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Global warming - impact of greenhouse gases
AgDM newsletter article, March 2008
by Eugene Takle, Professor of Atmospheric Science and Professor of Agricultural Meteorology, 515-294-9871, firstname.lastname@example.org and Don Hofstrand, value-added agriculture specialist, co-director AgMRC, Iowa State University Extension, 641-423-0844, email@example.com
(second in series)
Global warming will have a profound impact on global agriculture, with yet unknown influences on Midwest agriculture. As with most changes, this will provide both opportunities and threats for Midwest agricultural producers. This article discusses the role greenhouse gases play in global warming.
Solar energy heats the earth’s surface. But the energy does not stay bound up in the earth’s environment forever. Instead, as the earth warms, it emits thermal radiation. This thermal radiation, which is largely in the form of long-wave infrared rays, eventually finds its way out into space, leaving the earth and allowing it to cool. However, not all of the infrared rays pass into space. Some of the infrared rays are absorbed by greenhouse gases and warm the atmosphere. So the amount of greenhouse gases in the atmosphere is directly related to the temperature of the atmosphere. Increased concentrations of greenhouse gases increase the temperature of the atmosphere leading to the warming of the earth’s surface.
The natural carbon cycle
Carbon dioxide and other greenhouse gases go through a natural cycle. The carbon cycle is shown in Figure 1. Large amounts of carbon pass back and forth between the atmosphere and the earth’s surface. For example, growing crops and trees take in carbon dioxide (CO2) during photosynthesis. The carbon is the feedstock for making the plant and the oxygen (O2) is released into the atmosphere. When the plant dies and deteriorates or is processed, the carbon is combined with oxygen by microbial processes to become CO2 and is returned to the atmosphere. So these processes tend to keep the amount of carbon dioxide relatively constant over time.
However, burning fossil fuels takes carbon that has been stored deep in the earth and emits the carbon into the atmosphere in amounts that are too large for the earth’s plants to absorb. This is “new” carbon dioxide that is being pumped into the atmosphere.
Changing land-use has the effect of slightly increasing carbon dioxide atmospheric concentrations. Human activities such as burning fossil fuels, releasing chlorofluorocarbons, and deforestation have raised levels of greenhouse gases far above natural levels. Nature requires hundreds of years to remove these excessive amounts of greenhouse gases.
Types of greenhouse gases
Water vapor is the most prevalent greenhouse gas in the atmosphere. Water vapor doesn’t stay in the atmosphere very long. Although concentrations can change rapidly on a local basis, globally concentrations remain quite constant. The greenhouse gases that impact the gradual warming of the earth’s surface are those that stay in the atmosphere for a long period of time and build-up over time. In spite of their relatively low atmospheric concentrations, their long lifetime makes their influence on global warming large.
The warming impact of different types of greenhouse gases varies according to the warming power of the gas and the length of time it stays in the atmosphere. As shown in Table 1, carbon dioxide has an atmospheric life of 50 to 200 years. So once emitted into the atmosphere, it has a warming effect over a long period of time. Methane, for example, has a life of about 12 years, much shorter than carbon dioxide.
The warming power of each gas varies greatly. For example, methane is a much more powerful greenhouse gas than carbon dioxide. Over a 100 year period, a molecule of methane (CH4) has 21 times the warming effect as a molecule of carbon dioxide (CO2), even though it stays in the atmosphere for only about 12 years of the 100 year period.
To compare the impact of each gas, the warming potential of each gas is computed over a 100 year period as shown in Table 1. The Greenhouse Warming Potential (GWP) is computed for each gas based on its warming power and atmospheric lifetime. As a basis of comparison, carbon dioxide is assigned a GWP of one and the GWP of the other gases are computed in relationship to carbon dioxide. For example, relative to carbon dioxide, nitrous oxide has about 300 times the warming effect. The other gases (halocarbons, perfluorocarbons and sulfur hexafluoride) are also powerful gases.
The level of greenhouse gases emissions in carbon dioxide equivalents is shown in Figure 2. Although the warming potential of the other gases is more powerful than carbon dioxide, carbon dioxide emissions dwarf those of the other gases due to its large volume of emissions.
Atmospheric levels of greenhouse gases
The current rate of increase of greenhouse gas levels in the atmosphere is unprecedented. Focusing specifically on the major greenhouse gas, carbon dioxide, it has traditionally fluctuated from about 180 parts per million (ppm) to about 300 ppm. As shown in Figure 3, carbon dioxide emissions have increased from less than 320 ppm in 1960 to 380 presently. The atmosphere now contains more carbon dioxide than at any time in the last 420,000 years and possibly the last 20 million years.
We can calculate with confidence that, even with severe limits on emissions, carbon dioxide concentrations will be at least 450 ppm by 2050. If we allow for rapid economic growth based on continued use of fossil fuels, carbon dioxide concentrations will reach 600 ppm by 2050 and about 950 ppm by the end of the century (Intergovernmental Panel on Climate Changes, 2007 Report).
Impact on global temperatures
Average global temperature will rise 0.7 to 2.2 degrees Fahrenheit by 2030 and a 2.5 to 10.4 degrees Fahrenheit over the next 100 years (Intergovernmental Panel on Climate Change). Recent scientific reports conclude there is a 40% chance that warming will exceed this range and only a 5% chance that it will be less. There is no scientific evidence to suggest that global average temperatures will remain constant or decline in the next 100 years.
Although the earth has warmed and will continue to warm, the temperature increase has not and will not be distributed evenly. The temperature increases (decreases) that have occurred over the latter half of the 20th century are shown in Figure 4. The warming tends to be concentrated in certain parts of the world, especially the northern areas. There were areas that actually cooled slightly.
Projected temperatures increases over the next 100 years are shown in Figure 5. Once again the temperature increase is not expected to be distributed evenly. The warming tends to be concentrated in the far north. Also, because land is more responsive to atmospheric temperature changes than the oceans, the temperature increase will be greater over the continents than the oceans.
This article has focused on the role of greenhouse gases in global warming. The next article will focus on agriculture’s role in greenhouse gas emissions.