Model simulations of many climate phenomena remain highly uncertain despite scientific advances and huge amounts of data. Scientists must do more to tackle model uncertainty head-on.
Model uncertainty is one of the biggest challenges we face in Earth system science, yet comparatively little effort is devoted to fixing it.
A well-known example of persistent model uncertainty is aerosol radiative forcing of climate, for which the uncertainty range has remained essentially unchanged through all Intergovernmental Panel on Climate Change assessment reports since 1995. From the carbon cycle to ice sheets, each community will no doubt have its own examples.
We argue that the huge and successful effort to develop physical understanding of the Earth system needs to be complemented by greater effort to understand and reduce model uncertainty. Without such reductions in uncertainty, the science we do will not, by itself, be sufficient to provide robust information for governments, policy makers, and the public at large.
Model Wiggle Room
As British statistician George Box famously said, “all models are wrong, but some are useful.” Less known is what Box said next: “The scientist cannot obtain a ‘correct’ [model] by excessive elaboration” [Box, 1976].
Most modelers would probably disagree that current model developments are excessive. But the point that Box was making is that models are only representations of reality and are therefore full of uncertain numbers, many of which cannot be defined experimentally. Even elaborate representations based on good process-based understanding are uncertain. In other words, our model simulations have an enormous number of degrees of freedom. We can think of this as wiggle room.
So how much wiggle room do we have in our models, assuming we have decided what the key processes are? In a model with, conservatively, 20 important and uncertain processes, each associated with a single uncertain parameter, the model outputs can be sampled from 20-dimensional space. This is a hypercube with around a million corners.
Kenneth S. Carslaw, Lindsay A. Lee, Leighton A. Regayre and Jill S. Johnson – EOS (American Geophysical Union) – February 26, 2018.
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