A big question hanging over the wide-ranging greenhouse-gas proposals in the U.S. Congress is whether they will reduce emissions as much as advertised without significantly raising energy bills. One tool for achieving that goal, central to the House climate legislation, is the use of "offsets" that allow companies to avoid cutting their own greenhouse-gas emissions by investing in activities elsewhere that take carbon dioxide out of the atmosphere. That could mean planting trees in Brazil or encouraging corn farmers in Iowa to adopt better environmental practices. The Environmental Protection Agency estimates the price of emitting a ton of carbon under the House's Waxman-Markey bill would be 89% higher if it didn't allow companies to buy offsets overseas. Partly because of all the offsets the bill allows, the agency projects that conventional coal-power generation in the U.S. will fall only slightly during the program's initial phase, to 1,950 terawatt hours in 2020 from 1,992 terawatt hours in 2005, the baseline year set for measuring progress. "The way emissions-trading systems operate is that you make decisions on how much work you want to do at home, and how much you want to shift overseas," said Michael Wara, a law professor at Stanford University who has studied Europe's use of offsets. Because the Waxman-Markey bill is "extremely dependent" on offsets to deliver reductions and keep down costs, Mr. Wara said, "we're not going to know with much certainty the quantity of emissions reductions delivered." Many scientists and environmentalists endorse the logic behind offsets, reasoning that eliminating a ton of carbon emissions from a Brazilian forest or an Iowa farm benefits the climate just as much as cutting a ton of greenhouse-gas emissions from a coal plant in Ohio