By Duane Friend
In a previous article, I discussed the basics of the natural and enhanced greenhouse effect and global warming. In this article, I will talk about how climate models work. Much of the information in this article comes from the National Center for Atmospheric Research and the Environmental Protection Agency.
A global climate model is a mathematical representation of the interactions between and within the ocean, land, ice, and atmosphere. To construct climate models, scientists divide each of the Earth’s components into a set of boxes. The most complex models may have boxes about 30 miles on a side for land, and up to 30 vertical layers of cells for the atmosphere and that many for the oceans. The movement of energy, air, and water are represented as horizontal and vertical exchanges between the boxes. In this way, models represent interactions between different parts of the climate system and the world.
To make the huge number of calculations needed, the largest supercomputers in the world are used to run climate models. To test model accuracy, scientists simulate past climate conditions. They then compare the model results to observed conditions. The observed average global change in temperature over the past century is relatively well-replicated by models. Today’s models show a significant and increasing ability to represent many important features, such as the large-scale distributions of land surface temperature, precipitation, wind, ocean temperature, and sea ice cover. Scientists continually refine models as knowledge increases, trying to improve representations of features that are not
Models that look at only the effects of natural processes are not able to explain the recent warming of the climate. However, models that account for the greenhouse gases emitted by humans are able to explain this warming. Most of the observed increase in global average temperature since the mid-20th century is very likely due to the increase in human-generated greenhouse gas concentrations.
When using computer models to make predictions about future climate, there are two major sources of uncertainty. One is the science side. Though observations, theory, and climate models continue to improve, Earth’s climate is a very complex system. Any attempt at predicting future climate is bound to have at least a few uncertainties about the science involved. The other uncertainty is human activity. At what rate will world and regional populations continue to grow? What mix of renewable and fossil fuels will people use for energy in 10, 20, or 50 years’ time? Will people rely more on automobiles or public transportation to get around? Assumptions about the answers to such questions influence predictions of future climate change.
Most of the big unknowns about how Earth’s climate will change are due to the fact that we are not sure how humans will change their behaviors in the future. To try to account for several scenarios, such as a cut back of greenhouse gas emissions, models are run with high emission and low emission setups. These scenarios are a way to make a range of predictions of future climate change based on a range of ways that humans may live, work, and populate the Earth in the future.
The third article will discuss what models are predicting, especially for Illinois.