Computer models are everywhere.
- Engineers use them to design airplanes and buildings.
- Health experts use them to plan for epidemics.
- Fire researchers use them to improve safety regulations and protect firefighters.
- Farmers and agricultural researchers use them to increase crop yields and design best management practices.
Despite the prevalence (and acceptance) of models in so many aspects of our lives, many people still distrust - or are openly hostile to - climate models. In fact, of everything in climate science, what seems to spark the most skeptical questions is our use of computer models. Today and tomorrow, I want to tackle this issue with an overview of these vital tools. I hope that this two-parter (part one of which is also available at Climate411.org) will help you answer three common questions:
- How do climate models work?
- How accurate are they?
- What do they predict?
Follow me over the fold for question number one.
How do climate models work?
Let's start with why we use models in the first place. We want to project what will happen to our climate in the future – will it be warmer? How much warmer? Will it change in different ways in different places? Climate models use our knowledge of how the climate system works to calculate what different emissions scenarios mean for the future. Here's how the models are built.
General Circulation Models, or GCMs, are computer programs that use basic physics and chemistry to calculate properties of the atmosphere and ocean, such as temperature, precipitation and sea level.
GCMs represent the Earth’s ocean, biosphere, and atmosphere in three-dimensional cells. The models stack ocean and atmospheric layers in grids across the Earth’s surface. An example schematic of atmospheric cells is shown below. The models use differential equations to calculate transfers of energy, carbon, water, and other substances between cells.
A Coarse Example of Atmospheric Cells
This figure is from the Australian Bureau of Meteorology.
How we use a GCM depends on its complexity. The simplest models take less than a minute to complete a 100-year simulation. A more complex, high-resolution model can simulate phenomena such as regional weather extremes or atmospheric dust transport, but using these kinds of GCMs requires substantially more computing power and time.
To run a GCM, we need to give it input. This information includes solar radiation, volcanic emissions and human-produced emissions of greenhouse gases. We have this information for the 20th century, but of course we don’t know what emissions will be in the future. Scientists deal with that uncertainty by using a range of emissions scenarios.
Some emissions scenarios are "fossil fuel intensive", with emissions continuing to increase (for example, Scenario "A2" shown below in red). Others assume that greenhouse gas emissions decrease at various rates over time. For example, emissions cuts in Scenario "B1" (blue) are steeper than those in "A1B" (green). As shown in the graph below, using different scenarios results in different predictions for global temperature change.
Different scenarios of global warming
This is Figure SPM-7 in the summary of the latest IPCC report [2.9 MB PDF].
In the figure above, each scenario has a thick line with some shading on either side of it. The line represents the average of several model runs. The shading shows the range of temperature predictions from all the models.
All of these models were given exactly the same inputs — so why don’t they give the same results? Some of this has to do with the different levels of complexity from model to model. Some of the spread also comes from uncertainties in various factors such as the carbon cycle (the rate at which soils and plants store or release carbon), climate sensitivity (the amount of warming for a given amount of greenhouse gas) and cloud formation (which can lead to warming or cooling). Each model deals with these factors slightly differently, which leads to variations in their predictions.
These uncertainties lead to the most common question about climate models: How accurate are they? And that's the topic of part two. Stay tuned!
Further Reading