Observed Change In Atmospheric Composition And Climate

Carbon Dioxide

The concentration of carbon dioxide in the surface layer of the atmosphere was about 280 ppmv just before the industrial era started. This stood at 365 ppmv at the end of twentieth century. Thus the CO2 concentration in the atmosphere has increased by about 30 percent in a span of 200 years. Burning of oil, coal, and natural gas and the clearing and burning of vegetation are the main causes of the rise. This gas makes the biggest contribution (about 70 percent) to the enhanced greenhouse effect (World Meterological Organization/Global Atmosphere Watch [WMO/GAW] 116, 1998).

Acidifying Compounds

Sulfur dioxide (SO2) and nitrogen compounds are some of the major air pollutants emitted by industrial and domestic sources. Sulfur dioxide is further oxidized to sulfate, which exists in the atmosphere mainly as aerosols. Sulfate aerosols are found more in the Northern Hemisphere than in the Southern Hemisphere. Annual mean sulfur dioxide levels over land areas are estimated to be approximately 0.1 to 10 ug-m-3 (Ryaboshapko et al., 1998). Sulfate aerosols scatter (or reflect) sunlight, resulting in slight cooling at the earth's surface.

The main anthropogenic components of emissions of nitrogen compounds to the atmosphere are nitrogen oxides (NO2), nitrous oxide (N2O), and ammonia (NH3). Nitrous oxide (N2O), which is present in the atmosphere at a very low concentration (310 ppbv), is increasing slowly at a rate of about 0.25 percent per year. Despite its low concentration, it is an important greenhouse gas because of its longer lifetime (150 years) and much greater warming potential (about 30 times more than that of carbon dioxide). Burning of vegetation, industrial emissions, and effects of agriculture on soil processes have contributed to an increase of about 15 percent in the N2O concentration in the atmosphere over the past 200 years (WMO/GAW 116, 1998).

Methane makes the next largest contribution to global warming—some 20 percent of the total. Although the annual increase in the methane load in the atmosphere is 1/100 that of carbon dioxide, its contribution to global warming is quite high (WMO/GAW 116,1998). Its concentration has risen by about 145 percent over the past 200 years. The concentration of methane in the atmosphere (which is currently 1.74 ppm) is increasing at a rate of about 1 percent per year (WMO/GAW 116, 1998).

Tropospheric Ozone

Ozone is toxic for a wide range of living organisms. In the troposphere it is produced by a chain of chemical and photochemical reactions involving, in particular, nitrogen oxides, nitrous oxides, and volatile organic compounds (VOCs). Near the earth's surface, ozone concentrations are highly variable in space and time, with the highest values over industrial regions under suitable weather conditions. Global concentrations of ground-level ozone (yearly means) are about 45 ug-m-3 (Semenov, Kounina, and Koukhta, 1999). Measurements in Europe have shown that concentrations of ozone have increased from 20 to 30 ug-m-3 to 60 ug-m-3 during the twentieth century.

Anthropogenic emissions of chlorofluorocarbons (CFCs) and some other substances into the atmosphere are known to deplete the stratospheric ozone layer. This layer absorbs ultraviolet solar radiation within a wavelength range of 280 to 320 nm (UV-B), and its depletion leads to an increase in ground-level flux of UV-B. Enhanced UV-B negatively affects organic life in a number of ways. The current rate of increase of CFCs in the atmosphere is about 4 percent per year.

Ozone Hole

In 1985, large ozone losses were observed over the Antarctic region. NASA satellite observations showed that this ozone loss covered an extensive region, coining its name, the Antarctic ozone hole (Newman, 2000). The Antarctic ozone hole was subsequently shown to result from chlorine and bromine destruction of stratospheric ozone. The stratospheric chlorine and bromine levels primarily come from human-produced chemicals such as chlorofluorocarbons and halons, whose concentrations had been increasing throughout the 1970s and 1980s. Naturally occurring, extremely cold temperatures over Antarctica cause the formation of very tenuous clouds (polar stratospheric clouds, or PSCs). Certain chlorine and bromine compounds are then converted from benign forms into ozone-destructive forms when they come into contact with the surfaces of cloud particles. Hence, the massive ozone loss over Antarctica results from the unique meteorological conditions and the high levels of human-produced chlorine and bromine.

The Arctic stratosphere is considerably different from the Antarctic stratosphere. First, natural ozone levels in the Arctic spring are much higher than in the Antarctic spring. Second, Arctic spring stratospheric temperatures are much warmer than those in the Antarctic stratosphere. Because of the warmer Arctic stratospheric temperatures, polar stratospheric clouds are much less common over the Arctic than over Antarctica (Albritton and Kuijpers, 1999).

Changes in Temperature

Temperature anomalies that have been observed on a global and continental scale from the middle of the nineteenth century to the end of the twentieth century are shown in Figure 11.1. On a regional basis there are variations from these averages. The continent of Africa is warmer than it was 100 years ago (IPCC, 1996). Warming through the twentieth century has been approximately 0.7°C.

An average annual mean increase in surface air temperature of about 2.9°C in the past 100 years has been observed in boreal regions of Asia. During the cold winter season, mean surface air temperature increase is most pronounced at a rate of about 4.4°C/100 years (Gruza et al., 1997). In most of the Middle East region, the long time series of surface air temperature shows a warming trend. In Kazakhstan, the mean annual surface temperature has risen by about 1.3°C during 1894 to 1997 (IPCC, 1998). In temperate regions of Asia, covering Mongolia and northeastern China, temperature has increased at the global rate over the past 100 years. In Japan, the surface air temperature has shown a warming trend during the past century.

