Researchers : No evidence climate change played a role in the 2011 floods in Thailand

AP reports:

Last year brought a record heat wave to Texas, massive floods in Bangkok and an unusually warm November in England. How much has global warming boosted the chances of events like that?

Quite a lot in Texas and England, but apparently not at all in Bangkok, say new analyses released Tuesday.

Scientists can’t blame any single weather event on global warming, but they can assess how climate change has altered the odds of such events happening, Tom Peterson of the National Oceanic and Atmospheric Administration told reporters in a briefing. He’s an editor of a report that includes the analyses published by the Bulletin of the American Meteorological Society.

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A third analysis considered unusually severe river flooding last year in central and southern Thailand, including neighborhoods in Bangkok. It found no sign that climate change played a role in that event, noting that the amount of rainfall was not very unusual. The scale of the flooding was influenced more by factors like reservoir operation policies, researchers wrote.

The report is entitled “THE ABSENCE OF A ROLE OF CLIMATE CHANGE IN THE 2011 THAILAND FLOODS” and is available from here (PDF).

The introduction:

Thailand experienced severe flooding in 2011. During and after an unusually wet monsoon(July–September) in northern Thailand, rivers on the flood plains in the center and the south flooded their banks and inundated large parts of the country…

Flooding events are not uncommon in Thailand. However, the scale of the 2011 event was unprecedented. In this article we perform a first analysis of the meteorological component of the flood: how unusual was the rainfall in the catchment of the Chao Phraya river in northwestern Thailand,and are future monsoon rainfall trends expected due to climate change? It should be emphasized, however, that nonmeteorological factors were much more important in setting the scale of the disaster. Examples are the changing hydrography of the river (the levels of the Chao Phraya were in some places more than 0.5 m higher than in 1995 for even a slightly lower discharge), conversion of agricultural land to much more vulnerable industrial usage, and reservoir operation policies.

The trend of the time series in Fig. 4 is not significantly different from zero: the mean precipitation has not changed beyond the natural variability. The 20-yr running mean and standard deviation also do not show significant variations…

Fig. 4. (a) July–September precipitation (mm) in the upper catchment of the Chao Phraya river that flooded in 2011. The rainfall has been approximated by the 10 grid boxes in 15°–20°N, 99°–101°E in the GPCC V5 1° analysis 1915–2009, extended with the GPCC monitoring analysis linearly adjusted to agree on the overlap period. The red line denotes a 10-yr running mean. (b) Scatterplot of this precipitation against the HadISST1 Niño-3.4 index. The least squares regression line has been drawn red.
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They do show an increase of 10%–20% in both mean and standard deviation by 2100, indicating that the frequency of very active monsoons is projected to increase in the future by these models. We again stress that the credibility of any projected change depends on the simulation of climatology of the Asian monsoon, which is as yet untested in this ensemble and has been shown to be highly variable across models (Kim et al. 2008).

Conclusions. Although the damage caused by the 2011 floods on the Chao Phraya river in Thailand was unprecedented, the available data show that the amount of rain that fell in the catchment area was not very unusual. Other factors such as changes in the hydrography and increased vulnerability were therefore more important in setting the scale of the disaster. Neither in the precipitation observations nor in climate models is there a trend in mean or variability up to now, so climate change cannot be shown to have played any role in this event. Current models do project increases in both mean and variability in the future that would increase the probability of extremes. It may be advisable to take this into account when addressing current vulnerabilities.

BP: The report makes the situation about rainfall clearer than the AP article. The report states that the monsoon season was “unusually wet”, but not ”very unusual” to suggest that the floods were a result of  climate change. Rain is obviously relevant for floods as is changing hydrography, conversion of agricultural land for industrial use, and dam management policy. As the figures show – and they have even longer records than is available on the Metrological Department’s Web site – 2011 had a record rainfall. In fact, it was the most rain ever recorded dating back to 1915. The other years which had record rainfalls were 1942 and 1995, which were both years of great floods.

The researchers tentatively note though that the frequency of active monsoons will increase in the future. If so, this will increase the likelihood of more severe floods in the future. However, for 2012, BP still views that the analysis written the other day still holds. There has not been unusually high rainfall so far this year, the prediction is for not unusually high rainfall, and until there is any sign of such of such, the likelihood of a severe flood in 2012 is very, very low.

h/t to Krib for both the AP link and the link to the report