The Kyoto agreement would hold 180 countries to legally binding targets for curbing greenhouse emissions to below 1990 levels by 2012. Nations would be left alone to determine the relative proportions of each gas. However, the United States has resisted the terms. By itself, the U.S. produces nearly 25 percent of man-made carbon dioxide emissions worldwide. In that way, all greenhouse gases are treated equally under the treaty.
Economists object, saying that the gases behave differently in the atmosphere.
Combined with changing environmental and political circumstances, they say the value placed on as many as six different greenhouse gases cannot remain fixed. In their study, Richels and Stanford economist Arthur Manne recommend that governments establish a ceiling for how high the global average temperature should rise. Their formula pegs the temperature rise in 10-year increments over the next 200 years. That's a term established by scientists on the United Nation's Intergovernmental Panel on Climate Change. It establishes carbon dioxide as the yardstick by which all other greenhouse gases are measured.