July 30, 2015, by Tim Radford
Krymsk in July 2012: The cause of its woe lay at some distance.
Image: Маскитный via Wikimedia Commons
Researchers say devastating 2012 Russian flood occurred only because of global warming elsewhere. LONDON, 30 July, 2015 – Climate scientists, ever reluctant to link any one catastrophic weather event to global warming or climate change, have taken another step nearer the establishment of direct blame. They do not say that global warming triggered a devastating flood that swept through the Russian town of Krymsk and killed more than 150 people. But they do say the flood three years ago happened only because of decades of rising temperatures nearby. If the surface temperatures in the Black Sea had not been rising steadily for the last 30 years, researchers report, such a flood would not have been possible. In one of the first direct connections of this kind, Edmund Meredith of the Helmholtz Centre for Ocean Research in Kiel, Germany and colleagues spell it out. They report in Nature Geoscience that anthropogenic warming in the Black Sea region played “a crucial role” in the extreme and unprecedented flooding that made thousands homeless, turned a town into a sea of mud, and triggered a bitter political row in Russia. The scientists call the Krymsk floods “a showcase example” of the potential consequences of human-fuelled global warming because of rising emissions of the greenhouse gas carbon dioxide in the atmosphere in the last 200 years.
The reasoning goes like this: they looked at regional rainfall levels and found – using computer simulations – that precipitation was 300% higher on average with observed Black Sea surface temperatures, compared to simulations where the warming trend over the past 30 years was removed. That is, the simulations showed that without past surface ocean warming, what meteorologists call deep convection – the heat-driven transport of warm, moist air to the upper atmosphere, active in thunderstorms, which often results in heavy precipitation– does not develop in the region. What happened in July 2012 was that a cyclone moved across the Black Sea and carried moist air over to the foothills of the western Caucasus, followed by a “tongue” of saturated air over Krymsk in the form of a summer thunderstorm. Russian officials reportedly saw the calamity coming but failed to issue a warning: a five-metre wave of water swept through a town of 57,000 people, with catastrophic consequences. The official death toll reached 172. The Kiel researchers do not see the event as a statistical freak, but as an outcome of overall warming of the Black Sea and the Mediterranean, with the knock-on effect not just of more precipitation, but greater extremes of precipitation.
As the great American novelist Mark Twain once observed, climate is what we expect, but weather is what we get. Because extremes are rare events, meteorologists prefer to regard them as variations that in total add up to define an average. But as temperatures rise on average, the intensity and frequency of extremes should, on statistical reasoning, also be on the rise. Scientists have thought increasingly about the possibility of links between random events and background human activity and have warned that drought-stricken California’s vulnerability to drought is likely to increase with climate change. But it is rare for researchers to arrive at a connection as direct as the Nature Geoscience paper that spells out the process by which a global trend drives a local tragedy. And, very cautiously, and in very general terms, the scientists warn that such things could happen again. The climate projections, they say, promise increased sea surface temperatures and more summer cyclones in the Black Sea and Mediterranean regions. “This suggests a corresponding increased risk of intense convective precipitation events,” they conclude. “Other coastal regions with comparable geographical features may, where similar trends are projected, be similarly affected.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.