In tropical regions of Asia, several countries have reported increasing surface temperature trends in recent decades. The annual mean surface air temperature anomalies over India suggest a conspicuous and gradually increasing trend of about 0.36°C/100 years. The warming over India has been mainly due to increasing maximum temperatures rather than minimum temperatures, and the rise in surface temperature is most pronounced during winter and autumn (Rupakumar, Krishna Kumar, and Pant, 1994).

Warming trends in Australia are consistent with those elsewhere in the world. Australia warmed by 0.7°C from 1910 to 1990, with most of the increase occurring after 1950. Nighttime temperatures have risen faster than daytime temperatures (Whetton, 2001).

Most of Europe has experienced increases in surface air temperature during the twentieth century which, averaged across the continent, amounts to about 0.8°C in annual temperature (Beniston, 1997). The 1990-1999 decade has been the warmest in the instrumental record, both annually and for winter. Warming has been comparatively greater over northwestern Russia and

FIGURE 11.1. Trends in global and hemispherical temperature

the Iberian Peninsula, and stronger in winter than in summer. The warming in annual mean temperature has occurred preferentially as a result of nighttime rather than daytime temperature increases (Brazdil, 1996).

South American temperature records for many countries show temperatures have been warmer in the 1980s and 1990s, compared to the reference period from 1900 to 1940. Increasing trends have been found in the time series of daily mean and minimum air temperatures throughout Colombia, the Amazon region, and subtropical and temperate Argentina (Quintana-Gomez, 1999). North America, as a whole, has warmed by about 0.7°C/100 years, although this has been quite heterogeneous (Cubasch et al., 1995; Robinson, 2000). For example, the southeastern United States cooled slightly over that same period.

Significant warming in the Arctic since the beginning of the twentieth century has been confirmed by many different proxy measurements. Glaciers and ice caps in the Arctic have shown a retreat in low-lying areas since about 1920. Numerous small, low-altitude glaciers and perennial snow patches have disappeared. Greenland's ice sheet has thinned dramatically around its southern and eastern margins, many parts of which have lost 1.0 to 1.5 m per year in thickness since 1993 (Krabill et al., 1999). Snow cover extent in the Northern Hemisphere has reduced since 1972 by about 10 percent.

Summer sea ice extent has shrunk by 20 percent over the last 30 years in the Atlantic part of the Arctic Ocean (Walsh et al., 1998). Analysis of instrumental records has shown overall warming at permanently occupied stations on the Antarctic continent and Southern Ocean island stations. Sixteen Antarctic stations have warmed at a rate of 0.9 to 1.2 °C per century, and the 22 Southern Ocean stations have warmed at 0.7 to 1.0 °C per century.

Studies conducted by New Zealand Meteorological Service show that temperatures have been increasing by 0.1°C per decade in most of the small islands in the Pacific, Indian, and Atlantic Oceans and in the Caribbean Sea. Based on data from 34 stations in the Pacific from mostly south of the equator, surface air temperatures increased by 0.3°C to 0.8°C in the twentieth century.

Changes in Precipitation

Precipitation over North America increased by 70 mm per year during the later half of the twentieth century. These trends, like those of temperature, have been fairly heterogeneous. The largest increases have been in the northeastern and western coastal regions, with some regions of decreasing precipitation in the midcontinent (U.S. National Assessment, 2000).

Some parts of southern Mexico and Central America exhibit a trend toward less precipitation. In Colombia, long-term precipitation trends have been found with no preferred sign. For the Amazon region, recent studies based on the analysis of rainfall and river streamflow data show no significant trends toward drier or wetter conditions (Magana et al., 1997). In southern Chile and the Argentinean cordillera, a negative trend in precipitation and stream flow has been detected. Precipitation in subtropical Argentina, Paraguay, and Brazil exhibited an increasing tendency for the second half of the twentieth century (Magrin et al., 1999).

Trends in annual precipitation differ between northern and southern Europe. Precipitation over northern Europe has increased by between 10 and 40 percent in the twentieth century, whereas some parts of southern Europe have dried by up to 20 percent (Hulme and Carter, 2000). The time series of annual mean precipitation in Russia suggests a decreasing trend (Rankova, 1998).

For the long-term mean precipitation, a decreasing trend of about 4.1 mm/month during the last 100 years has been reported in boreal regions of Asia. In the arid and semiarid region of Asia, rainfall observations during the past 50-year period in some countries located in the northern parts of this region have shown an increasing trend on a mean annual basis. In Pakistan, the majority of stations have shown a tendency of increasing rainfall during the monsoon season. In the temperate region of Asia, covering the Gobi and northeastern China, the annual precipitation has been decreasing continuously since 1965. In the tropical region of Asia covering India and Sri Lanka, the long-term time series of summer monsoon rainfall has no discernible trends (Kothyari and Singh, 1996).

Most of the Arctic region has experienced increased rainfall since the 1950s. In the Antarctic region the rate of accumulation of ice shows increases in precipitation (Vaughan et al., 1999; Smith, Budd, and Reid, 1998).

In Australia, trends in rainfall are not very clear. The mean annual rainfall has increased by 6 percent (not statistically significant) since 1910. However, increases in the frequency of heavy rainfall and total rainfall are significant in many parts of southeastern Australia (Hennessy, Suppiah, and Page, 1999).

The average rise in sea level in the Australia-New Zealand region over the past 50 years is about 20 mm per decade (Salinger, Stigter, and Dasc, 2000). In the small island state regions of the Pacific and Indian Ocean, the rate of sea-level rise has also been approximately 2 mm per year.

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