PrepTest 84, Section 4, Question 3

The following passage was written in the mid-1990s.

Evidence that the earth's atmosphere has warmed has become quite compelling, in part because it has been reinforced recently by the development of accurate profiles of average annual temperatures throughout the last 1,000 years. These data, inferred from studies of geological patterns and samples of ice deposits, tree rings, and coral growth layers, indicate that the recent increase in average temperature—a rise of about one half of a degree Celsius over the last 100 years—is unprecedented in the previous 1,000 years. At the same time, other recent studies have strengthened the controversial link between this increase and the "greenhouse effect." Proponents of the greenhouse effect claim that the increase was caused by elevated levels in the atmosphere of certain gases that prevent heat from radiating back into space.

Early models charting the greenhouse effect were somewhat inconsistent with observed data; they estimated that the increase in the earth's atmospheric temperature over recent decades should have been higher than the increase observed in actuality, which led opponents to question the validity of the greenhouse theory. But new methods have enabled scientists to gauge the effect of greenhouse gases more accurately by taking into account an important factor that earlier studies overlooked: airborne sulfates. Sulfates from natural sources such as volcanoes as well as from human technological sources tend to counteract the heating effect of greenhouse gases by reflecting solar energy back into space. Taking into account the varying levels of airborne sulfates indicated by the concentration of sulfates in successive ages of glacial ice, these scientists have calculated theoretical temperatures for recent decades that are consistent with observed temperatures.

Another question for proponents of the greenhouse theory comes from scientists who have attempted to tie changes in the earth's atmospheric temperature to variations in solar energy. From observations of cycles in several types of solar phenomena, these scientists have developed models that chart variations in the sun's heating effects, and the models do show a strong decade-by-decade correspondence between solar activity and atmospheric temperature fluctuations. But the models cannot account for the entirety of the recent rise in atmospheric temperature. While researchers have found that the average annual atmospheric temperature fluctuates from one year to the next, its temperature over the long term has been relatively stable—deviations from the long-term average atmospheric temperature have inevitably reverted to this average, or equilibrium, temperature. But the current rise in temperature surpasses the most extreme fluctuations in temperature consistent with the models based on variations in solar energy. In light of all this, it seems reasonable to conclude that changes in the earth's atmosphere have raised its equilibrium temperature, and that greenhouse gases represent the best explanation of that shift.

The following passage was written in the mid-1990s.

Evidence that the earth's atmosphere has warmed has become quite compelling, in part because it has been reinforced recently by the development of accurate profiles of average annual temperatures throughout the last 1,000 years. These data, inferred from studies of geological patterns and samples of ice deposits, tree rings, and coral growth layers, indicate that the recent increase in average temperature—a rise of about one half of a degree Celsius over the last 100 years—is unprecedented in the previous 1,000 years. At the same time, other recent studies have strengthened the controversial link between this increase and the "greenhouse effect." Proponents of the greenhouse effect claim that the increase was caused by elevated levels in the atmosphere of certain gases that prevent heat from radiating back into space.

Early models charting the greenhouse effect were somewhat inconsistent with observed data; they estimated that the increase in the earth's atmospheric temperature over recent decades should have been higher than the increase observed in actuality, which led opponents to question the validity of the greenhouse theory. But new methods have enabled scientists to gauge the effect of greenhouse gases more accurately by taking into account an important factor that earlier studies overlooked: airborne sulfates. Sulfates from natural sources such as volcanoes as well as from human technological sources tend to counteract the heating effect of greenhouse gases by reflecting solar energy back into space. Taking into account the varying levels of airborne sulfates indicated by the concentration of sulfates in successive ages of glacial ice, these scientists have calculated theoretical temperatures for recent decades that are consistent with observed temperatures.

Another question for proponents of the greenhouse theory comes from scientists who have attempted to tie changes in the earth's atmospheric temperature to variations in solar energy. From observations of cycles in several types of solar phenomena, these scientists have developed models that chart variations in the sun's heating effects, and the models do show a strong decade-by-decade correspondence between solar activity and atmospheric temperature fluctuations. But the models cannot account for the entirety of the recent rise in atmospheric temperature. While researchers have found that the average annual atmospheric temperature fluctuates from one year to the next, its temperature over the long term has been relatively stable—deviations from the long-term average atmospheric temperature have inevitably reverted to this average, or equilibrium, temperature. But the current rise in temperature surpasses the most extreme fluctuations in temperature consistent with the models based on variations in solar energy. In light of all this, it seems reasonable to conclude that changes in the earth's atmosphere have raised its equilibrium temperature, and that greenhouse gases represent the best explanation of that shift.