Thailand's flood gradually subsiding; climate change increasing Thai flood risk

Unprecedented flood waters continue to besiege Thailand and its capital city of Bangkok this November. Heavy monsoon and tropical cyclone rains from July through October, enhanced by La Niña conditions, have led to extreme flooding that has killed 506 people and caused that nation's most expensive natural disaster in history, with a cost now estimated at $9.8 billion by re-insurance company AON Benfield. Thailand's previous most expensive natural disaster was the $1.3 billion price tag of the November 27, 1993 flood, according to the Centre for Research on the Epidemiology of Disasters (CRED). The floodwaters this year have hit 83% of Thailand's provinces, affected 9.8 million people, and damaged approximately 25% of the nation's rice crop. Thailand is the world's largest exporter of rice, accounting for 30% of the global total, and the flood has helped trigger an increase in world rice prices in recent months. Fortunately, the Food and Agriculture Organization of the U.N. reports that overall global food prices have been steadily falling since June, thanks in part to increases in wheat and corn production elsewhere in the world. Food prices had reached their highest levels since the late 1970s early in 2011.


Figure 1. The Advanced Land Imager (ALI) on NASA's Earth Observing-1 (EO-1) satellite acquired this pair of before-and-after natural-color images of Ayutthaya, Thailand, on October 23, 2011, and July 11, 2011. In both images, the Chao Phraya River curves through the outskirts of the city (north is to the left in these images). In October, however, the river has overflowed onto nearby floodplains. Fields, roads, and buildings have all been submerged by sediment-clogged flood water. Image credit: NASA Earth Observatory.

The Thailand floods in historical context
Large floods have occurred along Bangkok's Chao Phraya River in 1942, 1978, 1980, 1983, 1995, 1996, 2002, and 2006. The 1942 flood is considered to be the worst flood in modern times. Since 1942, land subsidence, sea level rise, deforestation, urbanization, and removal of wetlands have made floods more likely. However, this has been offset to a large degree by the construction of a series of dams in the upper watershed of the Chao Phraya River basin--including the Bhumibol Dam (1964) and Sirikit Dam (1971). According to wunderground's weather historian Christopher C. Burt, who blogged about the Thailand flood on October 29, the flood of 2011 rivals or exceeds the great flood of 1942 in terms of depth of the flood waters, and the 2011 Thailand flood is perhaps the greatest flood ever to swamp a city so large (population 10 million) in world history. The last time a flood of such a great magnitude affected a city so large occurred in 2004 in Dhaka, Bangladesh, which also has a population near 10 million. Dhaka was almost completely submerged in during massive monsoon floods in the summer of 2004.

The estimated $9.8 billion in damage to Thailand from the 2011 flood is nearly 4% of the country's GDP. Hurricane Katrina cost the U.S. about 0.7% of its GDP, so the Thailand floods can be thought of as a disaster five times worse than Katrina for that country. The most expensive natural disaster worldwide since 1970 in relation to a country's size and economy (for disasters with a high death toll) was the 2010 earthquake in Haiti, according to the Inter-American Development Bank. Haiti's earthquake cost $8 billion, which was 115% of Haiti's GDP. The most expensive flooding disasters in the world since 1970, relative to GDP, both occurred in Honduras. Category 5 Hurricane Mitch of 1998 did damage equivalent to 81% of GDP, and Category 3 Hurricane Fifi of 1974 cost 59% of GDP to Honduras.

Short range forecast: gradual improvement
Fortunately, the monsoon has been quiet so far in November over Thailand, and the latest GFS model precipitation forecast show little additional rain over the country during the coming week. One wild card might be the appearance of a typhoon; November typhoons often affect Thailand. However, the reliable models for predicting typhoon development are currently not forecasting any typhoon activity in the Western Pacific over the coming seven days.


Figure 2. Predicted global sea level rise for 2100 from three different studies.

Forecast for the remainder of the 21st century: more mega-floods for Bangkok
Bangkok lies in the Chao Phraya River basin, which covers about 35% of the country, and has an average elevation just 1 - 2 meters above sea level. Obviously, sea level rise, which averaged 7 inches world-wide during the 20th century, and is predicted to be at least that high during the 21st century, is a huge concern for Thailand. Loss of land due to a sea-level rise of .5 m and 1.0 m could decrease national GDP by 0.36% and 0.69% (US$300 to 600 million) per year, respectively (Ohno, 2001). Higher sea levels also block the flow of flood waters out of the Chao Phraya River, backing up these waters into the city, putting stress on levees and raising flood heights. Another major concern of climate change is the increase in rainfall expected in a warmer world. A 2010 study by the World Bank found that a global temperature rise of 1.2 - 1.9°C would likely increase precipitation over Thailand by 2 - 3%. This extra rainfall, when combined with predicted levels of sea level rise and land subsidence due to groundwater pumping, would likely make a 1-in-50 year flood occur once every fifteen years by the end of the century. Since the flood control system in Bangkok is "generally designed to protect against 1-in-30-year floods", the report concluded that "people living in Bangkok will be facing more frequent events that significantly disrupt daily life." This study assumed that sea level rise by 2100 would be what the IPCC predicted in 2007: 0.6 - 1.9 feet. A number of studies published since that report are predicting much higher levels of sea level increase: 3 - 6 feet (1 - 2 meters) by 2100. If these higher sea level rise estimates prove correct, mega-floods like the 2011 flood will occur several times per decade in Bangkok by the end of the century--unless the Thais can engineer a massive sea wall to keep the ocean at bay.

The city and national planners have plans to add further flood protection, which has been planned and budgeted through 2014. The long-term land use plan until 2057 calls for adding new buffer zones and canals to re-route flood waters. Intentional flooding of agricultural lands upstream of Bangkok during major floods will also be used to help reduce the flood profile in Bangkok.

References
Michael Lemonick of ClimateCentral.org has an op-ed in the L.A. Times called "Thailand's Heart Attack", where he shows how climate change increased the odds of the 2011 mega-flood in Thailand. He makes the point, "Increasing load of greenhouse gases we're pumping into the atmosphere doesn't 'cause' extreme weather. But it does raise the odds, just as a diet of triple bacon cheeseburgers raises the odds of heart disease."

Ohno, E., 2001: Economic evaluation of impact of land loss due to sea-level rise in Thailand. Proceedings of the APN/SURVAS/LOICZ Joint Conference on Coastal Impacts of Climate Change and Adaptation in the Asia Pacific Region, 14-16th November 2000, Kobe, Japan, Asia Pacific Network for Global Change Research, 231-235.

World Bank, 2010, Climate risks and adaptation in Asian coastal megacities: a synthesis report

Jeff Masters