Wednesday, August 04, 2010

Pakistan floods: anything to do with global warming?

The present tragic and politically dangerous flooding in Pakistan raises the question - Is it due to global warming?

No one single weather event can be taken as evidence of global warming. We have to look at the whole picture (something that global warming sceptics seem to find particularly difficult).  However, global warming theory predicts that floods and droughts will increase in frequency as the planet heats up.

The data on flooding is difficult to interpret, since they have to define what is meant by floods, and other factors, such as population numbers, land use changes (e.g. building on an area will reduce the ability of the ground to absorb water), flood defence measures, and ability to report events come in to play.

Researchers are conscious of these factors, and try to estimate or allow for them.

Having said that, here are a few findings:

The costs due to extreme weather events have certainly shot up. This figure covers 1950-99:
Source IPCC, who say: Part of the observed upward trend in historical disaster losses is linked to socioeconomic factors—such as population growth, increased wealth, and urbanization in vulnerable areas—and part is linked to climatic factors such as observed changes in precipitation, flooding, and drought events. Precise attribution is complex ...

The UK's Centre for Ecology and Hydrology says: "The latest research from the UK Climate Projections team (UKCP09) indicates that winter rainfall has increased in the north-west of the UK since the early 1960s.   Other research suggests that extreme rainfalls have also increased in frequency in northern and western areas of the UK, and one recent study which involved CEH scientists found that, over the last 50 years, the daily maximum rainfall has increased by 25% in northern and western areas relative to the previous 50 years. "
In Scotland: "It has been shown that river flows in the Clyde Catchment have increased at nearly twice the rate of increase in rainfall in the past 30 years." [.pdf]

Osborne and Hulme 2002: Over 100 weather stations with daily precipitation totals for the last four decades provide a record of precipitation characteristics across most of the UK. They indicate increased precipitation totals and increased frequency and contribution of heavy precipitation events during winter, and decreases in these characteristics during summer. These trends are consistent with changes in the full precipitation probability distributions and are spatially coherent across most of the UK. In relation to the entire 20th century, there is an indication that recent winter increases in heavy precipitation are unusual, while recent summer decreases may not be.[ ]..The sign of the change is consistent with the simulated climate-change signal due to increasing greenhouse gas concentrations, though the magnitude of the observed change is greater than that expected from the model simulations.

USA figures suggest an upward trend in large flood events:


Interestingly, deaths from flooding in the USA are falling over the same time frame, perhaps due to better flood warnings.

In Asia: "Overall, results showed an increasing likelihood of extreme floods [in the Mekong] during the last half of the century, although the probability of an average flood decreased
20 during the same period
." Source

A few straws in the wind, gathered from a quick Google search. Professional hydrologists will make cautious noises, but I, having no professional reputation to protect, would dare to say that events in Pakistan, and other flooding events brought to our attention by the news, are part of a global trend consistent with the predictions of global warming theory.

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