50 years of climate change — and possible futures

A new study using a computer climate model to simulate the last 50 years of climate changes, projects warming over the next 50 years regardless of whether or not nations curb their greenhouse gas emissions soon. If no emission reductions are made and they continue to increase at the current rate, global temperatures may increase by 1-2 degrees Celsius [2-4 degrees Fahrenheit]. But if the growth rate of carbon dioxide does not exceed its current rate and if the growth of true air pollutants (substances that are harmful to human health) is reversed, temperatures may rise by only 0.75 degrees Celsius [1.4 degrees Fahrenheit].

“Some continued global warming will occur, probably about 0.5 degrees Celsius [0.9 degrees Fahrenheit] even if the greenhouse gases in the air do not increase further, but the warming could be much less than the worst case scenarios,” says James E. Hansen, lead researcher on the study at NASA’s Goddard Institute for Space Studies (GISS), in New York, New York. The research was a collaborative effort among 19 institutions, including seven universities, federal agencies, private industry and other NASA centers, and was funded by NASA. The results appear this month in the Journal of Geophysical Research-Atmospheres, published by the American Geophysical Union.

The GISS SI2000 climate model provided a convincing demonstration that global temperature change of the past half-century was mainly a response to climate forcing agents, or imposed perturbations of the Earth’s energy balance, according to the researchers. This was especially true of human-made forcings, such as carbon dioxide and methane, which trap the Earth’s heat radiation as a blanket traps body heat; thus causing warming.

The computer model’s ability to simulate the past 50 years of global temperature change provided confidence in understanding the causes behind the climate changes that occurred over that time period. The sensitivity of the SI2000 model to a climate forcing is comparable to that of other climate computer models. Model results from 1951-2000 are in close agreement with observed changes: the surface has warmed by about 0.5 degrees Celsius [0.9 degrees Fahrenheit], while the upper atmosphere (15-25 kilometer [10-15 mile] altitudes) has cooled by about one degree Celsius [two degrees Fahrenheit].

The climate model then simulated global temperature change during the next 50 years, under two contrasting assumptions for future growth of human forcings. The first assumption for the emissions of greenhouse gases was the “business-as-usual” scenario, in which greenhouse gases continue to increase rapidly. This scenario leads to an accelerating rate of global warming, raising global temperature to levels that have not existed during the past several hundred thousand years.

In the “alternative” scenario, in which air pollution is decreased and fossil fuel carbon dioxide emissions are stabilized, further global warming is limited to 0.75 degrees Celsius [1.4 degrees Fahrenheit] over the next 50 years. Hansen cautioned that the “alternative” scenario would not be easy to achieve. It requires that the world begin to reverse the growth of true air pollution (especially “soot” and the gases that control surface ozone, including methane) and also that we flatten out and eventually begin to decrease carbon dioxide emissions.

The climate forcing agents that Hansen and his co-authors included in their climate simulations were: (1) long-lived greenhouse gases such as carbon dioxide, methane, and the chlorofluorocarbons; (2) stratospheric aerosols (fine particles) from volcanic eruptions; (3) variations in the Sun’s energy, indicated by sunspots; (4) ozone changes, both at the surface (a pollutant) and upper atmosphere (protection from the Sun’s ultraviolet rays); (5) stratospheric water vapor, and (6) lower atmosphere air pollution aerosols, including black and organic carbon (soot) and sulfates.

Achievement of stable carbon dioxide emissions, as required in the alternative scenario that yields minimal climate change, is likely to require some combination of increased energy efficiency, a growing role for renewable energies, capture and sequestration of carbon dioxide emissions, and/or increased use of nuclear power, says Hansen.

“Decision-makers, including the public, may need to consider all of these options as climate change becomes more apparent and as our understanding of the climate forcing agents improves,” Hansen said. “Halting and reversing the growth of air pollution is possible with existing and developing technologies. It would have other benefits, especially for human health and agricultural productivity.”