EGEE 401
Energy in a Changing World

Climate Modeling

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diagram of El Nino developeing across the tropical Pacific. See link in caption for details.
Figure 3.1: When a strong El Niño develops across the tropical Pacific, it can influence weather and climate as far away as the southern polar region. For full description see, iSGTW.

Simulation models

Simulation models are computer programs that represent actual systems. We use them to predict how the system would behave under different conditions. When I worked as an industrial engineer, we often used simulation models to help design and plan manufacturing processes. For example, suppose we had an assembly line of 20 workstations that made washing machine parts. We would "describe" the assembly line using data in a computer simulation program. Then we would enter information about each workstation—how long it took, number of defects (say, 1 out of 10 parts failed), how often the workstation needed to be shut down for repair or maintenance, etc. Then, we would run the model to see what would happen if we made changes. What if we added a workstation? or removed one? Would our output change? What if we retooled or retrained and quality improved so that we only had 1 in 50 defects? What if the number of orders we got suddenly doubled, how far behind would we get? We would use the output of these models to help us make business and management decisions. Each set of what if conditions is called a scenario.

"Mathematical models are used not only in the natural sciences and engineering disciplines (such as physics, biology, earth science, meteorology, and engineering) but also in the social sciences (such as economics, psychology, sociology and political science); physicists, engineers, computer scientists, and economists use mathematical models most extensively." (Wikipedia: Mathematical Model) (I don't usually use Wikipedia as a source, but in this case the information is general, well stated, and widely accepted.)

We model many, many things—business processes, financial systems, whole economies, Internet traffic, network traffic, air traffic, automobile traffic, human behavior, cellular behavior...and so on! The Weather Channel will often show multiple possible paths for an incoming storm and the reporter will refer to different "models." These are weather models. Each model is based on a set of equations and data representing the weather system. Given a set of conditions (a scenario), the model predicts what the weather will do.

Weather and Climate

In the previous section, I used weather models as an example of using models to simulate different "what-if" scenarios. In the following sections we are going to talk about using models to study climate change. Before going there, let's pause and consider the important differences between "weather" and "climate." NASA has devoted an entire page to "What's the Difference between Weather and Climate?" If you have time, check it out. Here are key excerpts:

  • The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time.
  • In addition to long-term climate change, there are shorter term climate variations. This so-called climate variability can be represented by periodic or intermittent changes related to El Niño, La Niña, volcanic eruptions, or other changes in the Earth system.
  • Weather is basically the way the atmosphere is behaving, mainly with respect to its effects upon life and human activities. The difference between weather and climate is that weather consists of the short-term (minutes to months) changes in the atmosphere. Most people think of weather in terms of temperature, humidity, precipitation, cloudiness, brightness, visibility, wind, and atmospheric pressure, as in high and low pressure.
  • In most places, weather can change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season. Climate, however, is the average of weather over time and space. An easy way to remember the difference is that climate is what you expect, like a very hot summer, and weather is what you get, like a hot day with pop-up thunderstorms.

IPCC Climate Models

The IPCC uses climate simulation models to predict future climate change and its impact. The IPCC Working Group I describes a climate model as,

Climate Model...

A numerical representation of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes, and accounting for some of its known properties. The climate system can be represented by models of varying complexity, that is, for any one component or combination of components a spectrum or hierarchy of models can be identified, differing in such aspects as the number of spatial dimensions, the extent to which physical, chemical or biological processes are explicitly represented or the level at which empirical parameterizations are involved. Coupled Atmosphere–Ocean General Circulation Models (AOGCMs) provide a representation of the climate system that is near or at the most comprehensive end of the spectrum currently available. There is an evolution towards more complex models with interactive chemistry and biology. Climate models are applied as a research tool to study and simulate the climate, and for operational purposes, including monthly, seasonal, and interannual climate predictions. (from Annex III: Glossary)

Reading Assignment

FAQ 9.1 Are Climate Models Getting Better, and How Would We Know? (AR5 WGI Chapter 9 Evaluation of Climate Models)