Are Recent Urban Floods Related to Global Warming?

Written by: Timothy H. Dixon

Original photo by Exile on Ontaria St: Rue Cousineau, Cartierville neighborhood of Montreal, 8 May 2017. Creative commons Licence

Are Recent Urban Floods Related to Global Warming?

In early May of this year (2017) severe flooding hit parts of the Canadian province of Quebec and adjacent US states of New York and Vermont.

A short answer to the title’s question is “Probably”

A short answer to the title’s question is “Probably – warmer air can hold more water vapor, which can lead to more intense rainfall events”. But notice the use of “related to” rather than “caused by” in the question, and “probably” in the answer. The immediate cause of this disaster was heavy rain that happened to coincide with spring snow melt season, a common time for this type of flooding to occur. If the intense rainfall had happened a month later, flooding probably could have been avoided, or at least would have been much less severe.

What’s the connection between a flood and an aircraft accident?

Disasters, both natural and human-caused, almost always have more than one contributing factor. Most reflect a combination of natural and human factors. Consider the 2009 crash of Air France Flight 447. What’s the connection between a flood and an aircraft accident? At first glance, this might seem to have nothing in common with recent flooding in Quebec, but there are some useful analogies. The aircraft was crossing the inter-tropical convergence zone (ITCZ) as it flew from Rio de Janeiro to Paris. The ITCZ is a well-known region of atmospheric turbulence and instability near the equator, where the northeast trade winds meet the southeast trade winds, forcing moist air upward. On Flight 447, the high altitude moisture caused the pitot tubes (instruments that measure the speed of the aircraft) to ice up, giving the pilots incorrect air speed information. Although pilots train for this eventuality, in this case they became confused, stalling the aircraft (putting it in a nose-up position so that the wings lose lift) and causing it to crash into the Atlantic. The combination of weather conditions, faulty aircraft design (in addition to the pitot tubes, confusing cockpit design was later implicated as a factor) and human error all contributed to this disaster.

Most disasters reflect a combination of natural and human factors

In the case of Quebec’s recent flooding, at least three other factors besides global warming and spring melt season likely contributed.

  1. Population growth in some cities has pushed development to low-lying areas, areas initially avoided by early inhabitants (probably because these areas tended to flood!). Unscrupulous developers and lazy city planners may have played a role in some towns and cities, but it is also true that weather conditions have changed in the last few decades, in ways that would have been tough to predict future flooding.
  2. Modern towns and cities have lots of “hardscape”, impermeable surfaces such as buildings and asphalt or cement roads, parking lots and sidewalks. This means that rainwater can’t percolate into the ground like it used to.
  3. Storm sewers that would normally channel rainwater to safer areas tend to plug up with debris during intense storms.

Globally, more and more cities are experiencing severe flooding of this type. In the US, the costliest disaster since Hurricane Sandy in 2012 occurred in Baton Rouge Louisiana, during a high rainfall event in August, 2016. More than 20 inches (510 mm) of rainfall fell over several days, killing 13 people and causing more than $10 billion (US) in damages. In the same year in western Europe, mainly in Germany and France, 20 people died during widespread flooding in late May and early June, also associated with extreme rainfall.

These won’t be the last high-rainfall flooding events

These won’t be the last high-rainfall flooding events. City planners need to act accordingly – more greenspace and wetlands to absorb storm run-off, more robust storm sewer systems, and less development in low-lying areas would all help.

What about the crash of Air France Flight 447-could it be related to global warming? No – issues related to aircraft design and pilot error provide sufficient explanation for this tragedy. But it’s interesting to speculate that as the atmosphere warms and becomes more energetic, the amount of water vapor advected to high altitude in the ITCZ and elsewhere could increase, leading to more blocked pitot tubes and other icing problems unless aircraft designers also act accordingly.

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About the Author: Timothy H. Dixon

Timothy H. Dixon, author of Curbing Catastrophe, 2016 is a Professor in the School of Geosciences and Director of the Natural Hazards Network at the University of South Florida. In his research, he uses satellite geodesy and remote sensing data to study earthquakes and volcano deformation, coastal...

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