The strong reaction is largely a function of the major role that Dr. Hansen has played in propelling the greenhouse debate onto the public stage. In 1981, he was a principal author of one of the first papers spelling out the links between rising atmospheric levels of carbon dioxide and rising global temperatures. In 1988, when a scorching American heat wave was at its peak and fires were consuming the Amazon, he testified before a Senate committee that human activities were changing the climate. In the analysis, Dr. Hansen and his colleagues culled data and scientific papers on topics from rice production, which releases methane, to urban pollution, a source of ozone and sooty particles, to obtain detailed estimates of the rate of change in different greenhouse emissions. He then used established formulas for calculating how different amounts of the gases in the future would affect the inflow of energy from the sun and escape of heat from the earth.
Carbon dioxide is by far the most abundant greenhouse gas in the atmosphere.
But the other gases Dr. Hansen and his team cite are more prodigious trappers of heat. Moreover, Dr. Hansen said, in contrast to the difficulties of controlling carbon dioxide, technologies already exist for capturing or eliminating many of the other kinds of emissions. Some scientists criticized aspects of the new study, but agreed that an initial focus on the other greenhouse gases could achieve significant slowing of climate warming, as long as carbon dioxide cuts were also made. Dr. John P. Holdren, a professor of environmental science and public policy at Harvard University, said any focus on cutting just one set of gases now would be a mistake. It's a both-and problem.