Emergent patterns in nature and society

Floods frequency: New regime shift coming soon

Floods frequency is tricky example of a regime shift. I have not idea yet whether it can be considered one. However, it seems so; and it seems to be driven by deforestation. The more deforested and fragmented a landscape is, the less likely it is to retain water coming from strong rainfall events. Vegetation speed down water drops, and root-rich soils with high porosity retain more humidity. When soils are clean or barely vegetated, one would expect water to run down faster.

On the top of this idea, it seems that climate change and green house gas emissions are playing an important role.  NewScientist recently reports a study by Pal and colleagues titled “Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000“. They comment:

This week, a study has shown that the devastating floods which damaged nearly 10,000 properties in England and Wales in 2000, and cost £1.3 billion in insurance losses, were made significantly more likely by climate change caused by humans.

It is the first study to quantitatively link a severe rainfall event and climate change. The team that carried out the work, led by Myles Allen of the University of Oxford, had earlier linked the 2003 European heatwave to climate change.

The bottom line of all this? Allen and his team found that human greenhouse gas emissions “significantly increased” the likelihood of the 2000 floods. They can say, with a 66 per cent confidence level, that emissions nearly doubled the risk of the 2000 floods.

Conversely, says Allen, there is only a 10 per cent chance that the increase in flood risk rose by just 20 per cent as a result of human contributions to climate.

Here some more comments from NatureNews:

The research directly links rising greenhouse-gas levels with the growing intensity of rain and snow in the Northern Hemisphere, and the increased risk of flooding in the United Kingdom […]

“We can now say with some confidence that the increased rainfall intensity in the latter half of the twentieth century cannot be explained by our estimates of internal climate variability,” she says.

The findings mean that Northern Hemisphere countries need to prepare for more of these events in the future. “What has been considered a 1-in-100-years event in a stationary climate may actually occur twice as often in the future,” says Allen.

“Governments plan to spend some US$100 billion on climate adaptation by 2020, although presently no one has an idea of what is an impact of climate change and what is just bad weather,” says Allen […]  “If rich countries are to financially compensate the losers of climate change, as some poorer countries would expect, you’d like to have an objective scientific basis for it.”

For the interested reader:

Nature 470, 382–385 (17 February 2011) doi:10.1038/nature09762
Nature 470,378–381 (17 February 2011) doi:10.1038/nature09763

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