Climate computer models (computer predictions of the Earth's climate) are one of the critical tools we use to determine the future of our Earth and the likelihood humans are having an impact on the environment.
So, what the heck is a climate model? If you are a scientist, it is a computer construct that expresses conservation of mass, energy, and momentum over a large number of grid boxes that cover the earth, including the atmosphere and the ocean. For people who aren't scientists, it is best to think of them as virtual-reality computer programs that predict the future.
Climate models are not the only reason scientists are so certain that humans are changing the climate. In fact, I argue they aren't even the best evidence. We also know humans are causing climate change because we can look at how the deep-past Earth has changed; we can extrapolate into the future. Additionally, we have recent instruments (since approximately 1880) that allow us to relate human emissions to the increase in temperatures. Climate models aren't limited to temperature. They allow prediction of species, precipitation, cloud cover, ice, and many other quantities.
The reason scientists spend so much time using models is because they allow us to make future predictions that are testable. Climate models can predict cooling of the stratosphere, they can predict loss of arctic sea ice, they can predict changes in weather patterns, they can predict acidification of the ocean; climate models can be used to study what if scenarios in a way that our other information sources cannot. In fact, computer models have an excellent track record predicting changes in the Earth's climate before they were detected by measurements. In this way, measurements have confirmed climate models.
The accuracy of a climate model is dictated in part by how many calculation points are used to cover the Earth. A fancy term for this is grid resolution. The more grid boxes used to subdivide the ocean and atmosphere, the better the results. Today's most powerful computer models are running 24-hour calculations of enormous climate models, making calculations over millions and millions of grid boxes. Even with today's powerful supercomputers, there is a limit to how large the number of calculation points can be and a limit to how much data can be stored.
Another limit to the accuracy of climate models deals with processes that do not obey basic laws of the universe (conservation of mass, momentum, and energy). Examples of processes that fall into this category are cloud formation, fluid turbulence, ocean oscillations, dispersion of particulates, reflection of sunlight by aerosols, transfer of water between oceans and the atmosphere, etc. Here, best guesses have to be used which are broadly called parameterizations. Parameterizations are equations that reflect our best understanding of the underlying physical process.
One of the nice things about climate models is that they are testable and they follow known governing laws of the universe that cannot be broken. The climate follows these laws now, has followed them in the past, and will follow them in the future.
At the recent American Geophysical Union meeting, I met a young climate modeler, Catalina Oaida. Her work deals with particulates impact in the cryosphere and her description of models in her discipline is a great view of how insiders view these computational tools.
"My research involves using what we call regional climate models to assess the impact of dust and black carbon deposited onto the mountain snowpack. Regional climate models are very similar to general climate models [GCMs - used in the IPCC]; they are based on the same universal principles of physics and conservation, but they have a higher resolution that allows us to study in greater detail, and often with better accuracy, the processes taking place at local and regional scales. What is great about physically-based regional climate models is that they allow us to study these processes as they have occurred in that past, and also allow us to assess how this human-nature system/interaction might change in the future."
My colleagues in business are not as fortunate. Worldwide and national business and economics do not follow universal governing laws. They follow human actions that are incredibly hard to predict.
Posted by John Abraham on Thursday, 19 December, 2013
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