by Judith Curry

[W]e presently find little evidence of trustworthy predictions at fine spatial scale and annual to decadal timescale from climate models. – Hargreaves and Annan

A new article in WIREs Climate Change:

Can we trust climate models?

JC Hargreaves and JD Annan

Abstract. What are the predictions of climate models, should we believe them, and are they falsifiable? Probably the most iconic and influential result arising from climate models is the prediction that, dependent on the rate of increase of CO2 emissions, global and annual mean temperature will rise by around 2–4∘C over the 21st century. We argue that this result is indeed credible, as are the supplementary predictions that the land will on average warm by around 50% more than the oceans, high latitudes more than the tropics, and that the hydrological cycle will generally intensify. Beyond these and similar broad statements, however, we presently find little evidence of trustworthy predictions at fine spatial scale and annual to decadal timescale from climate models.

WIREs Clim Change 2014. doi: 10.1002/wcc.288 Available online [here].

In the first two pages, they pose the following questions:

What are the predictions of climate models, should we believe them, and are they falsifiable?

Are the models sufficiently wrong that we should anticipate reality falling outside the range of model results?

On the agreement among climate models:

It cannot be argued on a rigorous basis that climate model agreement necessarily implies correctness, largely because of the ad hoc origins of the ensemble members, and unclear characterization of their inter-relationships. Models have shared code and ideas according to their origins,10 and therefore rather than considering them as independent sources of evidence concerning the climate system, it may be more realistic (albeit still perhaps optimistic) to interpret the ensemble collectively as representing (at least approximately) our range of beliefs and uncertainties regarding the behavior of the climate system.

On the falsifiability of climate models:

Nevertheless, one could argue that some combination of social pressure, and convenience, results in models sharing too much theory and even code, to the extent that they are little more than replicates. The validity of these competing arguments can hardly be decided on the basis of rhetoric, but there is yet little progress on how they can be assessed by analysis of the ensemble or other methods. Thus we consider this to be a particularly important area for future research. If it is the case that the pressure to conform in climate science has led to a serious disruption of the scientific process, then attention should indeed be focused toward building alternative models based on fundamentally contrasting physical hypotheses that perform equally well or better for the modern and past climates. However, proponents of such an exercise should note that while encouraging diversity in model design seems a laudable goal, alternative ideas cannot easily be conjured up from nothing, but are instead typically provoked by failures of the existing paradigm

Despite understanding the basic processes underlying the physics of the climate system, it is clear that the state-of-the-art climate models are not ‘good enough’, if we desire high resolution predictions with high temporal and spatial resolutions over coming decades. Thus far we seem to have only built sufficient confidence in the broad scale response of temperature and precipitation. The large-scale understanding of the physics seems to be sufficient, but the details are either not well understood, or are not being sufficiently well approximated by the model code. Given the spread of model results at the local scale, the issue is not so much one of falsification, but rather that current models do not provide much of a guide as to future climate change. Research to address this deficit in the models is required in order for the models to become truly trustworthy, but it is not clear when, if ever, this will be achieved.

JC comments

For background, a number of posts at Climate Etc on climate models are found [here]; my 2010 post What can we learn from climate models? provides the best introduction to my take on this issue.

Hargreaves and Annan’s article is an Opinion piece, not a scientific article; as such it is a relatively superficial treatment of the subject. In any event, I’m pleased to see their article published. The main thing that I disagree with is the statement in their abstract:

Probably the most iconic and influential result arising from climate models is the prediction that, dependent on the rate of increase of CO2 emissions, global and annual mean temperature will rise by around 2–4∘C over the 21st century. We argue that this result is indeed credible, as are the supplementary predictions that the land will on average warm by around 50% more than the oceans, high latitudes more than the tropics, and that the hydrological cycle will generally intensify.

They do not justify this statement in the main text; well since this is an opinion piece, I guess they can just state their opinion without justification. I guess it’s ok to be highly critical climate models, as long as you believe the 21st century prediction and the 20th century attribution.

I would like to see some serious thinking and discussion about assessment of fitness for purpose of climate models, on whatever timescales. Hargreaves and Annan say 100 years for ‘falsification’, and then dismiss the idea since we can’t wait in terms of decision making.

And finally, someone needs to get serious and discuss alternative climate model structural forms – not just adding more chemistry to the models, but rethinking the structural form of the dynamical core. Otherwise, we are spinning some very expensive wheels and potentially misleading decision makers.