Question: what is the quickest way to calculate a climate model?

James Pope answered on 29 Jun 2012:

Hi teenie,

The speed of a climate model depends on two factors.

Firstly, the speed of the computer, we use big supercomputers, which are thousands of times faster than a home PC, but you can run climate models on home PC’s (see http://climateprediction.net/ for people who allow you to do it and you can download and run the climate model yourself!).

The second factor is the resolution of climate model. Climate models split the world up into a series of cubes, they run all their calculations in each cube and then all the cubes share their information and the calculations are repeated again. The size of the cubes determines how complicated they are, with more smaller cubes taking much longer than fewer bigger cubes. I can run my model, whose cubes are 400km x 250km x 1km (vertically) at 25 model years a day, if I used one with cubes 1000km x 500km x 3km I could get 100 model years a day, if I used one that was 100km x 100km x 0.5km then I get about 1 year a day.

The final thing that affects a model, is how complicated it is. All climate models do some calculations which are the same across all the models, such as clouds and rain and how much sunshine their is, to calculate temperature and other climate variables. But some models include sub-models which model how ice sheets respond to the changes, or how the carbon cycle (the way that carbon and carbon dioxide moves around from plants and animals to the soils to the deep oceans to the atmosphere) responds to the changing climate. The biggest sub-model is the ocean, some climate models don’t include anythign other than the top 50m of the ocean as it takes a long time for the deeper ocean to be affected by climate, so it isn’t always needed. If you include a deeper ocean, or changing ice sheets or a carbon cycle, then there needs to be more calculations in each cube and so the model takes longer to run.

My model has the atmosphere and all of the ocean, but the ice sheets don’t change and the carbon cycle isn’t included, so we call it an Atmosphere-Ocean General Circulation Model. The very latest and most modern models include changing ice sheets, carbon cycle and atmospheric chemistry in all of them, so they are called Earth System Models.

However, an Earth System Model is just to slow for me, as I do thousands of years of simulations (climate model experiments), and the Earth Syetem Model would take about a year at present to run each of my simulations and I have 70 to do, so I use the quicker Atmosphere-Ocean model, which takes about 2 weeks to do a simulation. Luckily I can also do 6 simulations at a time, so that helps me also!

I hope this isn’t too complicated, let me know if you want anymore or anything explaining in more detail!