Emissions of greenhouse gases could rise enormously over the next 100 years, according to a draft report by leading climatologists on the Intergovernmental Panel on Climate Change. The IPCC researchers developed four general potential scenarios for how the climate system could develop over the coming century. Many scientists say that to avert a climatic disaster, we need to reduce greenhouse gas emission levels by 80 percent or more, but none of the four main scenarios in the IPCC report envisions anything close to that hoped-for number. Under the report’s most optimistic scenario, greenhouse gas emissions would be only a little lower than they are today, which would do scarcely anything to dent atmospheric concentrations.