WHY WE USE CLIMATE MODELS

Climate models have several purposes. They are used to understand present climate and what factors create a particular climate in any one region. They are used to project climatic conditions into the future. Finally, climate models are a tool to find out what natural processes or human activities may affect a region’s environment in the future.

To understand present climate:     Global climate models are used to understand the current climate, especially those processes that create a particular climate in each place. To become confident that a model correctly represents everything that shapes climate, scientists test models to see if they can accurately depict the current climate or recent historical climates on Earth.  In this way, developers can improve the computer code that represents important physical processes that shape climate in any region.

To project climatic conditions into the future:    Once developers have confidence that a model simulates present climate, they then use that model to project climate into the future. To do so, developers make assumptions about factors that will affect climate in the future. For example, they may assume that the amount of carbon dioxide (CO2) in the air will remain the same as it is now; or alternatively, they may assume that the level of CO2 will double.

Scientists call these factors “forcing factors” because they strongly influence the climate.
Other forcing factors, in addition to the level of CO2 , include the amount of dust, ash, or other particles in the air from volcanoes or pollution; the level of ozone; and the reflectivity of the land -- whether its surface is light or dark.

Tools to run experiments:    Models are a tool to run experiments that cannot be done in the real environment.   Models can simulate alternative futures and thus can answer questions about what processes may or may not shape the future environment of a region. Examples of model experiments include:

  • Oregon is now 48% forested. If all of this forest is removed, what will the state’s climate look like?

  • The Arctic Ocean is now one-half covered by sea ice in late summer. In the future, if no ice is present from July through September, what climatic conditions may we expect in the Arctic regions? Or in the United States?

  • If no greenhouse gases (GHG) are emitted by human activities after the year 2050, how much warmer will the climate be in 2100? How does that compare with conditions in 2100 if the present trend of increasing emissions of GHG continues to 2100?

An example of model experiments: the "Fingerprint" study:   A good example of model experiments is the “Fingerprint” study, in which scientists try to detect climate pattern changes that can be explained only by human activity. When model simulations of 20th century climate represent the major influences on climate, both natural and man-made, the results reproduce many of the important changes that have indeed been observed, such as:

  • Declining sea ice on the Arctic Ocean;

  • Agradual warming of the lower atmosphere over most of the Earth;

  • A cooling of the stratosphere;

  • Increasing amounts of water vapor in the atmosphere;

  • Dry regions becoming drier, and wet regions becoming wetter.

But when the models included all the natural influences and excluded the influences of human activity, the results depicted slight cooling of the surface of the Earth after about 1950. In fact, the surface has clearly warmed since about 1975.

Other fingerprint studies identified patterns of warming and cooling as one goes up through the layers of the atmosphere. These vertical patterns are what one expects with increasing amounts of heat-trapping greenhouse gases in the atmosphere.