Judith Curry has written a report for the Global Warming Policy Foundation called Climate Models for the layman. As you can imagine, the key conclusions is that climate models are not fit for the purpose of justifying political policies to fundamentally alter world social, economic and energy systems. I thought I would comment on the key points.
- GCMs have not been subject to the rigorous verification and validation that is
the norm for engineering and regulatory science.
Well, yes, this is probably true. However, it’s primarily because we only have one planet and haven’t yet invented a time machine. We can’t run additional planetary-scale experiments and we can’t go back in time to collect more data from the past.
- There are valid concerns about a fundamental lack of predictability in the complex
nonlinear climate system.
This appears to relate to the fact that the system is non-linear and, hence, chaotic. Well, that it is chaotic does not mean that it can vary wildly; it’s still largely constrained by energy balance. It will tend towards a state in which the energy coming in, matches the energy going out. This is set by the amount of energy from the Sun, the amount reflected, and the composition of the atmosphere. It doesn’t have to exactly match this state, but given the heat capacity of the various parts of the system, it is largely constrained to remain fairly close to this state. Also, for the kind of changes we might expect in the coming decades, the response is expected to be roughly linear. This doesn’t mean that something unexpected can’t happen, simply that it is unlikely. Also, that some non-linearity might trigger some kind of unexpected, and substantial, change doesn’t somehow reduce the risks.
- There are numerous arguments supporting the conclusion that climate models
are not fit for the purpose of identifying with high confidence the proportion
of the 20th century warming that was human-caused as opposed to natural.
This seems like a strawman argument. There isn’t really a claim that climate models can identify with high confidence the proportion of the 20th century warming that was human-caused as opposed to natural. However, they can be used to estimate attribution, and the conclusion is that it is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together (e.g., here ). Additionally, the best estimate of the human-induced contribution to warming is similar to the observed warming over this period (e.g., here ). One reason for this is that it is very difficult to construct a physically plausible, and consistent, scenario under which more than 50% of the warming is not anthropogenic.
- There is growing evidence that climate models predict too much warming from
increased atmospheric carbon dioxide.
This is mainly based on results from energy balance models. I think these are very interesting calculations, but they don’t rule out – with high confidence – equilibrium climate sensitivity values above 3K, and there are reasons to be somewhat cautious about these energy balance results. There are also indications that we can reconcile these estimates with estimates from climate models.
- The climate model simulation results for the 21st century reported by the Intergovernmental Panel on Climate Change (IPCC) do not include key elements of climate variability, and hence are not useful as projections for how the 21st century climate will actually evolve.
This seems to be complaining that these models can’t predict things like volcanic activity and solar variability. Well, unless we somehow significantly reduce our emissions, the volcanic forcing will probably be small compared to anthropogenic forcings. Also, even if we went into another Grand Solar Minimum, the reduction in solar forcing will probably only compensate for increasing anthropogenic forcings for a decade or so, and this change will not persist. Again, unless we reduce our emissions, these factors will almost certainly be small compared to anthropogenic influences, so this doesn’t seem like a particularly significant issue.
The real problem with this report is not that it’s fundamentally flawed; it’s just simplistic, misrepresents what most scientists who work with these models actually think, and ignores caveats about alternative analyses while amplifying possible problems with climate models. Climate models are not perfect; they can’t model all aspects of the system at all scales, and clearly such a non-linear system could respond to perturbations in unexpected ways. However, this doesn’t mean that they don’t provide relevant information. They’re scientific tools that are mainly used to try and understand how the system will evolve. Noone claims that reality will definitely lie within the range presented by the model results; it’s simply regarded as unlikely that it will fall outside that range. Noone claims that the models couldn’t be improved, it’s just difficult to do so with current resources; both people to develop/update the codes and the required computing resources. They’re also not the only source of information, so noone is suggesting that they should dominate our decision making.
Something to consider is what our understanding would be if we did not have these climate models. Broadly, our understanding would be largely unchanged. We’d be aware that the world would warm as atmospheric CO2 increased, and we’d still have estimates for climate sensitivity that would not be very different to what we have now. We’d be aware that sea levels would rise, and we’d be able to make reasonable estimates for how much. We’d be aware that the hydrological cycle would intensify, and would be able to make estimates for changes in precipitation. It would, probably, mainly be some of the details that would be less clear. If anything, without climate models the argument for mitigation (reducing emissions) would probably be stronger because we’d be somewhat less sure of the consequences of increasing our emissions.
I think it would actually be very good if laypeople had a better understanding of climate models; their strengths, their weaknesses, and the role they play in policy-making. This report, however, does little to help public understanding; well, unless the goal is to confuse public understanding of climate models so as to undermine our ability to make informed decisions. If this is the goal, this report might be quite effective.