Senate proposals on greenhouse gases would have roughly identical success in curbing global warming, but only if other nations also significantly cut heat-trapping emissions.
The Environmental Protection Agency examined the long-term impact of three climate change bills being considered in the Senate, each of which would cap carbon dioxide emissions from cars, industry and power plants with an aim to reduce greenhouse gas releases by 60 to 65 percent by mid-century.
By the end of the century all three bills would have reduced the concentration of CO2 in the atmosphere by roughly the same amount - by about 23 to 25 parts per million, said the EPA report which was sent to Congress on Tuesday.
"The three bills achieve similar levels of cumulative (greenhouse gas) emissions abatement," the EPA wrote in letters to Sens. Jeff Bingaman, a Democrat, and Arlen Specter, a Republican, co-sponsors of one of the bills.
The other two bills examined were a proposal by Sens. Joe Lieberman, an independent who usually votes with Democrats, and John McCain, a Republican, and one by Sens. John Kerry, a Democrat, and Olympia Snowe, a Republican.
Carbon dioxide from burning fossil fuels is the most prominent of greenhouse gases that scientists say are accumulating in the atmosphere where they act like a blanket to hold in the earth's warmth.
The EPA analysis, which had been sought by Bingaman and Specter, assumed that without global CO2 constraints, concentrations of the gas would nearly double to 718 parts per million, or ppm, by 2095. Scientists say that such levels would result in a serious warming of the Earth.
Adoption of any of the three Senate bills - which vary somewhat but each include a so-called cap-and-trade approach to limit CO2 emissions - would lower CO2 concentrations by 23 ppm 25 to ppm in 2095, the study said.
That itself would still leave concentrations far too high.
But if the U.S reductions under any of the three bills are accompanied by "aggressive international action" involving all countries - including rapidly developing nations such China - CO2 concentrations would be expected to stabilized and be no more than 496 ppm in 2095, according to the EPA.
The agency did not examine all the proposals being considered in Congress. Two other Senate bills would call for more aggressive reduction of CO2, of at least 80 percent by mid-century. A revised proposal offered by Lieberman, and joined by Sen. John Warner, a Republican, would require a 70 percent cut in emissions by 2050.
Concentrations of CO2 have increased from pre-industry levels of 280 ppm to about 380 ppm today. Climate scientists believe emissions must be curtailed to levels that stabilize atmospheric concentrations to between 450 and 550 ppm to avoid serious warming and climate disruptions this century.
President George W. Bush last week at a global warming summit reiterated that he believes voluntary actions - buttressed by the development of new technologies to expand use of non-fossil fuels, conservation and carbon capture - will be enough reduce the growth of greenhouse gases and forestall serious climate change.
Most environmentalists and many members of Congress contend such technologies will not be developed unless an economy-wide cap is put on CO2 emissions. While at least seven various CO2 cap and trade measures have been proposed in Congress, none has yet to emerge from any committees.
Rep. John Dingell, Democratic chairman of the Energy and Commerce Committee, last week suggested another approach: a carbon tax and a 50-cent (.71 euros) a gallon (3.8 liters) additional gasoline tax to reduce fossil fuel use. Dingell this week is expected to also unveil for discussions a proposal for a cap-and-trade measure as part of broader climate legislation.