Environment - Global Climate Change

Global Climate Change

Climate Change Facts
October 27, 1997

Issue 4 U.S. House of Representatives

Minority Staff, Committee on Government Reform and Oversight

How is Climate Predicted to Change in the Future?
The last fact sheet discussed the evidence that greenhouse gas emissions from human activities have already had a "discernible" effect on global climate. This fact sheet describes the effects greenhouse gas emissions are predicted to have on global climate in the future.

Methods of Predicting Climate Change. Mathematical models are the primary technique used to understand the earth's present climate system and to predict changes in the earth's climate. The most sophisticated climate models -- called atmospheric and oceanic general circulation models (GCMs) -- simulate large-scale climatic features, such as the physical properties of the atmosphere and oceans, global carbon and other chemical cycles, solar and infrared radiation, and snow and ice cover. Models also incorporate the effects of seasonal climatic cycles and periodic variations in climate (such as the El Nino).

Models are tested by running simulations of current climatic features (without future increased greenhouse gas emissions or other changes) to determine if they can accurately simulate current conditions. Comparisons of atmospheric models to actual observed temperature changes have demonstrated the ability of these models to accurately simulate climate processes. Scientific confidence in predictions for hemispheres and continents is much greater than it is for smaller regions because the size of the 3-dimensional modules mapped by the models is relatively large (typically 250 km in the horizontal by 1 km in the vertical), so resolution for smaller regions is less certain. Models are still being refined to better simulate cloud cover and its effect on local climate, nuanced land surface processes, and other smaller scale climatic processes.

To predict future climate change, atmospheric modelers "plug" various levels of greenhouse gas emissions into the models of the global climate system. The degree of "radiative forcing" -- the warming effect of heat-trapping greenhouse gases that collect in the atmosphere -- can then be forecasted based on the predicted concentration of these gases. To calculate the overall climatic effect of the increases in greenhouse gases, the models then incorporate an estimate of "climate sensitivity," a factor which describes the degree to which surface air temperature and other climatic factors will respond to the increased radiative forcing.

The IPCC Predictions. The Intergovernmental Panel on Climate Change (IPCC), the international body of scientists established by the United Nations Environment Programme and the World Meteorological Organization to examine the science of climate change, has used mathematical modeling to estimate a range of emission scenarios. These scenarios assume carbon dioxide emissions to be anywhere from their current levels of 6 billion metric tons (gigatonnes of carbon, or GtC) per year to 36 GtC per year by 2100. The IPCC used as its "best estimate" of future emissions a mid-range prediction of energy use, population growth, economic growth and technological change. Under this scenario, in which CO2 emissions will increase by roughly 300% over the next century, the atmospheric CO2 concentration will rise to roughly 700 parts per million by volume (ppmv) by 2100. This concentration is almost three times the concentration prior to industrialization (280 ppmv) and roughly double the current concentration of 360 ppmv. Under the "best estimate" of climate sensitivity, this corresponds to a 3.6ø F increase in global average temperature by 2100.

The IPCC's prediction for the increase in global mean surface air temperature relative to 1990 temperatures by 2100 was 6.3ø F in the worst-case emission scenario, in which carbon emissions increase by nearly 600%. The IPCC's best-case scenario, in which CO2 emissions stay approximately at their current levels, was projected to lead to a temperature change of 1.8ø F. Even in this best-case scenario, the average rate of warming would be greater than any in 10,000 years.

Potential Sudden, Dramatic Changes and Feedback Effects. The IPCC estimates described above assume that greenhouse gas emissions cause linear changes in climate. The IPCC report recognizes, however, that increases in greenhouse gas emissions could "non linear" changes. Increased runoff of fresh water in high latitudes (due to melting of glaciers), for example, could disrupt major oceanic circulation systems, leading to drastic and sudden global climatic shifts. Another prediction involves the rapid release of large concentrations of methane and carbon dioxide as warming occurs in high-latitude tundra. Prior to 1982, the arctic tundra was a "sink" for global CO2 and methane, but as temperatures in the arctic have increased this region has now become a substantial source of these greenhouse gases. These changes are difficult to predict with precision, but could be drastic and sudden contributions to future climate change.

Lag Time and Longer-Term Changes in Global Climate. While most climate models predict climatic changes over the next century, there are many factors in the climate system with long lag-times. Many greenhouse gases have long residence times in the atmosphere and thus continue to affect climate well after they are emitted. CO2 remains in the atmosphere for over 100 years, NOx emissions for 120 years, and CH4 for 9 to 15 years. Another similar factor is the temperature inertia of oceans. Because of this characteristic, a large percentage (10-50%) of the warming due to emissions of greenhouse gases over the next century will take place after 2100 -- even if greenhouse gas emissions are stabilized by 2100. If greenhouse gas emissions continue along the IPCC's best-estimate trajectory, CO2 concentrations are likely to quadruple relative to their pre-industrial levels by about 2150. With lag times taken into account, this means that global mean surface air temperatures have a 90% likelihood of increasing by 5 to 16 degrees F over the next 500 years.