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Friday, June 29, 2007

Trenberth: CGMs cannot predict climate

Some startling comments about the computer models that are "predicting" all the doom and gloom about man-made global warming. Startling considering the source. Via Professor Bob Carter at news.com.au:

KEVIN Trenberth is head of the large US National Centre for Atmospheric Research and one of the advisory high priests of the UN's Intergovernmental Panel on Climate Change (IPCC).

A New Zealander by birth, Trenberth has had a distinguished career as a climate scientist with interests in the use of computer General Circulation Models (GCMs), the basis for most of the public alarm about dangerous global warming.

When such a person gives an opinion about the scientific value of GCMs as predictive tools, it is obviously wise to pay attention.

In a remarkable contribution to Nature magazine's Climate Feedback blog, Trenberth concedes GCMs cannot predict future climate and claims the IPCC is not in the business of climate prediction.
Here's the full text of what Trenberth had to say [with emphasis added]:
I have often seen references to predictions of future climate by the Intergovernmental Panel on Climate Change (IPCC), presumably through the IPCC assessments (the various chapters in the recently completedWorking Group I Fourth Assessment report ican be accessed through this listing). In fact, since the last report it is also often stated that the science is settled or done and now is the time for action.

In fact there are no predictions by IPCC at all. And there never have been. The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. There are a number of assumptions that go into these emissions scenarios. They are intended to cover a range of possible self consistent “story lines” that then provide decision makers with information about which paths might be more desirable. But they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents. There is no estimate, even probabilistically, as to the likelihood of any emissions scenario and no best guess.

Even if there were, the projections are based on model results that provide differences of the future climate relative to that today. None of the models used by IPCC are initialized to the observed state and none of the climate states in the models correspond even remotely to the current observed climate. In particular, the state of the oceans, sea ice, and soil moisture has no relationship to the observed state at any recent time in any of the IPCC models. There is neither an El Niño sequence nor any Pacific Decadal Oscillation that replicates the recent past; yet these are critical modes of variability that affect Pacific rim countries and beyond. The Atlantic Multidecadal Oscillation, that may depend on the thermohaline circulation and thus ocean currents in the Atlantic, is not set up to match today’s state, but it is a critical component of the Atlantic hurricanes and it undoubtedly affects forecasts for the next decade from Brazil to Europe. Moreover, the starting climate state in several of the models may depart significantly from the real climate owing to model errors. I postulate that regional climate change is impossible to deal with properly unless the models are initialized.

The current projection method works to the extent it does because it utilizes differences from one time to another and the main model bias and systematic errors are thereby subtracted out. This assumes linearity. It works for global forced variations, but it can not work for many aspects of climate, especially those related to the water cycle. For instance, if the current state is one of drought then it is unlikely to get drier, but unrealistic model states and model biases can easily violate such constraints and project drier conditions. Of course one can initialize a climate model, but a biased model will immediately drift back to the model climate and the predicted trends will then be wrong. Therefore the problem of overcoming this shortcoming, and facing up to initializing climate models means not only obtaining sufficient reliable observations of all aspects of the climate system, but also overcoming model biases. So this is a major challenge.

The IPCC report makes it clear that there is a substantial future commitment to further climate change even if we could stabilize atmospheric concentrations of greenhouse gases. And the commitment is even greater given that the best we can realistically hope for in the near term is to perhaps stabilize emissions, which means increases in concentrations of long-lived greenhouse gases indefinitely into the future. Thus future climate change is guaranteed.

So if the science is settled, then what are we planning for and adapting to? A consensus has emerged that “warming of the climate system is unequivocal” to quote the 2007 IPCC Fourth Assessment Working Group I Summary for Policy Makers (pdf) and the science is convincing that humans are the cause. Hence mitigation of the problem: stopping or slowing greenhouse gas emissions into the atmosphere is essential. The science is clear in this respect.

However, the science is not done because we do not have reliable or regional predictions of climate. But we need them. Indeed it is an imperative! So the science is just beginning. Beginning, that is, to face up to the challenge of building a climate information system that tracks the current climate and the agents of change, that initializes models and makes predictions, and that provides useful climate information on many time scales regionally and tailored to many sectoral needs.

We will adapt to climate change. The question is whether it will be planned or not? How disruptive and how much loss of life will there be because we did not adequately plan for the climate changes that are already occurring?

Kevin Trenberth
Climate Analysis Section, NCAR
As Prof. Carter says, it would be wise to pay attention to what this man says and:
Trenberth's statements are a direct admission of the validity of similar criticisms that have been made of GCMs and the IPCC by climate rationalists for many years.

[...]

These various criticisms of climate modelling can be summed up in the following statement – there is no predictive value in the current generation of computer GCMs and therefore the alarmist IPCC statements about human-caused global warming are unjustified.
h/t: moonbattery

3 comments:

JR said...

Interesting! Trenberth’s frank statements are certainly welcome and confirm what many of the much abused “deniers” have been saying all along.

But I found some of his remarks, e.g. “In fact there are no predictions by IPCC at all. And there never have been” a bit puzzling. Though he seems to be drawing some esoteric distinction between “prediction” and “projection” it isn’t clear, to me, what he means. A projection with a probability attached to it’s occurrence certainly comes across as a prediction. The Fourth AR Summary for Policymakers is chock-a-block with statements using the language of probability including “likelihood”, “virtually certain”, “likely”. There’s a couple of charts at the end that give relative “probabilities” for different future scenarios. The AGW true believers, most disappointingly even our current Tory government, treat the IPCC’s alarmist pronouncements as reliable predictions - else why would they all be saying "the debate is over" and calling for immediate and drastic action?

Otherwise, good on Prof. Trenberth!

John Nicklin said...

In fact there are no predictions by IPCC at all. And there never have been.

If you believe that statement, you might be interested in some waterfront property I have in Nunavut.

jr is quite correct, the differnce between prediction and projection seems to be lost on Trenberth.

I'm affraid that the help this might provide skeptics will be small as the alarmist side will surely see it as yet another consensus, proving that IPCC is right.

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