Other economists, principally those who model the economy's overall operations, dispute this optimistic assessment, while acknowledging that some gains are possible through new technology. The disagreement, Richels said, is over how much technological fruit can be easily picked. Typically, the top-down models say reducing emissions to 1990 levels by 2010 would trim gross domestic product by 1 percent or 2 percent, with some predicting much greater losses. Repetto and Austin examined 162 estimates from 16 models and found that 80 percent of their differences could be explained by what they assumed about economic uncertainties. For example, the prediction turns out more optimistically if one assumes that the economy responds relatively easily to changes in policy, that noncarbon fuels are readily available and easily adopted and that damages from air pollution and climate change are averted as a result.
Opposite assumptions produce more pessimism.
Assessing the damages of climate change, and therefore the benefits of averting it, is perhaps the chanciest task of all, and relatively few economists have tried it. One who has, William Nordhaus of Yale University, says his best guess is that benefits to the overall market economy of the United States may just about balance costs over the next century. The most important uncertainties, said Jorgenson of Harvard, have to do with how much economic growth will take place in future decades even in the absence of any climate problem. Unlike some industry leaders, who argue that the uncertainties warrant doing nothing about emissions, Jorgenson says they are an argument for taking preventive action in the interest of future generations. But he also says he believes that the cost of reducing emissions to 1990 levels by 2010 are too high and advocates a more modest policy.