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Overview section
Global projections based
on population growth and assumptions about energy use indicate that the
CO2 concentration will continue to rise, likely reaching somewhere between
two and three times its pre-industrial level by 2100. |
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Climate and the
Greenhouse Effect
Earth's climate is determined by complex interactions between the sun,
oceans, atmosphere, land, and living things. The composition of the
atmosphere is particularly important because certain gases (including
water vapor, carbon dioxide, methane, halocarbons, ozone, and nitrous
oxide) absorb heat radiated from the Earth's surface. As the atmosphere
warms, it in turn radiates heat back to the surface, to create what is
commonly called the "greenhouse effect." Changes in the
composition of the atmosphere alter the intensity of the greenhouse
effect. Such changes, which have occurred many times in the planet's
history, have helped determine past climates and will affect the future
climate as well.
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Human
Activities Alter the Balance
Humans are exerting a major and growing influence on some of the key
factors that govern climate by changing the composition of the atmosphere
and by modifying the land surface. The human impact on these factors is
clear. The concentration of carbon dioxide (CO2) has risen about 30% since
the late 1800s. The concentration of CO2 is now higher than it has been in
at least the last 400,000 years. This increase has resulted from the
burning of coal, oil, and natural gas, and the destruction of forests
around the world to provide space for agriculture and other human
activities. Rising concentrations of CO2 and other greenhouse gases are
intensifying Earth's natural greenhouse effect. Global projections of
population growth and assumptions about energy use indicate that the CO2
concentration will continue to rise, likely reaching between two and three
times its late-19th-century level by 2100. This dramatic doubling or
tripling will occur in the space of about 200 years, a brief moment in
geological history.
The Climate Is Changing
As we add more CO2 and other heat-trapping gases to the atmosphere, the
world is becoming warmer (which changes other aspects of climate as well).
Historical records of temperature and precipitation have been extensively
analyzed in many scientific studies. These studies demonstrate that the
global average surface temperature has increased by over 1�F (0.6�C)
during the 20th century. About half this rise has occurred since the late
1970s. Seventeen of the eighteen warmest years in the 20th century
occurred since 1980. In 1998, the global temperature set a new record by a
wide margin, exceeding that of the previous record year, 1997, by about
0.3�F (0.2�C). Higher latitudes have warmed more than equatorial
regions, and nighttime temperatures have risen more than daytime
temperatures.
As the Earth warms, more
water evaporates from the oceans and lakes, eventually to fall as rain or
snow. During the 20th century, annual precipitation has increased about
10% in the mid- and high-latitudes. The warming is also causing permafrost
to thaw, and is melting sea ice, snow cover, and mountain glaciers. Global
sea level rose 4 to 8 inches (10-20 cm) during the 20th century because
ocean water expands as it warms and because melting glaciers are adding
water to the oceans.
According to the Intergovernmental
Panel on Climate Change (IPCC), scientific evidence confirms that
human activities are a discernible cause of a substantial part of the
warming experienced over the 20th century. New studies indicate that
temperatures in recent decades are higher than at any time in at least the
past 1,000 years. It is very unlikely that these unusually high
temperatures can be explained solely by natural climate variations. The
intensity and pattern of temperature changes within the atmosphere
implicates human activities as a cause.
The relevant question is not
whether the increase in greenhouse gases is contributing to warming, but
rather, what will be the amount and rate of future warming and associated
climate changes, and what impacts will those changes have on human and
natural systems.
Records of Northern
Hemisphere surface temperatures, CO2 concentrations, and carbon
emissions show a close correlation. Temperature Change:
reconstruction of annual-average Northern Hemisphere surface air
temperatures derived from historical records, tree rings, and
corals (blue), and air temperatures directly measured (purple).
CO2 Concentrations: record of global CO2 concentration for the
last 1000 years, derived from measurements of CO2 concentration in
air bubbles in the layered ice cores drilled in Antarctica (blue
line) and from atmospheric measurements since 1957. Carbon
Emissions: reconstruction of past emissions of CO2 as a result of
land clearing and fossil fuel combustion since about 1750 (in
billions of metric tons of carbon per year). |
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