Yet another in my ongoing posts about our inability to accurately predict the impact of emissions. What’s interesting about this one is that it is a blog post by one of the CLAs (basically, big-shot authors of a full chapter) in the latest IPCC report, Kevin Trenberth.
Here are key excerpts of what he has to say about the current state of model-based temperature forecasts [my comments inserted in italicized brackets]:
[This is a critical, if nerdy, issue. Essentially, if climate prediction (or as the author says here a specific part of climate prediction, namely prediction of climate at the regional level) is an Initial Value Problem rather than a Boundary Value Problem, it means that errors in measuring the initial state today will propagate into future predictions, and therefore the distance into the future that we can accurately predict climate change will be dictated by how accurately we can measure the current physical variables that drive climate change. In practice, if climate prediction is a true Initial Value Problem, we’ll probably not be able to make meaningful multi-decadal climate change predictions within the lifetime of anybody reading this.]
…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. … 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….
[The key word here is “assumes”. In summary, he is saying that many complex feedback effects can be approximated with simplifying assumptions. With “enough” GHG emissions this is probably correct. The trick, of course, is knowing a priori what is “enough”.]
…the science is not done because we do not have reliable or regional predictions of climate. But we need them. (bold added)
[We sure do.]