The northeastern USA has just experienced a fairly intense snowstorm that has seen more than 20 inches of snow fall in Boston. There have been a couple of interesting articles about this. Eric Holthaus points out that this is just another sign of global warming and Nate Silver does an analysis for New York and shows that, although the total amount of snowfall hasn’t changed much, the number of days on which it has snowed has reduced; concluding that this indicates an increase in the intensity of the precipitation when it does happen.
There’s also the standard pushback against these links between extreme events and global warming – in which Roger Pielke Jr appears to insult Nate Silver. One problem I have with most of these pushbacks is that they’re typically associated with rather simplistic interpretations of past data. The increase in the intensity of extreme precipitation events is something that we expect to see in a warmer world and is – in a sense – an aspect of physical climatology that is understood quite well.
In a warmer world, the atmosphere can hold more water vapour. This is essentially the Clausius-Clapeyron relation. Additionally, in a warmer world, we expect the evaporation rate to increase. If the rate at which we add water vapour to the atmosphere increases, then the rate at which it falls out of the atmosphere has to also increase. Additionally, we don’t expect this to increase precipitation uniformly; we expect it to predominantly increase the intensity of extreme events.
So, even if we can’t specifically attribute this snowstorm to anthropogenic global warming (something that I suspect is probably virtually impossible to actually do anyway) the increase in the intensity of extreme events is what we would expect under increased anthropogenic forcings. I even did a Google Scholar search for trends in extreme precipitation and found this paper (Kunkel et al. 2013) which says
There is strong evidence for a nationally averaged upward trend in the frequency and intensity of extreme precipitation events.
I accept that this is (I think) for the whole US, not just the North East, but it is consistent with what we’d expect under increased anthropogenic forcings. Eric Holthaus’s article also links to a resource that says
The Northeast has experienced a greater recent increase in extreme precipitation than any other region in the United States; between 1958 and 2010, the Northeast saw more than a 70% increase in the amount of precipitation falling in very heavy events (defined as the heaviest 1% of all daily events)
Both Eric Holthaus’s and Nate Silver’s articles also point out that there is a temperature sweetspot for snowfall (between -7oC and ooC). Therefore, we could see increased precipitation in a warmer world, but a reduction in snowfall if winter temperatures are less often – in future – in the optimal range for snowfall.
So, the point I’m trying to get at is that an increase in the intensity and, possibly, frequency of extreme precipitation events is something that we expect under increased anthropogenic forcings. The physical processes involved are also well understood, and it would be extremely surprising if the climate didn’t behave in this way. Additionally, there is already evidence for an increase in the frequency and intensity of these extreme events. So, those who want to downplay the link between global warming and these events should probably at least take this into account when doing so; if they want to be taken seriously that is. I guess, in my view, context is important. Others may, of course, disagree.
Update : I had a brief chat with Mauri Pelto on Twitter, which made me realise that I should clarify something. The issue here is not the storm itself, but the precipitation. So, the suggestion is not necessarily that AGW has made the storm more intense (measured by wind speed, for example), but that anthropogenic influences have increased the amount of available water vapour and hence makes intense precipitation events more likely.