Testing Hansen’s Climate Predictions
Posted by softestpawn on June 28, 2009
The case for harmful global warming largely rests, understandably, on the climate models required to emulate the world’s complex climate and how it is expected to behave as humans continue on our merry way.
Validating these models is tricky as we only have one real world to test them against. Sometimes they can be checked against new historical data, but since this data is usually another aspect or refinement of existing knowledge this isn’t a proper validation. Where the models are found to deviate from the data, they are adjusted to fit which is a Good Thing, but this is merely a retrospective refit and can be done with any arbitrary multivariable model; it’s not a validation.
Which is why James “Death Trains” Hansen’s predictions from 1988 are seen as an important test for the Man-Made CO2-induced Global Warming case.
Setting the Scene
Essentially Hansen ran three predictions, based on three scenarios (abstract here):
“Scenario A assumes continued exponential trace gas growth, scenario B assumes a reduced linear linear growth of trace gases, and scenario C assumes a rapid curtailment of trace gas emissions”
Now Hansen claims that, if we look at the observations:
then because the observed temperatures match Scenario B, his predictions have been validated.
Which is a novel approach.
Everyday prediction tests
There’s nothing scarily complicated about testing models. Let me suggest a model for you:
“The more you eat, the fatter you get”
It sounds plausible, let’s test it. Here are three scenarios:
A: I will eat steak and chips and cheese omelettes and Big Macs, three times a day. I predict I will put on 4″ to my waistline by a month next Friday.
B: I will eat cereal for breakfast, meat and two veg for lunch and a sandwich for supper. I predict no great change.
C: I will drink watery soup and have muesli for supper. I predict my ribs will be showing and I shall be in a really foul temper. But it’s OK, I won’t have the energy to do anything about it.
So, after a month, we measure my waistline and see that I have slimmed down quite a bit. Brilliant! Scenario C, my prediction was correct!
Which is a bit daft, as it ignores the premise for the scenario; the whole point of the model is to test that the scenario prediction matches the observation. In this particular case I ate a high calorie diet but trained every day for the marathon; that’s Scenario A. I ate a lot, but slimmed; my model was wrong (or at best, very incomplete).
Hansen Prediction Tests
So when we look at Hansen’s predictions, we should be comparing the observations with those predictions from the right scenario.
Naively, we could pick the scenario from his abstract, which would give us scenario A, as our CO2 emissions have exponentially increased (which is the major component). Hansen gave this scenario as his “business as usual” one, and indeed that is what happened; we didn’t start reducing emissions as he would have needed to make Scenario B the ‘plausible’ one.
(update) However we can look at it in more detail if we look at pages 9631-2 of his paper. There he has CO2 emission increase at 1.5%/year in Scenario A, and has it reducing in steps to 0% in 2010 for Scenario B. CO2 emissions have been 2%/year until 2000ish, then increasing to nearly 3% until last year, as you may recall causing panic and predictions of even worse effects of global warming with no eye on actual observations at all.
CH4, the next biggest forcing, is not so clear as there was low/no growth in the 1990s, but otherwise is overall consistent 1985-2005 with a 1% growth year-on-year which is lower than Scenario A (1.5%) and higher than Scenario B ( 1% 1990-2000, then 0.5%)
Scenario A had no volcanos, Scenario B had two or three (though I’m not clear on this), and in the end there was one of the right scale.
So it looks like mostly Scenario A-ish; possibly ‘somewhere between A and B’ but given the above, heavily weighted toward A:
(via Paul Macrae here)
Although Hansen and his friends get very upset when people compare observations with that scenario. Apparently it’s ‘fraud’
And he’s not only way off Scenario A, but if we plot out the later years (and remove that convenient estimate for 2005 and replace with the observation), we see that the observations drop lower (for a short period) than even Scenario C; where CO2 emissions were to stop increasing altogether:
This image is a clip from Hansens report, overlaid with the observed temperatures, from Climate Skeptic. Feel free to check with Hadley’s temperature measurements. Note that in the text Hansen gives Scenario A as ‘growth… typical of the last 30 years’ though these days he insists it was Scenario B that he gave as the ‘most plausible’, which is not quite the same thing.
So really, there’s no positive validation for his 1988 models (although of course modern models are a good 20 years more up to date…)
“It doesn’t matter how elegant your theory is, it doesn’t matter how smart you are, if the experiment says it’s wrong, it’s wrong” – Feynman
It gets more complicated though. It may be that actually there’s little difference in the CO2 forcing paramaters between Hansen’s Scenario A and B; Steve McIntyre thinks it’s mostly down to CFC forcings.
Similarly, it may be that the scenarios are actually based on forcings rather than emissions as Hansen gives in his abstract. This would make for a different story, as the forcing strength decreases as CO2 increases (ie, as you add more and more CO2 the effect it has becomes a bit less each time).
Hansens predictions remain broken, these would just explain where.
This is hard to verify as, like a lot of academic research, climatologists have yet to embrace “full disclosure” or many of the other practices that would be associated with the ‘open’ and ‘welcoming criticism’ working environment they claim to have.