GREENHOUSE GASES AND GLOBAL WARMING

14.2. GREENHOUSE GASES AND GLOBAL WARMING

This section deals with infrared-absorbing trace gases (other than water vapor) in the atmosphere that contribute to global warming. These gases produce a “green- house effect” by allowing incoming solar radiant energy to penetrate to the earth’s surface while reabsorbing infrared radiation emanating from it. Levels of these “greenhouse gases” have increased at a rapid rate during recent decades and are continuing to do so. Concern over this phenomenon has intensified since about 1980. This is because ever since accurate temperature records have been kept, the 1980s were the warmest 10-year period recorded and included several record warm years. In general, the 1990s continued the warming trend. All months in 1998 except for October (which missed by about 0.1˚C) set record monthly temperature highs, and

1998 was the warmest year on record as of 1999. 3 Figure 14.1 shows global tempera- ture trends since 1880. In addition to being a scientific issue, greenhouse warming of the atmosphere has also become a major policy, political, and economic issue.

The analysis of fossil ice provides evidence of past variations in temperature. 4 One characteristic of ice that indicates the temperature at which it was deposited is conductivity, which declines with declining temperature of ice formation. The other

characteristic is the 18 O/ 16 O ratio, which is higher with increasing temperature of ice formation. Ice from the Vostok core taken in Antarctica dates back as far as 500,000

years, providing a valuable record of past climatic conditions.

There are many uncertainties surrounding the issue of greenhouse warming.

However, several things about the phenomenon are certain. It is known that CO 2 and other greenhouse gases, such as CH 4 , absorb infrared radiation by which earth loses heat. The levels of these gases have increased markedly since about 1850 as nations have become industrialized and as forest lands and grasslands have been converted

to agriculture. As shown in Figure 14.2 , per capita carbon dioxide emissions are high-

Figure 14.1. Global temperature trends. Earlier values are less certain because of the lack of sophisticated means of measuring temperature. More recent values are very accurate because of the use of satellite-based technologies for measuring temperature.

United States

Australia Canada Russia

Germany Japan

Global average

China India

0 1 2 3 4 5 6 Metric tons of carbon dioxide emitted per

person each year

Figure 14.2. Per capita emissions of carbon dioxide. Rapid industrialization of high-population countries, particularly China and India, can be expected to add much larger quantities of carbon dioxide to the atmosphere in the future.

est for industrialized countries, and development of countries with high populations, such as China and India, can be expected to add large quantities of carbon dioxide to the atmosphere in the future. Chlorofluorocarbons, which also are greenhouse gases, were not even introduced into the atmosphere until the 1930s. Although trends in est for industrialized countries, and development of countries with high populations, such as China and India, can be expected to add large quantities of carbon dioxide to the atmosphere in the future. Chlorofluorocarbons, which also are greenhouse gases, were not even introduced into the atmosphere until the 1930s. Although trends in

Carbon dioxide is the gas most commonly thought of as a greenhouse gas; it is responsible for about half of the atmospheric heat retained by trace gases. It is produced primarily by the burning of fossil fuels, and deforestation accompanied by burning and biodegradation of biomass. On a molecule-for-molecule basis, methane,

CH 4 , is 20–30 times more effective in trapping heat than is CO 2 . Other trace gases that contribute are chlorofluorocarbons and N 2 O. The potential of such a gas to cause greenhouse warming may be expressed by a global warming potential (GWP), originally defined by the United Nations’ Intergovernmental Panel on Climate Change, which is a function of both the infrared sorption characteristics and the lifetime of the gas.

Analyses of gases trapped in polar ice samples indicate that preindustrial levels of CO 2 and CH 4 in the atmosphere were approximately 260 parts per million and

0.70 ppm, respectively. Over the last 300 years these levels have increased to current values of around 360 ppm, and l.8 ppm, respectively; most of the increase by far has taken place at an accelerating pace over the last 100 years. (A note of interest is the observation based upon analyses of gases trapped in ice cores that the atmospheric

level of CO 2 at the peak of the last ice age about 18,000 years past was 25 percent below preindustrial levels.) About half of the increase in carbon dioxide in the last 300 years can be attributed to deforestation, which still accounts for approximately

20 percent of the annual increase in this gas. Carbon dioxide is increasing by about 1 ppm per year. Methane is going up at a rate of almost 0.02 ppm/year. The comparatively very rapid increase in methane levels is attributed to a number of factors resulting from human activities. Among these are direct leakage of natural gas, byproduct emissions from coal mining and petroleum recovery, and release from the burning of savannas and tropical forests. Biogenic sources resulting from human activities pro- duce large amounts of atmospheric methane. These include methane from bacteria degrading organic matter such as municipal refuse in landfills; methane evolved from anaerobic biodegradation of organic matter in rice paddies; and methane emitted as the result of bacterial action in the digestive tracts of ruminant animals.

In addition to acting as a greenhouse gas, methane has significant effects upon atmospheric chemistry. It produces atmospheric CO as an intermediate oxidation product and influences concentrations of atmospheric hydroxyl radicals and ozone.

