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Atmospheric
Change
Humanity has been adding gases to the
atmosphere that tend to warm the earth, known as "greenhouse gases."
We are also adding small particles and droplets called aerosols that reflect
light back into space and tend to cause some areas to cool. In the coming
decades, we are likely to continue to change our atmosphere. Because the
greenhouse gases that warm the earth stay in the atmosphere longer than the
aerosols that cool the earth, the earth's average temperature is likely to
continue to warm.
Past
The temperatures of the Earth and
any other planet depends mainly on (1) the amount of sunlight received, (2)
the amount of sunlight reflected into space, and (3) the extent to which the
atmosphere retains heat. Over the last two million years, changes in the
timing and amount of sunlight striking the earth has been responsible for
inducing ice ages (see glossary),
during which temperatures were about 5°C (9°F) colder than today, and
interglacial warm periods during which temperatures have been approximately
the temperature of today. During the 20th and 21st century, however, the other
two factors may be more important.
The water vapor and carbon dioxide found
naturally in the atmosphere keeps the earth warmer than it would otherwise be.
Our clear atmosphere allows sunlight to penetrate to the earth's surface and
warm it. The surface releases this energy as infrared radiation, which is
absorbed by water vapor and CO2 in the atmosphere. This
mechanism is commonly known as the "greenhouse effect." Without the
greenhouse effect, the earth would be about 33°C (60°F) colder than it is
currently. (For a more detailed explanation of the greenhouse effect, see The
Natural Greenhouse Effect.)
Humanity is altering the energy balance
of our planet by adding gases that absorb infrared radiation to the
atmosphere, and thereby strengthening the greenhouse effect. The chief
"greenhouse gases" are CO2, methane, and
nitrous oxide. Whenever oil, coal, gas, or wood are burned, carbon dioxide is
released into the atmosphere. Approximately half of the CO2
that is released is soon absorbed by the oceans or by increased plant
photosynthesis. The other half remains in the atmosphere for many decades. As
a result, the atmospheric concentration of CO2 is
increasing. The average concentration of carbon dioxide has increased from
around 275 parts per million before the industrial revolution, to 315 ppm when
precise monitoring stations were set up in 1958, to 361 ppm in 1996. This
change has increased the amount of energy striking the earth's surface by
about 1.5 watts for every square meter of the earth's surface. This increased
energy is equal to about 1 percent of the energy in the sunlight that reaches
the earth's surface.
About two thirds of the current
emissions of methane into the atmosphere result from cattle farming, rice
paddies, landfills, coal mining, oil and gas production, and several other
human activities. The other third comes from natural sources, particularly
wetlands and termites. The total greenhouse effect from methane has increased
by about 0.5 watts (0.3%) the energy striking each square meter of the earth's
surface.
Several other gases collectively may
have as much of a greenhouse effect as methane. Nitrous oxide (also known as
"laughing gas") is released by the use of nitrogen fertilizers, the
burning of wood, and some industrial processes. Higher levels of ozone, an
urban pollutant regulated by EPA NAAQS,
also add to the greenhouse effect. (The loss of ozone in the upper
atmosphere tends to reduce the greenhouse effect.) Other gases with a
greenhouse effect include methyl chloroform, carbon tetrachloride, and
numerous halocarbons (see glossary).
Humanity is also increasing the extent
to which the atmosphere and the surface of the earth reflect light back into
space, which tends to cool the earth. Changes in land use can change the
reflectivity of the earth's surface; tropical deforestation appears to be the
most important change, but has only reduced the amount of sunlight absorbed by
about 0.1 watt per square meter. Changes in the reflectivity of the
atmosphere, however, appear to have an impact that could be as great as the
impact of carbon dioxide, albeit in the opposite direction. Most importantly,
society is adding very fine particles and droplets known as aerosols (see glossary).
The most important aerosols are
sulfates. Power plants that burn coal, as well as copper, lead, and zinc
smelters, release sulfur dioxide, which reacts with water vapor in the
atmosphere to form sulfates. Sulfates currently reflect enough light back into
space to reduce the amount of energy striking the earth's surface by somewhere
between 0.5 and 1.5 watts per square meter. Unlike CO2,
methane, and other greenhouse gases, which remain in the atmosphere for
decades or longer, most of the sulfates are removed by precipitation within a
few weeks of being emitted. As a result, sulfates tend to be concentrated in
the areas immediately downwind of major industrial areas. (There is more on sulfates
in the Climate System – Trends section.) The ability of sulfates to
scatter light also causes visibility
problems in the Grand Canyon and other scenic vistas. Sulfates also cause acid
rain.
