Thunderstorms in the IPCC AR5

It’s been a while since I blogged; I hope you didn’t think I’d forgotten you! My workload has “shifted” recently and I’m doing a bit more teaching/supervision/management these days. Blogging has taken a bit of a backseat. So I’m a bit late on this one but thought that it was still interesting. Anyway, enough of the excuses…

I’ve often thought it was odd that the potential changes in frequency and/or intensity of small scale severe storms/thunderstorms – one of my areas of research – was absent from the IPCC TAR, AR4 and SREX.

This has been put right in the IPCC AR5, which was published in late 2013 but, if anything, it highlights some of the problems with the slow and rigidly structured IPCC process.

So here’re a few sentences from IPCC AR5 that deal with severe thunderstorms:

The large-scale environments in which [severe thunderstorms] occur are characterized by large Convective Available Potential Energy (CAPE) and deep tropospheric wind shear (Brooks et al., 2003; Brooks, 2009). Del Genio et al. (2007), Trapp et al. (2007; 2009), and Van Klooster and Roebber (2009) found a general increase in the energy and decrease in the shear terms from the late 20th century to the late 21st century over the United States using a variety of regional model simulations embedded in global-model SRES scenario simulations. The relative change between these two competing factors would tend to favour more environments that would support severe thunderstorms, providing storms are initiated.

Overall, for all parts of the world studied, the results are suggestive of a trend toward environments favouring more severe thunderstorms, but the small number of analyses precludes any likelihood estimate of this change.

It’s a pretty good, concise summary of work in this area up to 2012/13. (I’ve not included some of the text on examples and the few studies outside of the US, you can find the full text here towards the end of section 12.4.5.5 Extreme Events in the Water Cycle. There’s another bit in 2.6.2.4 Severe Local Weather Events as well.)

However, whilst the IPCC report was being published, this paper came out:

Diffenbaugh, N. S., Scherer, M. and Trapp, R. J. (in press) “Robust increases in severe thunderstorm environments in response to greenhouse forcing” PNAS, doi: 10.1073/pnas.1307758110

They say:

We use an ensemble of global climate model experiments to probe the severe thunderstorm response. We find that this ensemble exhibits robust increases in the occurrence of severe thunderstorm environments over the eastern United States. In addition, the simulated changes in the atmospheric environment indicate an increase in the number of days supportive of the spectrum of convective hazards, with the suggestion of a possible increase in the number of days supportive of tornadic storms.

It’s a much more up-to-date and robust analysis of the problem and even uses the CMIP5 climate projections that form the backbone of the IPCC AR5. (I’ve been working on something similar for the Northern Hemisphere but not quite finished it yet!) I guess that this paper must have been accepted for publication after the deadline for the IPCC process so it isn’t mentioned. It’s a shame as a citation to this paper would have added something to the argument.

And this seems to be a problem with the IPCC. Climate science research is a much bigger area now than when the IPCC process started in the late 1980s/early 1990s. So a whole area of research (e.g. severe thunderstorms in a changing climate) becomes a couple of sentences with the most up-to-date paper missing.

As good as the IPCC has been over the years, perhaps it’s time to move on. The SREX example seems to be a good one: a multi-disciplinary, timely analysis of an important area. I think that a series of special reports like SREX would be a better use of valuable time than an AR6.

Tags: Climate change, IPCC, storms