In the stratosphere, it produces hydrogen and H 2 O, but acts to remove ozone- destroying chlorine.

A term called radiative forcing is used to describe the reduction in infrared radiation penetrating outward through the atmosphere per unit increase in the level of gas in the atmosphere. Using this measure it can be shown that the radiative

forcing of CH 4 is about 25 times that of CO 2 . Increases in the concentration of forcing of CH 4 is about 25 times that of CO 2 . Increases in the concentration of

a comparatively small incremental effect because the gas is already absorbing such a high fraction of infrared radiation in regions of the spectrum where it absorbs, an increase in the concentration of methane, chlorofluorocarbon, or other greenhouse gases has a comparatively much larger effect.

Both positive and negative feedback mechanisms may be involved in determining the rates at which carbon dioxide and methane build up in the atmos- phere. Laboratory studies indicate that increased CO 2 levels in the atmosphere cause accelerated uptake of this gas by plants undergoing photosynthesis, which tends to slow the buildup of atmospheric CO 2 . Given adequate rainfall, plants living in a warmer climate that would result from the greenhouse effect would grow faster and take up more CO 2 . This could be an especially significant effect of forests, which have a high CO 2 -fixing ability. However, the projected rate of increase in carbon dioxide levels is so rapid that forests would lag behind in their ability to fix additional CO 2 . Similarly, higher atmospheric CO 2 concentrations will result in accelerated sorption of the gas by oceans. The amount of dissolved CO 2 in the oceans is about 60 times the amount of CO 2 gas in the atmosphere. However, the times for transfer of carbon dioxide from the atmosphere to the ocean are of the order of years. Because of low mixing rates, the times for transfer of oxygen from the upper approximately 100-meter layer of the oceans to ocean depths is much

longer, of the order of decades. Therefore, like the uptake of CO 2 by forests, increased absorption by oceans will lag behind the emissions of CO 2 . A concern with increased levels of CO 2 in the oceans is the lowering of ocean water pH that will result. Even though such an effect will be slight, of the order of one tenth to several tenths of a pH unit, it has the potential to strongly impact organisms that live in ocean water. Severe drought conditions resulting from climatic warming could cut

down substantially on CO 2 uptake by plants. Warmer conditions would accelerate release of both CO 2 and CH 4 by microbial degradation of organic matter. (It is important to realize that about twice as much carbon is held in soil in dead organic matter—necrocarbon—potentially degradable to CO 2 and CH 4 as is present in the atmosphere.) Global warming might speed up the rates at which biodegradation adds these gases to the atmosphere.

It is certain that atmospheric CO 2 levels will continue to increase significantly. The degree to which this occurs depends upon future levels of CO 2 production and the fraction of that production that remains in the atmosphere. Given plausible projections of CO 2 production and a reasonable estimate that half of that amount will remain in the atmosphere, projections can be made that indicate that sometime during the middle part of the next century the concentration of this gas will reach 600 ppm in the atmosphere. This is well over twice the levels estimated for pre- industrial times. Much less certain are the effects that this change will have on climate. It is virtually impossible for the elaborate computer models used to estimate these effects to accurately take account of all variables, such as the degree and nature of cloud cover. Clouds both reflect incoming light radiation and absorb outgoing infrared radiation, with the former effect tending to predominate. The magnitudes of It is certain that atmospheric CO 2 levels will continue to increase significantly. The degree to which this occurs depends upon future levels of CO 2 production and the fraction of that production that remains in the atmosphere. Given plausible projections of CO 2 production and a reasonable estimate that half of that amount will remain in the atmosphere, projections can be made that indicate that sometime during the middle part of the next century the concentration of this gas will reach 600 ppm in the atmosphere. This is well over twice the levels estimated for pre- industrial times. Much less certain are the effects that this change will have on climate. It is virtually impossible for the elaborate computer models used to estimate these effects to accurately take account of all variables, such as the degree and nature of cloud cover. Clouds both reflect incoming light radiation and absorb outgoing infrared radiation, with the former effect tending to predominate. The magnitudes of

Drought is one of the most serious problems that could arise from major climatic change resulting from greenhouse warming. Typically, a three-degree warming would be accompanied by a ten percent decrease in precipitation. Water shortages would be aggravated, not just from decreased rainfall, but from increased evaporation as well. Increased evaporation results in decreased runoff, thereby reducing water available for agricultural, municipal, and industrial use. Water shortages, in turn, lead to increased demand for irrigation and to the production of lower quality, higher salinity runoff water and wastewater. In the U. S. such a problem would be especially intense in the Colorado River basin, which supplies much of the water used in the rapidly growing U. S. Southwest.

A variety of other problems, some of them unforeseen as of now, could result from global warming. An example is the effect of warming on plant and animal pests —insects, weeds, diseases, and rodents. Many of these would certainly thrive much better under warmer conditions.

