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Flooded house in Brisbane, Australia.

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RealClimate

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Dr. Dick van der Wateren

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Climate Change, Climate Modelling, Floods, Weather


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Climate change and extreme weather. Linked?

19.02.2011, Age: 2860 days

Two new papers in this week's Nature investigate how much human-induced greenhouse gas increase contributes to precipitation extremes and increasing flood risks.

Lately, the news has been full of extreme weather events - rivers flooding Australian cities, devastating landslides after heavy rains in Rio de Janeiro's favelas, exceptional snow storms in the US - begging the question whether global warming may be behind these disasters. To many in the popular press the answer to this question is a clear yes. Yet, however likely the relationship may be, proving a causal link is far from straightforward.

In his RealClimate blog Gavin Schmidt, commenting on the two Nature articles, states that the issue is surrounded by a lot of confusion:

"Not all extremes are the same. Discussions of ‘changes in extremes' in general without specifying exactly what is being discussed are meaningless. A tornado is an extreme event, but one whose causes, sensitivity to change and impacts have nothing to do with those related to an ice storm, or a heat wave or cold air outbreak or a drought.
There is no theory or result that indicates that climate change increases extremes in general. [...]
Some extremes will
become more common in future (and some less so). [...]
Attribution of extremes is hard. There are limited observational data to start with, insufficient testing of climate model simulations of extremes, and (so far) limited assessment of model projections."

The two articles clearly show how much work is needed to test causal relationships between global warming and weather extremes. It is also clear that today's climate models are not yet able to answer this question.

Seung-Ki Min, Xuebin Zhang, Francis W. Zwiers1 & Gabriele C. Hegerl. Human contribution to more-intense precipitation extremes. Nature, Volume 470, Pages: 378-381. DOI: doi:10.1038/nature09763. Received 15 March 2010. Accepted 17 December 2010. Published online 16 February 2011. Link to article.

Abstract

Extremes of weather and climate can have devastating effects on human society and the environment. Understanding past changes in the characteristics of such events, including recent increases in the intensity of heavy precipitation events over a large part of the Northern Hemisphere land area, is critical for reliable projections of future changes. Given that atmospheric water-holding capacity is expected to increase roughly exponentially with temperature-and that atmospheric water content is increasing in accord with this theoretical expectation it has been suggested that human-influenced global warming may be partly responsible for increases in heavy precipitation. Because of the limited availability of daily observations, however, most previous studies have examined only the potential detectability of changes in extreme precipitation through model-model comparisons. Here we show that human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land areas. These results are based on a comparison of observed and multi-model simulated changes in extreme precipitation over the latter half of the twentieth century analysed with an optimal fingerprinting technique. Changes in extreme precipitation projected by models, and thus the impacts of future changes in extreme precipitation, may be underestimated because models seem to underestimate the observed increase in heavy precipitation with warming.

Pardeep Pall, Tolu Aina, Dáithí A. Stone, Peter A. Stott, Toru Nozawa, Arno G. J. Hilberts, Dag Lohmann & Myles R. Allen. Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000. Nature, Volume 470, Pages: 382-385. Date published: (17 February 2011). DOI: doi:10.1038/nature09762. Received 30 March 2010. Accepted 10 December 2010. Published online 16 February 2011. Link to article.

Abstract

Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion. Although the flooding was deemed a ‘wake-up call' to the impacts of climate change at the time, such claims are typically supported only by general thermodynamic arguments that suggest increased extreme precipitation under global warming, but fail to account fully for the complex hydrometeorology associated with flooding. Here we present a multi-step, physically based ‘probabilistic event attribution' framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000. Using publicly volunteered distributed computing, we generate several thousand seasonal-forecast-resolution climate model simulations of autumn 2000 weather, both under realistic conditions, and under conditions as they might have been had these greenhouse gas emissions and the resulting large-scale warming never occurred. Results are fed into a precipitation-runoff model that is used to simulate severe daily river runoff events in England and Wales (proxy indicators of flood events). The precise magnitude of the anthropogenic contribution remains uncertain, but in nine out of ten cases our model results indicate that twentieth-century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales in autumn 2000 by more than 20%, and in two out of three cases by more than 90%.


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