Greenhouse effect is a warming of the lower atmosphere and
surface of a planet by a complex process involving sunlight,
gases, and particles in the atmosphere. On the earth, the
greenhouse effect began long before human beings existed.
However, recent human activity may have added to the effect.
The amounts of heat-trapping atmospheric gases, called
greenhouse gases, have greatly increased since the
mid-1800’s, when modern industry became widespread. Since
the late 1800’s, the temperature of the earth’s surface has
also risen. The greenhouse effect is so named because the
atmosphere acts much like the glass roof and walls of a
greenhouse, trapping heat from the sun.
The atmosphere reflects toward space about 30 percent of
the energy in incoming sunlight. The atmosphere absorbs
about another 30 percent, and the remaining 40 percent or so
reaches the earth’s surface.
The earth’s surface reflects about 15 percent of the
solar energy that reaches it back toward space. The
remaining energy heats the lands and seas. The warmed lands
and seas then send most of the heat back into the
atmosphere, chiefly as infrared rays and in evaporated
water. Infrared rays are much like light waves but are
invisible to the human eye.
When the rays from the lands and seas strike certain
substances in the atmosphere, such as greenhouse gases and
particles, those substances absorb the rays. As a result,
the gases and particles are heated. They then are cooled by
sending out infrared rays of their own. Some of the rays go
into space. The remainder radiate back toward the earth’s
surface, adding to the warming of the surface layer of air.
Without the natural greenhouse effect, the average
temperature of the earth’s surface would be about 59 degrees
Fahrenheit (33 degrees Celsius) colder than it is now.
The chief greenhouse gases are made up of atoms of carbon
(C), hydrogen (H), and oxygen (O). These gases are water
vapor, carbon dioxide, methane, and ozone. The greenhouse
particles include cloud droplets, soot, and dust.
Since the early to mid-1800’s, the amount of carbon
dioxide in the atmosphere has increased by about 25 percent
and the methane concentration has risen by about 150
percent. Most of the increase has been due to human
activities-chiefly the burning of fossil fuels (coal, oil,
and natural gas) and the clearing of land. Fossil fuels
contain carbon, and burning them creates carbon dioxide.
Trees and other plants absorb the gas through the process of
photosynthesis. As land is cleared and forests are cut down,
carbon dioxide levels rise.
The average temperature of the earth’s surface has
increased by about 0.9 to 1.6 degrees Fahrenheit (0.5 to 0.9
degree Celsius) since the late 1800’s. Scientists have not
yet proved that an increase in atmospheric carbon dioxide
has raised the surface temperature. But in the likely event
that this relationship does exist, the eventual results
could be severe. Many scientists estimate that by about
2050, the amount of carbon dioxide in the atmosphere will
have doubled from the preindustrial level. If this increase
were to add to the natural greenhouse effect, the earth’s
surface temperature might rise 2.5 to 10.4 degrees
Fahrenheit (1.4 to 5.8 degrees Celsius) by 2100.
The increase in surface temperature, which is called
global warming, could alter the ecology of many parts of the
earth. For example, global warming could change rainfall
patterns, melt enough polar ice to raise the sea level,
increase the severity of tropical storms, and lead to shifts
in plant and animal populations. Ocean currents and wind
patterns could change, making some areas cooler than they
are now. One remote possibility is that a warming of
northern regions will result in more winter snowfall,
causing some ice sheets to advance.
Researchers use high-speed computers to study how carbon
dioxide concentration may affect surface temperature. The
computers manipulate mathematical models, sets of equations
that describe relationships between changeable factors.
Scientists do not have enough data to prove that variations
in carbon dioxide and other human-caused changes to
atmospheric composition cause shifts in surface temperature.
They may need until the 2010’s to gather enough data. But
certain models suggest that the 2010’s may be too late to
avoid some damage from global warming.
