Gavin Schmidt on Climate Models | Edge
Gavin Schmidt is a climatologist working with Jim Hansen at NASA's Goddard Institute for Space Studies and one of the bloggers at Real Climate. In an Edge video, he describes the intricacies of climate modeling:
We have been quite successful at building these models on the basis of small-scale processes to produce large-scale simulation of the emerging properties of the climate system. We understand why we have a seasonal cycle; we understand why we have storms in the mid latitudes; we understand what controls the ebb and flow of the seasonal sea ice distribution in the Arctic. We have good estimates for all the things that are going on. But we don't have perfect estimates. Instead, we have maybe 20 different groups around the world who have put together their best shot at what all those process are, which ones are important and which ones are not important, and they have all produced their own separate digital world, their digital climate. They are all a little bit different and they all have a little bit different sensitivity. So if I change one element in those models, for instance the amount of carbon dioxide in the atmosphere, then they all react in slightly different ways.
In some respects they all act in very similar ways — for instance, when you put in more carbon dioxide, which is a green house gas, it increases the opacity of the atmosphere and it warms up the surface. That is a universal feature of these models and it is universal because it is based on very, very fundamental physics that you don't actually need a climate model to work out. But when it comes to aspects which are slightly more relevant – I mean, nobody lives in the global mean atmosphere, nobody has the global mean temperature as an important part of their expectations – things change. When it comes to something like rainfall in the American Southwest or rainfall in the Sahel or the monsoon system in India, it turns out that those different assumptions that we made in building those models (the slightly different decisions about what was important and what wasn't important) have a very important effect on the sensitivity of very complex elements of the climate.
Some models suggest very strongly that the American Southwest will dry in a warming world; some models suggest that the Sahel will dry in a warming world. But other models suggest the exact opposite. Now, let's just imagine that the models have an equal pedigree in terms of the scientists who have worked on them and in terms of the papers that have been published — it's not quite true but it's a good working assumption. With these two models, you have two estimates — one says it's going to get wetter and one says it's going to get drier. What do you do? Is there anything that you can say at all? That is a really difficult question.