Interestingly, another air pollutant, acid-rain-forming sulfur dioxide (see Section 14.3), may have a counteracting effect on greenhouse gases. 5 This is because sulfur dioxide is oxidized in the atmosphere to sulfuric acid, forming a light-reflecting haze. Furthermore, the sulfuric acid and resulting sulfates act as condensation nuclei upon which atmospheric water vapor condenses, thereby increasing the extent, density, and brightness of light-reflecting cloud cover. Sulfate aerosols are particularly effective in counteracting greenhouse warming in central Europe and the eastern United States during the summer.

Some evidence of the effects of global warming may have been manifested by the powerful El Niño phenomenon that occurred during the late months of 1997 and early months of 1998. El Niño is the name given to the warming of surface water in the eastern Pacific Ocean that commonly takes place around Christmas time. The 1997/98 El Niño was particularly powerful and caused many marked weather phenomena. It also increased confidence in global climate models because of the generally accurate forecasts of its effect on climate. Forecasts of a warmer and wetter winter than normal in the continental United States with particularly heavy rains in California and the Gulf Coast regions were fulfilled. In fact, Los Angeles experienced more than 33 cm of rainfall during February 1998, setting a new record for the month, and the southeastern U. S. had the most rainfall in more than a cen- tury of record keeping. Eastern equatorial Africa experienced torrential rains, as did parts of Peru. Indonesia experienced a severe drought that resulted in extensive destruction of forests by fires. The central and northern continental U.S. benefitted from a warmer winter than usual with record warmth in some eastern and mid- western regions.

As it affected North America, the stronger El Niño enhanced the intensity of the eastward flowing high altitude jet stream, the result of a greater temperature differ- ential between warmer southern tropical waters and the colder northern regions. The intense jet stream carried storms rapidly across the southern U. S., causing intense rainstorms in these regions and keeping cold Arctic air to the north. Some authorities contend that the effects of the 1997/1998 El Niño were increased by global warming.

Serious Concern Over Changes in Climate

As outlined in an article entitled “Storm Warnings Rattle Insurers,” 6 insurance companies have become quite concerned about the possibility of significant changes in global climate, especially because of potential effects on the frequency and sever- ity of damaging storms. In 1996-97 there were at least six weather disasters that cost over a billion dollars each. These included (1) a catastrophic drought in the Southern Plains that began in the fall of 1995 and lasted through the summer of 1996; (2) a blizzard followed by flooding that occurred in the northeastern U.S., the mid- Atlantic states, and the Appalachian mountain areas in January of 1996; (3) flooding in the Pacific Northwest in February 1996; (4) Hurricane Fran, which caused 36 deaths and over $5 billion in damage during September 1996; (5) severe flooding in the northern west coast region of the U.S. in December 1996, and January, 1997; and (6) an unprecedented 500-year flood complicated by freezing weather and ice jams that hit the Dakotas and Minnesota in April 1997, virtually wiping out the city of Grand Forks, North Dakota. One of the most destructive storms of all time was Hurricane Mitch, which struck Central America in 1998. Estimates of loss of life in this storm ranged up to 13,000. Property damage in Honduras was approximately 3 billion dollars, and in Nicaragua the damage was about one billion dollars. On May

3, 1999, an F5 tornado, the largest class of this kind of treacherous storm, took a number of lives and caused about $1 billion damage in Central Oklahoma. Although drought is the most frequently mentioned possible effect of greenhouse warming, the frequency and severity of storms, often accompanied by high levels of precipitation, have the insurance companies particularly concerned. During the 1990s “100-year” weather events, those that are expected to occur statistically only once each century, have become so common in the U.S. that the term has begun to lose its meaning. As shown in Figure 14.3 , the last century has seen a significant increase in precipitation in the lower 48 continental United States. These observations are consistent with currently accepted models of the weather effects of greenhouse warming, which predict that more precipitation will come in the form of brief, heavy precipitation events such as thunderstorms (heavy convective storms) rather than through gentle rainfall that comes over a longer time period. The debate continues over whether the apparent weather anomalies observed during recent years denote a marked change in climate or are simply normal fluctuations in weather. However, the insurance companies, whose prosperity and even survival depend upon accurate statistical analysis of often subtle risk factors, seem to be concerned that human effects on climate are real and that the resulting losses may be substantial.

International concern over global warming led to a meeting of 160 nations in Kyoto, Japan, in December 1997. At that meeting, the U.S. proposed stabilizing emissions of greenhouse gases to 1990 levels during the period 2008-2012. If this were achieved, levels of greenhouse gases would be 23% below those projected from trends projected from the 1990s without remedial action. The proposed agreements resulting from this meeting have met with severe criticism in the U.S. because of exemptions for developing countries, which can be expected to produce increasing fractions of greenhouse gases in the future. Meanwhile, the increased popularity of large “sport utility vehicles” in the U.S., which emit disproportinate amounts of carbon dioxide per unit distance traveled, continue to add to levels of atmospheric carbon dioxide.

Figure 14.3. Sections of the lower 48 United States in which precipitation levels have increased by 10-20% since about 1900 (shown as shaded regions). Some areas, particularly North Dakota, eastern Montana, Wyoming, and California have experienced decreases in precipitation of a similar magnitude. This map is based on data gathered by the National Oceanic and Atmospheric Administration’s National Climatic Data Center.