Scientists have also examined evidence from the distant
past to determine whether changes in carbon dioxide
concentration cause temperature changes. Cores of ice
drilled from great depths in Greenland and Antarctica
provide a record for the past 160,000 years. During those
years, the climate warmed and cooled several times.
Researchers analyzed the gases and other substances that
were trapped in the ice when it formed. During the cooler
periods, the atmosphere contained about 30 percent less
carbon dioxide and 50 percent less methane than during the
Climatologists are studying ways to limit global warming.
Two key methods would be (1) limiting CO2 emissions and (2)
carbon sequestration-either preventing carbon dioxide from
entering the atmosphere or removing CO2 already there. Two
effective techniques for limiting CO2 emissions would be (1)
to replace fossil fuels with energy sources that do not emit
CO2, and (2) to use fossil fuels more efficiently.
Alternative energy sources that do not emit CO2 include
the wind, sunlight, nuclear energy, and underground steam.
Devices known as wind turbines can convert wind energy to
electric energy. Solar cells can convert sunlight to
electric energy, and various devices can convert solar
energy to useful heat. Geothermal power plants convert
energy in underground steam to electric energy. Alternative
sources of energy are more expensive to use than fossil
fuels. However, increased research into their use would
almost certainly reduce their cost.
CO2 emissions could be greatly reduced if automobiles and
trucks utilized fuel more efficiently. Some scientists and
engineers are working on engines with improved fuel
efficiency. Other inventors are developing devices to
replace fuel-burning engines or to use with smaller engines.
Cars known as hybrids have already entered the market. A
hybrid has all the components of a battery-driven electric
car plus another power source, usually a small gasoline
engine. Fuel cells, devices that convert chemical energy to
electric energy, may be used in future automobiles.
Global warming is the increase in the average temperature
of Earth's near-surface air and oceans since the mid-20th
century and its projected continuation. According to the
2007 Fourth Assessment Report by the Intergovernmental Panel
on Climate Change (IPCC), global surface temperature
increased by 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 20th
century. Most of the observed temperature increase since the
middle of the 20th century has been caused by increasing
concentrations of greenhouse gases, which result from human
activities such as the burning of fossil fuel and
deforestation. Global dimming, a reduction of sunlight
reaching the surface as a result of increasing atmospheric
concentrations of human-made particulates, has partially
countered the effects of warming induced by greenhouse
Climate model projections summarized in the 2007 IPCC
report indicate that the global surface temperature is
likely to rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F)
during the 21st century. The uncertainty in this estimate
arises from the use of models with differing sensitivity to
greenhouse gas concentrations and the use of differing
estimates of future greenhouse gas emissions. An increase in
global temperature will cause sea levels to rise and will
change the amount and pattern of precipitation, probably
including expansion of subtropical deserts. Warming is
expected to be strongest in the Arctic and would be
associated with continuing retreat of glaciers, permafrost
and sea ice. Other likely effects of the warming include
more frequent occurrence of extreme weather events including
heatwaves, droughts and heavy rainfall events, species
extinctions due to shifting temperature regimes, and changes
in agricultural yields. Warming and related changes will
vary from region to region around the globe, though the
nature of these regional changes is uncertain.
The scientific consensus is that global warming is
occurring and is mostly the result of human activity. This
finding is recognized by the national science academies of
all the major industrialized countries and is not rejected
by any scientific body of national or international
standing. According to a recent Gallup poll, people in most
countries are more likely to attribute global warming to
human activities than to natural causes. The major exception
is the U.S., where nearly half of the population (the
largest percentage of any country) attributes global warming
to natural causes.
The Kyoto Protocol is aimed at stabilizing greenhouse gas
concentration to prevent a "dangerous anthropogenic
interference". As of November 2009, 187 states had signed
and ratified the protocol. Proposed responses to global
warming include mitigation to reduce emissions, adaptation
to the effects of global warming, and geoengineering to
remove greenhouse gases from the atmosphere.