Tuesday, February 2, 2010

More knowledge, less certainty

Kevin Trenberth in Nature Reports comments on the next iteration of the IPCC’s work: The climate scientists that comprise the Intergovernmental Panel on Climate Change (IPCC) don't do predictions, or at least they haven't up until now. Instead the scientists of the IPCC have, in the past, made projections of how the future climate could change for a range of 'what-if' emissions scenarios. But for its fifth assessment report, known as AR5 and due out in 2013, the UN panel plans to examine explicit predictions of climate change over the coming decades. In AR5's Working Group I report, which focuses on the physical science of climate change, one chapter will be devoted to assessing the skill of climate predictions for timescales out to about 30 years. These climate forecasts, which should help guide decision-makers on how to plan for and adapt to change, will no doubt receive much attention.

Another chapter will deal with longer-term projections, to 2100 and beyond, using a suite of global models. Many of these models will attempt new and better representations of important climate processes and their feedbacks — in other words, those mechanisms that can amplify or diminish the overall effect of increased incoming radiation. Including these elements will make the models into more realistic simulations of the climate system, but it will also introduce uncertainties.

So here is my prediction: the uncertainty in AR5's climate predictions and projections will be much greater than in previous IPCC reports, primarily because of the factors noted above. This could present a major problem for public understanding of climate change. Is it not a reasonable expectation that as knowledge and understanding increase over time, uncertainty should decrease? But while our knowledge of certain factors does increase, so does our understanding of factors we previously did not account for or even recognize….

...It has been said that all models are wrong but some are useful. A climate model is a tool, albeit a very sophisticated one that includes complexity and nonlinearities in ways that are impossible to comprehend analytically. Ideally, a model should encapsulate the state of our knowledge. When that knowledge is incomplete, one strategy is to omit certain complex processes and to assume that they are constant, even when it is known that they cannot be. Adding complexity to a modelled system when the real system is complex is no doubt essential for model development. It may, however, run the risk of turning a useful model into a research tool that is not yet skilful at making predictions....

Model of the late 17th / early 18th century frigate Das Wappen von Bremen (“the Coat of Arms of Bremen”), a convoy ship of the city of Bremen. The model belongs to the yachting club “Das Wappen von Bremen” founded in 1934 and was manufactured by a former member of the club. It is probably not an exact representation of the ship. Shot by Till F. Teenck, Wikimedia Commons, under the Creative Commons Attribution 3.0 Unported license

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