fully implement the carbon dioxide reduction pledges made from Copenhagen, they will fall 5 gigatons short of the emissions level needed to keep the increase in the average global temperature below 3.6 degrees Fahrenheit (2 degrees Celsius), according to a report released Nov. 23 by the United Nations Environment Program.

Scientists have calculated that in order to have a likely and cost-effective chance of pegging temperature increases to 3.6 degrees F, global carbon dioxide emissions will need to be around 44 gigatons by 2020 — a 4-gigaton reduction from 2009 emissions.  

If no changes are made — the business-as-usual scenario — emissions could reach 56 gigatons by 2020. [ Earth in the Balance: 7 Crucial Tipping Points ]

U.N. Undersecretary-General Achim Steiner, the Environment Program executive director, sees a glass-half-full scenario.

"There is a gap between the science and current ambition levels," Steiner said. "But what this report shows is that the options on the table right now in the negotiations can get us almost 60 percent of the way there. This is a good first step."

Nearly 200 nations are being represented at the U.N. Climate Change Conference, which runs until Dec. 10.

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