Modelling the climate

Tom James @ 02-07-2009

weatherAn interview with Gavin Schmidt over on Edge explores the nature and development of climate modelling:

What we have decided, as a scientific endeavor, is to extrapolate as much as we can from our knowledge of the individual processes that we can measure: evaporation from the ocean, the formation of a cloud, rainfall coming from a cloud, changes in the wind patterns as a function of the pressure field, changes in the jet stream. What we have tried to do is encapsulate those small-scale processes, put them altogether, and see if we can predict the emerging properties of that fundamental complex system.

He explores the sometimes contradictory predictions of different climate models:

In the same way that you can’t make an average arithmetic be more correct than the correct arithmetic, it’s not obvious that the average climate model should be better than all of the other climate models. So for example if I wanted to know what 2+2 was and I just picked a set of random numbers, the answer by averaging all those random numbers is unlikely to be four. Yet when you come to climate models, that is kind of what you get. You get all the climate models and they give you some numbers between three and five and they give you something that is very close to four. Obviously, it’s not pure mathematics — it’s physics, it’s approximations, there is empirical tuning that goes on.

You need to have some kind of evaluation. I don’t like to use the word validation because it implies a kind of binary/true-false set up. But you need an evaluation; you need tests of the model’s sensitivity compared to something in the real world that can give you some credibility that that model has the right sensitivity. That is very difficult.

It is a lengthy essay/video interview but well worth the read/watch, as it is refreshing to hear firsthand from a professional climatologist.

[at Edge][image from Nicholas T on flickr]


Cosmic ray global warming debunked; deep ocean conveyors rethought

Paul Raven @ 18-05-2009

sun, clouds and seaOne of the more popular alternatives to anthropic global warming theories has been the cosmic ray hypothesis – the notion that changes in the sun’s output of cosmic rays are responsible for the planet’s recent changes in temperature. However, it’s always been short on evidence (much shorter than the theories it is intended to topple, funnily enough), and now new research has put another nail in its coffin lid:

In research published in Geophysical Research Letters, and highlighted in the May 1 edition of Science, Adams and Pierce report the first atmospheric simulations of changes in atmospheric ions and particle formation resulting from variations in the sun and cosmic rays. They find that changes in the concentration of particles that affect clouds are 100 times too small to affect the climate.

[…]

Despite remaining questions, Adams and Pierce feel confident that this hypothesis should be laid to rest. “No computer simulation of something as complex as the atmosphere will ever be perfect,” Adams said. “Proponents of the cosmic ray hypothesis will probably try to question these results, but the effect is so weak in our model that it is hard for us to see this basic result changing.”

As the researchers point out, these results are based on a computerised model of phenomena, and it could (and doubtless will) be asserted that it may not have any bearing on reality. In the absence of a model of similar complexity and expertise that supports the solar wind warming theory, however, I think I’m going to accept it as having been laid to rest. YMMV. [via DailyGalaxy]

While we’re talking about complex climate models, though, it looks like some rethinking will be required with respect to the ways in which deep-ocean circulation functions; experiments involving the dispersal of sensor-laden floats have revealed that a ‘conveyor belt’ of cold water flowing southward from the Labrador Sea doesn’t actually form a loop with the Gulf Stream as previously assumed.

I’d lying if I said I totally understood what this means (I’m not an oceanographer, nor do I play one on television), but what’s clear is that scientists aren’t just cherry-picking evidence that suits their models; they’re actively looking to improve the accuracy of their calculations all the time. Who’d have thought, eh? [via SlashDot; image by notsogoodphotography]

[Welcome back, JasperPants. ;)]