Readers following recent threads know that I’ve been looking for instances where scientists make claims that some observations are “inconsistent with” the results from climate models. The reason for such a search is that it is all too easy for modelers to claim that anything and everything under the sun is “consistent with” their predictions, sometimes to avoid the perception of a loss of credibility in the political battle over climate change.

I am happy to report that claims of “inconsistent with” do exist. Here is an example from a paper just out by Knutson et al. in Nature Geoscience:

Our results using the ensemble-mean global model projections (Fig. 4) are inconsistent with the notion of large, upward trends in tropical storm and hurricane frequency over the twentieth century, driven by greenhouse warming.

The climate modelers at Real Climate apparently don’t like the phrase “inconsistent with” in the context of models and try to air brush it away when they write of Knutson et al.:

. . .we know that (i) the warming [of the oceans] is likely in large part anthropogenic, and (ii) that the recent increases in TC frequency are related to that warming. It hardly seems a leap of faith to put two-and-two together and conclude that there is likely a relationship between anthropogenic warming and increased Atlantic TC activity.

Knutson et al. respond in the comments that this in fact is not how to interpret their paper, and — kudos to them — take strong, public issue with the weaselly words implying a connection that they don’t show (emphasis added in the below, and I’ve copied the whole comment for the entire context):

Mike [Mann],

Statement (i), that “the warming [of the tropical Atlantic Ocean] is likely in large part anthropogenic.” is reasonable, taking “anthropogenic” to mean “greenhouse gas”, given the work of Santer et al (2006, PNAS), Knutson et al (2006, J. Clim.), and Gillett et al (2008, G.R.L.). To quote from Gillett et al:

…our results indicate that greenhouse gas increases are indeed likely the dominant cause of [tropical Atlantic] warming…

However, statement (ii), that “the recent increases in [Atlantic] TC (tropical cyclone) frequency are related to that warming” is vague – with “related to” allowing an interpretation that includes anything from a negative relationship, to a minor contribution, to local SST warming being the dominant dynamical control on TC frequency increase. Some might interpret “related to” to mean “are dominantly controlled by”, and we think the evidence does not justify such a strong statement. In particular, the results of Knutson et al (2008) do not support such an attribution statement,if one focuses on the greenhouse gas part of the anthropogenic signal. Quoting from page 5 of the paper:

Our results using the ensemble-mean global model projections (Fig. 4) are inconsistent [emphasis added] with the notion of large, upward trends in tropical storm and hurricane frequency over the twentieth century, driven by greenhouse warming

We agree that TC activity and local Atlantic SSTs are correlated but do not view this correlation as implying causation. The alternative, consistent with our results, is that there is a causal nonlocal relationship between Atlantic TC activity and the tropical SST field. The simplest version uses the difference between Atlantic and Tropical-mean SST changes as the predictor (Swanson 2008, Non-locality of Atlantic tropical cyclone intensities, G-cubed, 9, Q04V01). This picture is also consistent with non-local control on wind shear (e.g. Latif et al 2007, G.RL.), atmospheric stability (e.g., Shen et al 2000, J. Clim.) and maximum potential intensity (e.g., Vecchi and Soden, 2007, Nature).

We view the SST change in the tropical Atlantic relative to the rest of the tropics as the key to these questions. Warming in recent decades has been particularly prominent in the northern tropical Atlantic, but such a pattern is not evident in the consensus of simulations of the response to increasing greenhouse gases. So, whether changes in Atlantic SST relative to the rest of the tropics – that according to our hypothesis have resulted in the changes in hurricane activity – were primarily caused by changes in radiative forcing, or whether they were primarily caused by internal climate variability, or (most likely) whether both were involved, is obviously an important issue, but this is not addressed by our paper

Now a word of caution — Knutson et al. 2008 is by no means the last word on hurricanes and global warming, and the issue remains highly contested, and will remain so for a long time. Of course, you heard that (accurate) assessment of the state of this particular area of climate science here a long time ago (PDF

Knutson et al. is notable because it clearly identifies observations “inconsistent with” what the models report which should give us greater confidence in research focused on generating climate predictions. We should have greater confidence because if practically everything observed is claimed to be “consistent with” model predictions, then climate models are pretty useless tools for decision making.