Observations show that warming of the climate is unequivocal. The global
warming observed over the past 50 years is due primarily to human-induced
emissions of heat-trapping gases. These emissions come mainly from the
burning of fossil fuels (coal, oil, and gas), with important contributions from
the clearing of forests, agricultural practices, and other activities.
Warming over this century is projected to be considerably greater than
over the last century. The global average temperature since 1900 has risen
by about 1.5ºF. By 2100, it is projected to rise another 2 to 11.5ºF. The U.S.
average temperature has risen by a comparable amount and is very likely
to rise more than the global average over this century, with some variation
from place to place. Several factors will determine future temperature
increases. Increases at the lower end of this range are more likely if global
heat-trapping gas emissions are cut substantially. If emissions continue to
rise at or near current rates, temperature increases are more likely to be near
the upper end of the range. Volcanic eruptions or other natural variations
could temporarily counteract some of the human-induced warming, slowing the rise in global
temperature, but these effects would only last a few years.
Reducing emissions of carbon dioxide would lessen warming over this century and beyond. Siz-
able early cuts in emissions would significantly reduce the pace and the overall amount of climate
change. Earlier cuts in emissions would have a greater effect in reducing climate change than com-
parable reductions made later. In addition, reducing emissions of some shorter-lived heat-trapping
gases, such as methane, and some types of particles, such as soot, would begin to reduce warming
within weeks to decades.
Climate-related changes have already been observed globally and in the United States. These
include increases in air and water temperatures, reduced frost days, increased frequency and inten-
sity of heavy