Tuesday, January 20, 2009

Some time ago I was asking the question: ‘How good are these climate models? What sort of predictive value have they shown in modelling future climate? After all, we’ve been doing them for a few decades now.’
A nice person on realclimate.org (there are some, not all of them treat people who disagree with them as the demonised other) directed me to a classic paper by Hansen et al.
"If you want an indication of how well these models do you can go get (J. Geo Res. 93 (1988) 9341) the Hansen GCM paper that people talk about, and compare their results with observed patterns of warming and other things."
Here is the plot from that paper showing the response of overall global temperature (which the authors argue convincingly is a much better parameter than any subset of the data, e.g., whether it snowed at my house or not in a given year) for three different scenarios- A being continued exponential growth, B being a more subdued form of business as usual, and C if drastic cuts are implemented starting a few years ago.

I went and got the Hadcrut3 data set and plotted it on top of this one, as near as I was able, and got this.

There are other data sets out there. I shall plot some of the others and put them up for you.

The Hansen et al. model predicts the greatest degree of warming at high latitudes, fitting observations, but the model also reproduces another feature of observed weather, that those latitudes have the highest natural variability from one year to another.

Update 2012:
Here is another three years of data. I do realise I haven't plotted any of the other data sets. Bad me. The red points are the average of 13 monthly data points averaged on each month, while the blue points are the actual Hadcrut3 monthly global averages you can download yourself.


Jenny said...

does this mean we started some years ago?
I've always thought the whole climate change thing was a bit open to interpreting the outcome anyway you like.

change: our efforts to stop this have not done anything, therefore its a natural cycle.
No change: your model was wrong, there is no climate change.

change: we didn't try to fix it soon enough, we're all doomed
no change: Hey, we fixed it by our actions.

There's no controls, I'd be criticized for designing an experiment like this. We need a second planet Earth.

Chris Fellows said...

Well, of course, that would be nice...

Unfortunately it is the sort of curve that you can look at either way, but my point was just- I think- is that the motivation for taking drastic action is entirely from the output of climate models, and the best model from 20 years ago can't really be said to have done a very good job at predicting what would happen.

I think the one thing we can say with absolute certainty, looking at the graphs in the post before this one, is that there are lots of factors influencing the climate, and it is not easy to say how important different ones are.

Marco said...

Back in 2007 I mentioned:

Clearly, having an economy tank is an obvious way to reduce emissions, but no environmentalist is seriously suggesting it (except Peter Garrett before he became a politician:)).

One of the other assumptions of climate models is that BAU is predictably continuing exponential growth in emmissions, while several examples since 1990 (Russia and China come to mind) show respectively that BAU with a collapse in industry/economy, and BAU with high economic growth of a previously underdeveloped economy dwarf the discretionary decisions of leaders on policy.

Some policy decisions, such as whether to save the American auto industry (letting it collapse would reduce US emmissions considerably) are more relevant than others, like subsidising ethanol (arguably is an emmissions neutral policy).

Where the economies go, the emmissions statistics will follow.