Water vapor causes about 36-70% of the greenhouse effect and carbon dioxide causes 9-26%.
Fossil fuel burning has produced approximately three-quarters of the carbon dioxide from
human activity over the past 20 years, while most of the rest is due to land-use change,
particularly deforestation (Figure 25.6). Methane causes 4-9% of the greenhouse effect
and ozone causes 3-7%. Some other naturally occurring gases contribute very little to the
greenhouseeffect; oneofthese, nitrousoxide, isincreasinginconcentrationduetoanincrease
in such human activities as agriculture.
Figure 25.6: Deforestation, shown here as a result of burning for agriculture in southern
Mexico, has produced significant carbon dioxide production over the past 20 years. ( 38 )
The effect of global warming is to increase the average temperature of the Earth’s near-
surface air and oceans. This increase in global temperature will cause the sea level to rise
and is expected to cause an increase in intensity of extreme weather events and to change
the amount and pattern of precipitation. Other effects of global warming include changes in
agricultural yields, trade routes, glacier retreat, and species extinctions.
Other environmental problems caused by human-caused air pollution includeglobal dim-
ming (a reduction in the amount of radiation reaching the Earth’s surface) andozone
depletion(the latter being two related declines in stratospheric ozone). Particulate matter
from the burning of wood and coal andaerosols(airborne solid particles or liquid droplets)
cause global dimming, by absorbing solar energy and reflecting sunlight back into space.
Environmental effects of global dimming include less photosynthesis, resulting in less food
for all trophic levels; less energy to drive evaporation and the hydrologic cycle; and cooler
ocean temperatures, which may lead to changes in rainfall and drought.
Ozone is both a benefit and detriment. As a component of the upper atmosphere, it has