Andrew Montford, who runs the Bishop Hill blog has produced a report for the Global Warming Policy Foundation/Forum (GWPF) on precipitation, deluge and flood. It’s the standard kind of message; climate models have failed, we can’t find a trend in some data, cherry-pick a few papers that support this position, argue that we shouldn’t do anything. I can’t really face going into it in much detail. However, if anyone does wish to read the report, it might be worth reading this RealClimate post that discusses one of the papers that Andrew Montford’s report relies on (H/T Gavin Cawley), and having a look at this Met Office webpage that suggests that 1 in 100 precipitation events may have become more frequent.
What really bugs me about these type of reports is that they fail to acknowledge our understanding of the underlying physics/science. There’s a reason why climate scientists are quite confident about the increase in precipitation in a warmer world : we understand the underlying physics quite well. Firstly, there’s a water cycle. Water evaporates from oceans, sea, and lakes, is released by plants and sometimes sublimates from ice and snow. This produces atmospheric water vapour. The water vapour then condenses to form clouds, and eventually precipates as rain or snow. The figure below, from NOAA, illustrates the basic processes.
Something else we understand quite well is the relationship between the water vapour content of the atmosphere and atmospheric temperature. As it warms, the atmosphere can hold more water vapour. This in itself may suggest that we’d expect warming to produce more precipitation. The only way it wouldn’t is if somehow the rate at which it precipitated stayed the same, despite the increase in atmospheric water vapour. The problem with this is that we expect the rate of evaporation to also increase with increasing temperature. If we’re increasing the rate at which we add water vapour to the atmosphere, then we’d expect the rate at which it precipitates to also increase. Therefore, we not only have a good understanding of the water cycle, but also of the relationship between atmospheric water vapour and temperature and the relationship between evaporation rate and temperature. If we manage to warm without precipitation increasing, it will be remarkably surprising.
Of course, you could argue that even if the above is true, it doesn’t necessarily tell us where precipitation will increase. However, our planet has climate zones, and these are largely set by the distribution of Solar insolation and the Coriolis effect. Therefore, we have a good understanding of where evaporation dominates, how the water vapour will be transported in the atmosphere, and where it will most likely precipitate. Unless our warming produces some extremely unexpected changes, we expect precipitation to increase in regions where it is already quite high. Of course, there is a chance that warming could produce completely unexpected outcomes, but assuming that this is likely and that these unexpected outcomes will be beneficial would seem remarkably optimistic (foolish?). If anything, it’s these high-risk, low-probability outcomes that are most concerning.
So, the GWPF commissioning reports from someone who appears not to understand the underlying science is, one might think, a little surprising. Surely they could commission these reports from an actual expert (say, someone from the Met Office) rather than someone who has written a couple of books and mainly concentrates on writing blog posts that appear to either mock mainstream climate scientists or simplistically criticise mainstream climate science. It might make you think that they don’t actually want people to realise just how well we understand the basic science/physics.