A>E
As energy from the sun passes through the atmosphere, 26% is reflected back to space by
clouds and atmospheric particles, 19% is absorbed by clouds, gases (ex. Ozone) and particles,
and 4% is reflected back to space by
Earth’s surface. This leaves 51% of the
sun’s radiation actually being absorbed by
Earth (see Figure 1). The Earth’s surface
then re-radiates the Sun’s energy as
infrared radiation. Greenhouse gases in
the atmosphere then absorb this infrared
radiation, and radiate the energy in all
directions. This once again heats Earth’s
surface, which once again re-radiates the
infrared energy (see Figure 2).
Greenhouse gases include water vapor,
Figure 1: Fate of solar radiation passing through Earth’s
carbon dioxide, and methane. While the
atmosphere (Pidwirny, 2006)
greenhouse effect is a naturally occurring
event, human activities have
increased the concentration of
greenhouse gases in the atmosphere,
and magnified the greenhouse effect.
The concentration of carbon dioxide
in the atmosphere has risen from
about 280ppm in pre-industrial times
to 386 ppm in 2008 (Hofmann, Butler,
Tans, 2009). Methane concentrations
have risen from 700 ppb in preindustrial times to 1775ppb in 2005
(IPCC, 2007) (see Figure 3). Over the
past century, average global
temperature has risen 1.3oF and will
reach an increase of 3 oF to 7 oF by
2100 if emission of greenhouse gases
stays at the current rate (EPA, 2009).
Figure 2: The Greenhouse Effect
Figure 3: Atmospheric concentrations of carbon dioxide and
methane (IPCC, 2007).
E>A
Global warming and climate change has lead to an increase in atmospheric temperature.
Temperature of the lower troposphere is rising at a rate slightly greater than warming rates at
the Earth’s surface (IPCC, 2007). The height of the tropopause, the division between the
troposphere and stratosphere, has also risen due to warming in the troposphere due to higher
greenhouse gas concentrations and cooling of the stratosphere due to stratospheric ozone
depletion (IPCC, 2007).
Normal atmospheric circulations are driven by temperature differences between different
regions of the earth, the turning of the earth, and temperature differences between Earth’s
surface and aloft. In a simplified model, warm moist air rises at the equator, moves towards
the pole, where cold dry air descends. Global warming has lead to surface temperatures rising
most drastically at the poles. This can lead to weakened wind patterns, as the temperature
difference between the equator and poles is lessened. This can lead to changes in precipitation
patterns, as wind patterns move moisture (Vecchi, Soden, et al, 2006).
L>A
Aerosols in the atmosphere also play a role in global climate change. There are many natural
sources of aerosols. For example, wind storms over deserts lift large amounts of fine mineral
particles into the atmosphere. Volcanic eruptions are also a major source of aerosols, including
black carbon (soot), which also results from wildfires or other types of burning. Aerosols in the
atmosphere have various effects on incoming solar radiation. While some aerosols reflect the
sun’s energy, other types, especially black carbon, absorb sunlight and radiate infrared
radiation, warming the atmosphere (UCAR, 2007). Aerosols also alter cloud formation by acting
as “cloud seeds,” particles upon which water can condense and form clouds. Any alteration in
the atmosphere’s natural aerosol content can alter the number and type of clouds forming in
the affected area. (UCAR, 2007).
H>A
Rising global temperatures leads to greater rates of evaporation. Total column water vapor has
increased 4% since 1970, and similar upward trends have been found in upper-troposphereic
specific humidity (IPCC, 2007). Water vapor is a powerful greenhouse gas, and increased water
vapor in the atmosphere due to increased evaporation may be part of a positive feedback cycle,
as increased water vapor leads to increased effects of global warming and greater evaporation
(UCAR, 2007). However, a negative feedback loop is also possible, as increased evaporation
could lead to greater cloud formation. Greater presence of clouds would increase Earth’s
albedo, decreasing the amount of solar radiation reaching Earth’s surface (UCAR, 2007).
B>A
By carrying out photosynthesis, growth of plants takes carbon dioxide out of the atmosphere
and stores the carbon in plant tissue. However, as natural ecosystems are cleared, carbon
locked in vegetation is returned to the atmosphere. This return can be immediate, as with
slash-and-burn clearing techniques, or slow as lumber or wood products gradually decay (Jain,
Yang, 2005). Farm animals, especially cattle, and rice paddies also release large amounts of
methane, a greenhouse gas, into the atmosphere (UCAR, 2007).
References
United States Environmental Protection Agency (EPA). (2009). Frequently asked questions about
Global Warming and Climate Change: Back to basics. Retrieved from
http://www.epa.gov/climatechange/downloads/Climate_Basics.pdf
Hofmann, D.J., Butler, J.H., Tans, P.P. (2009). A new look at atmospheric carbon dioxide.
Atmospheric Environment 43(12), 2084-2086
Intergovernmental Panel on Climate Change (IPCC). (2007). Changes in Atmospheric
Constituents in Radiative Forcing. Climate Change 2007: The Physical Science Basis.
Cambridge University Press: Cambridge, U.K.
Jain, A.K., Yang, X. (2005). Modeling the effects of two different land cover change data sets on
the carbon stocks of plants and soils in concert with carbon dioxide and climate change.
Global Biogeochemical Cycles 19, 1-20.
Pidwirny, M. (2006). The Greenhouse Effect. Fundamentals of Physical Geography. Retrieved
from http://www.physicalgeography.net/fundamentals/7h.html
University Corporation for Atmospheric Research (UCAR). (2007). Climate and global change.
Windows to the Universe. Retrieved at
http://www.windows.ucar.edu/tour/link=/earth/climate/climate.html
Vecchi, G.A., Soden, B.J., Wittenberg, A.T., Meld, I.M., Leetmaa, A., Harrison, M.J. (2006).
Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing.
Nature 441(1), 73-76.