Future
Carbon
Dioxide
The extent and speed at which humanity changes the climate will depend to a
large extent on the rate at which society adds additional greenhouse gases to
the atmosphere. If we continue to use today's technology, then as populations
and the economy grow, emissions of CO2 will continue to
increase. Until a few decades ago, energy consumption grew at about the same
rate as the Gross National Product. After oil supplies were disrupted in 1973
and 1979, however, people made substantial efforts to decrease energy
consumption, including better insulation, smaller cars, and more energy
efficient appliances. As a result, U.S. emissions are only growing at about
half the rate of economic growth. Although lower gasoline prices in the 1990s
have led people to buy larger cars, many businesses are continuing to
institute energy conservation measures.
Will the trend be toward increasing
energy efficiency? In projecting future emissions of carbon dioxide, most
studies use a set of scenarios proposed by the Intergovernmental
Panel on Climate Change. The lowest of these scenarios assumes that the
world's population will increase to 6.4 billion by 2100;
 the
economy will grow by an average of 2.0% per year by 2025 but slow
substantially thereafter; and nuclear power costs will decline by 0.4% per
year. With these assumptions, CO2 emissions would
increase from about 7.4 gigatons (GT--see glossary) per year today, to 8.8 GT
in 2025, and then decline to 4.6 GT by 2100. At the other extreme is a
scenario "E" in which world population grows to 11.3 billion people,
the economy grows by about 3 percent per year over the next century, and
nuclear power costs increase. With these assumptions, CO2
emissions would increase to 15.1 GT by 2025 and 35.8 GT by 2100. Under a
middle scenario "A", annual emissions will rise to 20.3 GT by 2100.
All of these scenarios imply that CO2
concentrations will increase throughout the next century. Under the mid-range
scenario A, the concentration will reach 700 parts per million – double
today's level – by the year 2100. The other two scenarios imply
concentrations of 450 and 900 ppm by the year 2100. Today, the concentration
of 360 ppm is about 85 ppm greater than the pre-industrial concentration.
Thus, IPCC's scenarios imply that in the next century humanity will increase
atmospheric CO2 by 2 to 7 times as much as we increased
CO2 in the last century.
Methane
Methane emissions are not projected to rise as rapidly as carbon dioxide
emissions. Rice paddies are a major source; and there is a limit to how much
additional land can be used to grow rice. Therefore, IPCC's middle scenario
assumes that emissions only rise from 60 teragrams (see glossary) today to 84
TG in 2100. Nevertheless, the concentration may increase by a greater
percentage than emissions, because methane will remain in the atmosphere
longer. Thus, IPCC estimates that the concentration of methane would
approximately double from 1700 parts-per-billion (ppb) today to 3600 ppb in
2100, with a range of 2100 to 4700 ppb.
Nitrous Oxide
Emissions from fertilizer are projected to approximately double, as are the
other emissions from human activities. The current concentration of 314 ppb is
approximately 40 ppm greater than the pre-industrial concentration. The IPCC
projects that by 2100, the concentration of nitrous oxide could be 391-433
ppb, 120-160 ppb greater than the pre-industrial concentration. Thus, in the
next century, humanity may increase the greenhouse effect of nitrous oxide by
3-4 times the impact of the last century.
Sulfur Dioxide
Scenarios of future sulfur dioxide emissions vary considerably. The IPCC
mid-range assumption is that these emissions will increase by about 70
percent. However, the low scenario implies that acid-rain
and other air pollution policies will reduce these emissions. In the United
States, the Clean Air Act
requires a 40% reduction. Unlike carbon dioxide and most greenhouse gases,
sulfur dioxide is not accumulating in the atmosphere. Thus, a 70 percent
increase in emissions would lead to a 70 percent increase in concentrations.
While not trivial, this would be much less than the projected 200-700 percent
increase in humanity's contribution of CO2 to the
atmosphere. As a result, the cooling impact of sulfate aerosols in the next
century is likely to be less important relative to greenhouse gases than in
the past. The IPCC scenarios generally imply that sulfates will only offset
about 7 percent of the greenhouse gases added to the atmosphere over the
century.
U.S. Environmental Protection
Agency
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