Wednesday, June 10, 2015

GCMs: Explicative value is strong, predictive only in the long run

Twitter, can be succinct

Damn well better, but Steve Easterbrook has a nice explanation of what GCMs are and are not.  As he points out,
you don’t actually need a computer model to predict climate change. The first predictions of what would happen if we keep on adding carbon dioxide to the atmosphere were produced over 120 years ago. That’s fifty years before the first digital computer was invented. And those predictions were pretty accurate – what has happened over the twentieth century has followed very closely what was predicted all those years ago. Scientists also predicted, for example, that the arctic would warm faster than the equatorial regions, and that’s what happened. They predicted night time temperatures would rise faster than day time temperatures, and that’s what happened.

So in many ways, the models only add detail to what we already know about the climate. They allow scientists to explore “what if” questions. For example, you could ask of a model, what would happen if we stop burning all fossil fuels tomorrow. And the answer from the models is that the temperature of the planet will stay at whatever temperature it was when you stopped. For example, if we wait twenty years, and then stopped, we’re stuck with whatever temperature we’re at for tens of thousands of years. You could ask a model what happens if we dig up all known reserves of fossil fuels, and burn them all at once, in one big party? Well, it gets very hot.
Post Script: Isaac Held has some essays on the speakable and unspeakable in climate models. Last The quality of the large scale flow simulated in GCMs, instructions on How Not to Evaluate Climate Models, and a bug-a-boo of Eli's, Addicted to Global Mean Temperature.


Barton Paul Levenson said...

Russell Seitz said...

"You could ask a model what happens if we dig up all known reserves of fossil fuels, and burn them all at once, in one big party? "

But you hsould ask how hot it would get on the way to the party, because it would take a fair percentage of fossil fuel reserves to excavate the rest.

Chase Stoudt said...

Best seminar I ever took was reading "The Warming Papers"

BBD said...

Well-known model sceptic James Hansen sets the record straight:

TH: A lot of these metrics that we develop come from computer models. How should people treat the kind of info that comes from computer climate models?

Hansen: I think you would have to treat it with a great deal of skepticism. Because if computer models were in fact the principal basis for our concern, then you have to admit that there are still substantial uncertainties as to whether we have all the physics in there, and how accurate we have it. But, in fact, that's not the principal basis for our concern. It's the Earth's history-how the Earth responded in the past to changes in boundary conditions, such as atmospheric composition. Climate models are helpful in interpreting that data, but they're not the primary source of our understanding.

TH: Do you think that gets misinterpreted in the media?

Hansen: Oh, yeah, that's intentional. The contrarians, the deniers who prefer to continue business as usual, easily recognize that the computer models are our weak point. So they jump all over them and they try to make the people, the public, believe that that's the source of our knowledge. But, in fact, it's supplementary. It's not the basic source of knowledge. We know, for example, from looking at the Earth's history, that the last time the planet was two degrees Celsius warmer, sea level was 25 meters higher.

And we have a lot of different examples in the Earth's history of how climate has changed as the atmospheric composition has changed. So it's misleading to claim that the climate models are the primary basis of understanding.