WASHINGTON (September 20, 2007) – To avoid the most severe effects of climate change, the world must stabilize the concentration of heat trapping gases in the atmosphere at no more than 450 parts per million, according to a study by the Union of Concerned Scientists (UCS) and scientists at Stanford University and Texas Tech University. This limit aims to avoid exceeding a 2 degree Celsius increase in a global average temperature above pre-industrial levels (roughly equivalent to a 2 degree Fahrenheit rise above current temperatures).

Stabilizing above this level likely would lead to severe risks to natural systems and human health. Sustained warming of this magnitude could, for example, result in the extinction of many species and increase the threat of extensive melting of the Greenland and West Antarctic ice sheets.

"Hitting this target for heat trapping gases would give us a fighting chance to avoid the worst consequences of global warming," said Amy Luers, UCS California climate manager and one of the study authors. "The study assumes both developing and industrialized countries would cut their emissions to avoid such a temperature increase. However, even with other countries taking aggressive action, the United States must make deep cuts."

The study found the United States must cut its emissions by at least 80 percent below 2000 levels by 2050 if the world is to stay within the prescribed atmospheric concentration limit. According to the study, cutting emissions soon is essential.

"The cost of delay is high," said Michael D. Mastrandrea, research associate at the Woods Institute for the Environment at Stanford University. "If we wait until 2020 to start emission reductions, we'll have to cut twice as fast than if we start in 2010 to meet the same target."

Policies under consideration in the United States vary in the timing and levels of emissions cuts they call for and many fail to achieve the minimum pollution cuts needed.

"This report makes clear that the United States must make meaningful cuts in global warming pollution, and soon, to reduce the risk of severe climate impacts," said Alden Meyer, UCS director of strategy and policy. "President Bush should drop his opposition to mandatory emissions limits, and put forward a specific proposal to aggressively reduce U.S. emissions at the meeting of major emitting countries that he is hosting next week."

Congress also must act to help the world avoid the worst consequences of global warming. Several pieces of legislation have been introduced that set mandatory reductions, but only two bills would keep U.S. emissions within the overall limits called for in the UCS study. One measure was introduced by Rep. Henry Waxman (D-Calif.), and the other by Sens. Bernie Sanders (I-Vt.) and Barbara Boxer (D-Calif.).