Global Climate Change
Energy from the Sun passes through the atmosphere as light and is absorbed by soil, rock, and water at the surface of Earth. The energy is reradiated as heat and absorbed in the atmosphere by greenhouse gases, including carbon dioxide (CO 2 ), water vapor, methane, ozone, nitrous oxide, and the human-made chemicals chlorofluorocarbons (CFCs). This atmospheric warming is called the greenhouse effect; without it Earth's average global temperature would be about –18 degrees Celsius (0 degrees Fahrenheit). Greenhouse gases are added to the atmosphere by natural events including volcanic eruptions, the decay and burning of organic matter, and respiration by animals. They are also removed from the atmosphere. CO 2 is absorbed by seawater and stored in plant tissue. When plants die and gradually are transformed into fossil fuels—coal, oil, natural gas—deep in the earth, their CO 2 is stored with them. The removal of greenhouse gases from the atmosphere keeps the planet from overheating.
Besides the concentrations of greenhouse gases in the atmosphere, other factors affect global climate including Earth's orbital behavior, the positions and topography of the continents, the temperature structure of the oceans, and the amount and types of life. During much of Earth's history the climate was warm and humid with ice-free poles; global average temperatures were about 5 degrees Celsius (9 degrees Fahrenheit) higher than today. Several times glaciers covered the higher latitudes, most recently during the Pleistocene (1.6 million to 10,000 years ago), when up to 30 percent of the land was covered by ice. During the four glacial advances of the Pleistocene, average global temperature was 5 degrees Celsius lower than today and 10 degrees Celsius (18 degrees Fahrenheit) lower than the ancient global average. During the three interglacial periods, global temperature was a degree or two warmer than today. Many scientists think that Earth is in an interglacial period, and the ice sheets will return.
Since the peak of the last glacial advance 18,000 years ago, average global temperature has risen 4 degrees Celsius (7 degrees Fahrenheit), including 1 degree Celsius (1.8 degrees Fahrenheit) since the beginning of the Industrial Revolution. It is difficult to know how much of the recent warming is the result of the end of the Pleistocene and how much is the result of human activities that add greenhouse gases to the atmosphere. CO 2 is the most abundant greenhouse gas, a by-product of burning fossil fuels and modern forests. In the early twenty-first century, there is greater than 30 percent more CO 2 in the atmosphere than in 1850. There have also been significant increases in methane and CFCs. Some projections show a doubling of CO 2 over preindustrial levels by 2050 and additional increases in methane. (CFCs are being phased out by international agreement because they destroy Earth's protective ozone layer.)
Adding greenhouse gases to the atmosphere is like throwing another blanket on Earth; the consequent rise in global temperature is known as global warming. Since climate is a complex system and climate models are difficult to construct, scientists can only speculate on the effect large increases in greenhouse gases will have on global climate. Some models show average global temperature increasing as much as 5 degrees Celsius by 2100. Any temperature increase will not be uniform. Since ocean water absorbs more heat than land, the Southern Hemisphere (which has more water) will warm less than the Northern. Atmospheric circulation patterns will bring the greatest warming, as much as 8 to 10 degrees Celsius (14 to 18 degrees Fahrenheit), to the poles.
A rapid increase in global average temperature could have profound effects on social and natural systems. Warmer temperatures would cause ocean water to expand and polar ice caps to melt, increasing sea level by as much as 50 centimeters (1.6 feet) by 2100. This would flood coastal regions, where about one-third of the world's population lives and where an enormous amount of economic infrastructure is concentrated. It would destroy coral reefs, accelerate coastal erosion, and increase salinity to coastal groundwater aquifers. Warmer temperatures would allow tropical and subtropical insects to expand their ranges, bringing tropical diseases such as malaria, encephalitis, yellow fever, and dengue fever to larger human populations. There would be an increase in heat-related diseases and deaths. Agricultural regions might become too dry to support crops, and food production all over the world would be forced to move north; this would result in a loss of current cropland of 10 to 50 percent and a decline in the global yield of key food crops of from 10 to 70 percent.
Wild plant and animal species would need to move poleward 100 to 150 square kilometers (60 to 90 miles) or upward 150 meters (500 feet) for each 1 degree Celsius rise in global temperature. Since most species could not migrate that rapidly and since development would stop them from colonizing many new areas, much biodiversity would be lost. The decrease in the temperature difference between the poles and the equator would alter global wind patterns and storm tracks. Regions with marginal rainfall levels could experience drought, making them uninhabitable. Overall, since warmer air holds more moisture, an increase in global air and sea temperatures would increase the numbers of storms. Higher sea surface temperatures would increase the frequency and duration of hurricanes and El Niño events.
Many scientists believe that global warming is the most serious threat to our planet. By 2025 the world's energy demand is projected to be 3.5 times greater than in 1990, with annual CO 2 emissions nearly 50 percent higher. Thus far, attempts at international agreements to curb the emissions of greenhouse gases (for example, the Kyoto Protocol) have failed. This is due to several factors: (1) the scientific uncertainty of the role humans play in global warming; (2) the lifestyle changes necessary to reduce fossil fuel consumption in developed nations; (3) the possible slowdown in the economic development of developing nations; and (4) the need for true international cooperation. A high-technology alternative to decreasing greenhouse gas emissions is to sequester CO 2 . Experiments are underway to inject liquid CO 2 deep into the earth, thereby effectively removing it from Earth's carbon cycle.
Drake, Frances. Global Warming: The Science of Climate Change. Edward Arnold, 2000.
Stevens, William K. The Change in the Weather: People, Weather, and the Science of Climate. Delta, 2001.