The estimate is substantially higher than the maximum temperature rise of 6 degrees predicted in the 1995 edition of the United Nations report and will renew debate over what causes global warming and how to address it. The report goes further than the 1995 edition in tying humans to the warming of the planet. It says ''it is likely'' that gases produced by human activity ''have contributed substantially'' to the warming seen so far. It is indeed a much stronger statement'' than five years ago, said Kevin Trenberth of the National Center for Atmospheric Research. The Earth has warmed about 1 degree in the past 100 years.
Many scientists believe that part of the cause is a buildup of greenhouse gases.
These gases, emitted by factories and fossil-fuel use, linger in the atmosphere and trap heat. However, a minority of scientists say there is no proof that the warming of the 20th century is anything but natural, and they argue that the computer models used to predict climate change are not reliable. The new report, a draft summary prepared by the United Nations' Intergovernmental Panel on Climate Change, was being circulated for review to scientists this week. The report says that the Earth could warm 2.5 to 10.4 degrees by 2100, compared with the 1.8 to 6.3 degrees estimated in the 1995 report. Scientists say that the new estimates are higher partly because they assume developing nations will cut emissions of sulfur, an air pollutant that causes health problems and haze.