Greenhouse Effect

The physical mechanism of the greenhouse effect is conceptually simple, and this phenomenon is acknowledged by scientists as helping to keep Earth's temperature within the comfort zone for organisms. It is also known that the concentrations of CO₂ and other RAGs have increased in Earth's atmosphere, and will continue to do so. However, it has proven difficult to demonstrate that the observed warming of Earth's surface or lower atmosphere has been caused significantly by a stronger greenhouse effect rather than by some still-unknown process of natural climate change.

Since the beginning of instrumental recordings of surface temperatures around 1880, almost all of the warmest years on record have occurred since the late 1980s. Typically, these warm years have averaged about 1.5–2.0°F (0.8-1.0°C) warmer than occurred during the decade of the 1880s. Overall, Earth's surface air temperature has increased by about 0.9°F (0.5°C) since 1850.

However, the temperature data on which these apparent changes are based suffer from some important deficiencies, including: (1) air temperature is variable in time and space, making it difficult to determine statistically significant, longer-term trends; (2) older data are An atmosphere with natural levels of greenhouse gases (left) compared with an atmosphere of increased greenhouse effect (right). Illustration by Hans & Cassidy. Courtesy of Gale Group.
generally less accurate than modern records; (3) many weather stations are in urban areas, and are influenced by "heat island" effects; and (4) climate can change for reasons other than a greenhouse response to increased concentrations of CO₂ and other RAGs, including albedo-related influences of volcanic emissions of sulfur dioxide, sulfate, and fine particulates into the upper atmosphere. Moreover, it has long been thought that the interval 1350 to 1850, known as the Little Ice Age, was relatively cool, and that global climate has been generally warming since that time period. (The data one which this claim was based, however, have recently been called into question; no instrumental or global data at all are available from the period in question.)

However, some studies have provided evidence for linkages between historical variations of atmospheric CO₂ and surface temperature. Important evidence comes, for example, from a core of Antarctic glacial ice that represents a 160,000–year period. Concentrations of CO₂ in the ice are determined by analysis of air bubbles in ice layers of known age (determined by counting annual snowfall layers back from the present), while changes in air temperature are inferred from ratios of oxygen isotopes in the ancient ice. (Because atoms of various isotopes differ in weight, their rates of diffusion are affected by temperature differently; differences in diffusion rate, in turn, affect their relative abundance in the ice). Because changes in CO₂ and surface temperature are positively correlated, a greenhouse mechanism is suggested. However, this study could not determine causal direction—that is, whether increased CO₂ might have resulted in warming through an intensified greenhouse effect, or whether, conversely, warming (caused by something unknown) could have accelerated CO₂ release from ecosystems by increasing the rate of decomposition of biomass, especially in cold regions.

Because of the difficulties in measurement and interpretation of climatic change using real-world data, computer models have been used to predict potential climatic changes caused by increases in atmospheric RAGs. The most sophisticated simulations are the socalled "three-dimensional general circulation models" (GCMs), which are run on supercomputers. GCM models simulate the extremely complex mass-transport processes involved in atmospheric circulation and the interaction of these processes with other variables that contribute to climate. To perform a simulation "experiment" with a GCM model, components are adjusted to reflect the probable physical influence of increased concentrations of CO₂ and other RAGs.

Many simulation experiments have been performed using a variety of GCM models. Their results have, of course, varied according to the specifics of the experiment. However, a central tendency of experiments using a common CO₂ scenario (i.e., a doubling of CO₂ from its recent concentration of 360 ppm) is an increase in average surface temperature of 1.8–7.2°F (1–4°C). This warming is predicted to be especially great in polar regions, where temperature increases could be two or three times greater than in the tropics.

One of the best-known models was designed by the International Panel on Climate Change (IPCC). This GCM model makes assumptions about population and economic growth, resource availability, and management options that result in increases or decreases of RAGs in the atmosphere. Scenarios were developed for emissions of CO₂, other RAGs, and sulfate aerosols, which may cool the atmosphere by increasing its albedo and by affecting cloud formation. For a simple doubling of atmospheric CO₂, the IPCC estimate was a 4.5°F (2.5°C) increase in average surface temperature. The estimates of more advanced IPCC scenarios (with adjustments for other RAGs and sulfate) were similar, and predicted a 2.7–5.4°F (1.5–3°C) increase in temperature by the year 2100, compared with 1990. Thus, theoretical studies tend to back the claim that CO₂ can cause global warming, whether or not the reverse process may also occur.