Greenhouse Effect and Global Warming

Since the early 1960s, climate change and air quality have become major and often controversial issues in many countries and among groups from governments to various scientific communities. Prominent among these issues is the greenhouse effect, in which the gradually increasing tropospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N20) are believed to trap an excessive amount of solar radiation reflected from the earth.

The trapped radiation is predicted to cause significant ambient temperature increases. Other issues include ozone (03) formation over populated areas due to photochemical interactions of hydrocarbon, carbon monoxide (CO), and nitrogen oxide (NOx) emissions, primarily from motor vehicles; natural ozone layer destruction in the stratosphere by photochemical reactions of organic chlorofluorocarbon compounds (CFCs) resulting in increased penetration to the earth’s surface of shorter-wavelength ultraviolet light that can cause skin cancers; and acid rain, which has harmful effects on buildings and the growth of biomass and is caused by sulfur oxide (SOx) emissions from the combustion of sulfur-containing fossil fuels.

The predictions of some of the resulting environmental effects are quite dramatic. In the U.S. National Research Council’s first assessment of the greenhouse effect in 1979, one of the primary conclusions was that if the CO2 content of the atmosphere is doubled and thermal equilibrium is achieved, a global warming of between 2 and 3.5~ can occur, with greater increases occurring at higher latitudes. Some of the earlier predictions indicated that this increase is sufficient to cause warming of the upper layers of the oceans and a substantial rise in sea level, a pronounced shift of the agricultural zones, and major but unknown changes in the polar ice caps.

There has by no means been universal acceptance among the experts of many of the predictions that have been made, and there are many who have opposing views of the causes of some of the phenomena that have been observed and experimentally measured. However, several detailed reports were issued in the 1990s in which the consensus of large groups of experts is that human activities, largely the burning of fossil fuels, are affecting global climate. At any one location, annual variations can be large, but analyses of meteorological and other data over decades for large areas provide evidence of important systemic changes.

One of the first comprehensive estimates of global mean, near-surface temperature over the earth’s lands and oceans was reported in 1986 (Jones et al., 1986). The data showed a long-timescale warming trend. The three warmest years were 1980, 1981, and 1983, and five of the nine warmest years in the entire 124-year record up to 1984 were found to have occurred after 1978. It was apparent from this study that over this period, annual mean temperature increased by about 0.6 to 0.7~ and that about 40 to 50% of this increase occurred since about 1975.

According to many analysts, the warmest year on record up to 1995 is 1995, and recent years have been the warmest since 1860 (cf. Intergovernmental Panel on Climate Change, 1991 and 1995). Nighttime temperatures over land have generally increased more than daytime temperatures, and regional changes are also evident. Warming has been the greatest over the mid-latitude continents in winter and spring, with a few areas of cooling such as the North Atlantic Ocean. Precipitation has increased over land in the high latitudes of the Northern Hemisphere, especially during the cold season. Global warming mean surface temperature has increased by between 0.3 and 0.6~ since the late nineteenth century and average global surface temperature increases of I to 3.5~ somewhat lower than originally predicted, are expected to occur by the middle of the twenty-first century. Global sea level has risen by between 10 and 25 cm over the past 100 years, and much of the rise may be related to the increase in global warming mean temperature.