Flood Impacts: Don’t Forget Other Factors

by Lakis Polycarpou | February 23, 2011

Two recent studies recently appearing in Nature have given added weight to the assertion that global warming is already causing extreme weather events. According to Nature, “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.”

The research has created something of a stir in the media, which is always looking for definitive, smoking-gun-type statements from scientists when talking about the impact of global climate change. The New York Times called one of the studies “the first major paper of its kind,” to use models to analyze whether the rise in severe rainstorms, heavy snowfalls and similar events over the last half of the 20th century could be explained by natural climate variability. The study found that it could not—suggesting that human-caused global warming was at least partly to blame. That suggestion, in turn, led to a discussion of last year’s catastrophic floods in Pakistan, Australia, Brazil and elsewhere, and wondering if scientists could attribute those events to climate change.

However, as Gavin Schmitt, a climate modeler at NASA’s Goddard Institute and blogger for RealClimate pointed out, popular overgeneralizations about climate change can be misleading. “This is a complex issue, and one not well-suited to soundbite quotes and headlines,” he writes, “while questions of attribution come up whenever something weird happens to the weather, these papers demonstrate clearly that the instant pop-attributions we are always being asked for are just not very sensible.” Needless to say, while there’s no denying that climate change is an unprecedented challenge for the human race, the scientific discussion of just how much climate change contributes to particular extreme events (including floods) is complex and ongoing.

Meanwhile, as scientists continue to try to sort out climate change effects, it’s important to remember also that when it comes to the impact of floods, there are many factors outside of global climate change that affect outcomes for people. Here are a few.

Deforestation. In an April, 2010 study published in Journal of Water Resource Protection, Ali Panahi compared the intensity of peak floods from 1960 to 2002 in the Madarsu Basin in Golestan province of Iran. The results showed that the intensity of the peak flood increased over that period even as natural forests, rangelands and bare lands decreased in favor of agriculture. At the same time, the capacity of the soil to retain moisture also decreased. As a result the “discharge rate” (water that runs off) in 2003 was about 10 times larger than that the 1964 flood, even though the precipitation was the same.

Panhi’s study is not the first to draw the connection between deforestation and floods; scientists have known the deforestation increases flood risk for years, if not centuries. A study by Corey Bradshaw and colleagues looked at the deforestation/flood connection in 56 developing countries and found that a 10 percent decrease in forest cover could cause 4 to 28 percent increase in flood risk.

Mismanagement of Rivers. In the summer of 2010 after a week of an unprecedented deluge of monsoon rains, the Indus River flooded Pakistan—by some estimates covering as much as one-fifth of the nation’s land area, killing around 2,000 people and causing untold economic damage and human misery.

While unprecedented rains are the most obvious immediate cause of the flood, some experts also blame the mismanagement of the Indus as well by building levees that cause the river to silt up with sediment washed down from the Himalayas, which has the effect of causing bigger floods when the levees break. As Rajiv Sinha of the Indian Institute of Technology told the BBC, “what we’ve done is apply a system from the West that just doesn’t work in South Asia.” The erosion that leads to silting is made worse by upstream deforestation as well.

Loss of Wetlands. According to the EPA, wetlands function as “natural sponges” that trap and slowly release surface water, mitigating the effect of floods. Unfortunately, the world lost over half its wetlands in the 20th century. Bottomland hardwood-riparian wetlands along the Mississippi River once stored at least 60 days of floodwater; now they only store 12.

Urbanization. According to the USGS, “Common consequences of urban development are increased peak discharge and frequency of floods. Typically, the annual maximum discharge in a stream will increase as urban development occurs.” Urban development has a relatively larger effect on smaller floods. A typical example is Salt Creek, Illinois, where urbanization has caused 100 percent increase in the peak flow of large floods and a 200 percent increase in the peak flow of smaller events.

Given that from 1900 to 2005 the percentage of people living in urban areas went from 13 percent to 49 percent even as population grew from 1.7 to over 6 billion in the same period, it’s not surprising that flood impacts have gotten more pronounced.

What can be done?

The good news is that these and many other human impacts can be mitigated or redesigned. Rivers can be managed better; urban environments can be better designed to take the water cycle into account. As for restoring wetlands and reforestation, both of these activities have potential as carbon sinks as well, which in turn could mitigate the effects of climate change, in the long term making extreme floods less likely than they might have been otherwise.

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