I watched, for my sins, a new Global Warming Policy Foundation (GWPF) video called Extreme Weather Events & Global Warming: How Good Is The Evidence? To be fair, it was more balanced than I had expected but really just seemed to be Roger Pielke Jr versus the rest. The basic message seemed to be that there was evidence for more heatwaves and heavier precipitation, but no real evidence for more extreme events like hurricanes, cyclones, tornadoes, and flooding. I will add, though, that the video very clearly mis-represents what Connie Hedegaard (from the European Commission) says. The narrator (David Whitehouse) says
she admits there’s little evidence for an increase in climate extremes, before claiming that there is an increase. Contradicting herself and the IPCC.
No, what Connie Hedegaard says is that it is difficult to establish whether a particular event is actually linked to climate change ……. but the overall pattern of events seems to be bearing out the forecast that climate change will bring more frequent and more extreme weather. This isn’t a contradition. It’s really just the very obvious point that an individual event is not a trend.
Anyway, I’m no expert at this, but a quick search of Google Scholar returned a paper by Holland & Bruyère (2013). This paper seems to develop an Anthropogenic Climate Change Index (ACCI) and then considers how the different categories of hurricanes vary with ACCI. This is shown in the figure below which seems to indicate that although the fraction of Category 1 hurricanes decreases with ACCI, the fraction of Category 4 and Category 5 hurricanes seem to increase. It also appears that Aslak Grinsted finds that there is a trend in extreme hurricane damage. A point that Aslak makes in his post, though, is that it is simply impossible for any normalization procedure to remove all non climatic influences / societal changes that has taken place over the 20th century.
Now, I don’t know enough about this to really know whether or not there is evidence for a trend, associated with global warming, in extreme events like hurricanes and cyclones. I was, however, going to comment on something related though. The GWPF video also included Jennifer Francis who says
I think what we’re seeing happening just in these last few years seems to be very consistent with what the models are projecting for the end of the century.
This may sound a bit wishy-washy but, to a scientist, I think this makes sense and maybe illustrates an issue with the approach that I suspect some take with respect to issues like this. I’m thinking, for example, of those economists who regularly claim to understand statistics much better than typical climate scientists. Essentially, I get the impression that there are some who think that data is paramount. For example, if you want to claim/suggest that there is a rising trend in extreme events, you need to find a statistically significant trend in the data. If not, then you really can’t say anything.
I suspect there are many who think that that is indeed the right approach. It, however, misses – in my opinion – something important. If you have models that suggest that extreme events should increase over the coming decades and that also suggests we should already be able to detect a weak trend (possibly masked by natural variability) then that is extra information. Finding a weak, statistically insignificant, trend when models predict such trends is different to simply finding such a trend in some data without any associated modelling. It doesn’t necessarily prove it beyond any measure of doubt, but does allow scientists to say consistent with what the models are projecting. I would add that analysing data, without having some kind of underlying theory or model, is really largely meaningless.
So, I don’t know enough about extreme events (both in terms of modelling or data) to really know what the evidence suggests or how consistent the models are with the data. It does seem, however, that there are a lot of people who want to treat the models and data as completely independent entities when, in fact, we should be using both to understand what’s happening. Many, of course, are insistent that models need to be validated using data, but seem to completely ignore that you also need models/theories in order to understand the data itself. It’s not a one-way street.