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.