By the year 2100, global temperatures are expected to rise by between 0.8 and 3.5 degree Celsius. That may not seem like much, but such an increase in temperature would cause a rise in sea levels large enough to put the lives of up to 100 million people at risk. For the first time in the scientific community, there is total agreement that the activity of humans is at least partly responsible for the problem—specifically the emission of greenhouse gases like carbon dioxide, which is released by the burning of wood, coal and petroleum products. Reducing harmful emission is just one area in which the United Nations Intergovernmental Panel is decidedly optimistic. In the short term it might not prove that difficult. Efficiency improvements alone could cut energy needs by as much as 30 percent at virtually no extra cost and, in developed countries, emission reductions of up to 60 percent "are technically feasible". In the longer term, harmful emissions will be reduced as the world changes over to cheaper, less environmentally damaging energy sources.