8.1. How do gases affect the earth's temperature ?
"The Earth's surface temperature would
be about 34°C (61°F) colder than it is now if it were
not for the natural heat trapping effect of greenhouse gases
like carbon dioxide, methane, nitrous oxide, and water vapor.
Indeed, water vapor is the most abundant and important of
these naturally occurring greenhouse gases. In addition to
its direct effect as a greenhouse gas, clouds formed from
atmospheric water vapor also affect the heat balance of the
Earth by reflecting sunlight (a cooling effect), and trapping
infrared
radiation (a heating effect). "
"However, just because water vapor is the most
important gas in creating the natural greenhouse
effect does not mean that human- made greenhouse gases are unimportant.
Over the past ten thousand years, the amounts of the various greenhouse
gases in the Earth's atmosphere
remained relatively stable until a few centuries ago, when the concentrations
of many of these gases began to increase due to industrialization,
increasing demand for energy, rising population, and changing land
use and human settlement patterns. Accumulations of most of the
human-made greenhouse gases are expected to continue to increase,
so that, over the next 50 to 100 years, without control measures,
they will produce a heat-trapping effect equivalent to more than
a doubling of the pre-industrial carbon dioxide level."
8.3. How could water vapor amplify global warming ?
"Increasing amounts of human-made greenhouse
gases would lead to an increase in the globally averaged surface
temperature. However, as the temperature increases, other aspects
of the climate will alter, including the amount of water vapor in
the atmosphere.
While human activities do not directly add significant amounts of
water vapor to the atmosphere, warmer air contains more water vapor.
Since water vapor is itself a greenhouse gas, global warming will
be further enhanced by the increased amounts of water vapor. This
sort of indirect effect is called a positive feedback.
It has been suggested that as greenhouse gases accumulate,
the atmospheric events that generate cumulus clouds in tropical
areas would cause a drying rather than moistening of the upper layers
of the troposphere (the lowest region of the atmosphere). However,
observations of the current atmosphere provide evidence for the
conclusion that on a global scale, a warmed atmosphere will moisten
and this will enhance greenhouse warming."
"Clouds are another important factor in determining
climate. The increased levels of water vapor in the atmosphere,
as well as changes in temperatures and winds, will also cause changes
in clouds that will alter the amount of energy from the sun that
is absorbed and reflected by the Earth, at some locations enhancing
and at others diminishing the warming due to greenhouse gases. The
response of clouds to global warming is a major uncertainty in determining
the magnitude and distribution of climate change."
