When the rain comes…

By Gareth Renowden 22/06/2010

The extreme weather flavour of the moment is without doubt heavy rain and flooding. As I write, severe flooding has caused 132 deaths in southern China and 19 in Burma. The Var region in southern France is recovering from spectacular flash flooding that killed 25 people (watch the BBC video), Tennessee’s recovering from a 1000 year flood in May, and NZ hasn’t escaped. The Metservice blog reports that the flooding in Whakatane a few weeks ago was caused by rainfall of 89.8mm in one hour (with more heavy rain either side of that hour). As the blog notes, that’s tropical rain happening well outside the tropics. But what struck me at the time was a comment from a Whakatane resident included in the TV3 News coverage of that flooding. I can’t remember his precise words, but it was something along the lines of “should be a wake-up call for anyone who doesn’t think global warming’s an issue, because this is what global warming delivers…” Perceptive, I thought, because one of the more robust predictions of climate science is sometimes described as an intensification of the hydrological cycle.

It works like this. Increasing CO2 in the atmosphere warms the planet. As the oceans warm up, more water vapour enters the atmosphere, and because it is itself a heat-trapping gas this adds to the warming. This positive feedback is important because it increases the amount of warming triggered by the CO2, but it’s also important because of impacts of the increase in water vapour itself. The increase has been measured: there’s about 4% more water vapour in the atmosphere now than there was 30 years ago, and I suspect that we’re now seeing the effects of that on our day to day weather.

Water vapour is sometimes described by meteorologists as the “fuel” that drives storms. As water evaporates from a warm ocean, it cools the surface and transfers energy into the atmosphere. As the water vapour condenses into clouds and rain, that energy is released, intensifying the storm. More water vapour, stronger storms, heavier rainfall.

4% extra water vapour doesn’t sound like a lot, does it? But it’s enough to change the probabilities of heavy rainfall events in two ways. Firstly, the frequency of heavy rainfall events will increase, and secondly the amount of rain that falls in the heaviest events will increase. Take a look at this graph (from NASA’s Earth Observatory feature on the costs of climate change):


The graph considers temperature extremes, but the same principle applies to rainfall (except that the probability distribution is pegged at zero — no rain). The top curve shows what happens if the climate warms but the variance — the size of the swings between warm and cold — remain the same. The middle curve shows a stationary climate (ie, not warming or cooling) but with a variance increase — more dramatic swings between hot and cold, but no new records. The effect is to squash the curve and create more warm and cold events. Combine the two, and you get increases in both the hot weather and in record heat. This is already being seen with heatwaves in Australia, for instance, and I suspect we’re now seeing the same effect happening with rainfall.

Joe Romm at Climate Progress has been diligently pursuing the issue of weather extremes as a symptom of climate change, and last week interviewed NZ scientist Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado, on the subject. It’s worth reading the full interview, and following the references, but here’s Trenberth making an important point. Romm asks about the best way to describe the increasing extremes:

I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is ’Well you can’t attribute a single event to climate change.’ But there is a systematic influence on all of these weather events nowadays because of the fact that there is this extra water vapour lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.

Earlier in Romm’s interview Trenberth had drawn attention to research that demonstrated that in the US, “the really heavy rainfall events – the top 1% and the top 0.3% – had gone up at even more alarming levels something like 27% as I recall over the last 30 or 40 years [actually 1967 to 2006].” Ring any bells? Jeff Masters comments and amplifies the point here. Then try a Google News search for the term “flood“, and see what pops up.

From my personal perspective, sitting in a farmhouse in North Canterbury on the east coast of the South Island of New Zealand, deep down in the South Pacific ocean, I think we’re seeing this effect quite clearly. I haven’t crunched the numbers for New Zealand, but I think they might show the same thing as the research Trenberth quotes. I’ve pondered the subject before, when considering the evidence of my rain gauge in 2008 and the climatic swings of 2009, but I can’t help but feel that as we head into the second half of a year that might set another global temperature record, we’re already seeing the concomitant impacts of increased water vapour in the atmosphere in the dramatic flood events happening around the world.

The lesson: climate change is not an abstract thing, a problem reserved for the future. It’s not only a slow but steady increase in long term averages, it’s also a change in the probability of extreme events, which are themselves becoming more extreme. The impact of climate change will be delivered by changes in the weather that we experience — and those changes are already happening. Worse, when the weather’s bad, it can and will be very bad indeed.

[Update 23/6: The Guardian reports on a new paper which suggests intensification of the hydrological cycle will persist, due to the thermal inertia of the global ocean, for a considerable period after CO2 levels have stabilised and been reduced. And Nature News discusses the terrifying power of flash flooding…]