No Evidence That Climate Change Causes Weather Extremes: (2) Floods

Widespread flooding and devastating tornadoes in the U.S. Midwest this May only served to amplify the strident voices of those who claim that climate change has intensified the occurrence of major floods, droughts, hurricanes, heat waves and wildfires. Like-minded voices in other countries have also fallen into the same trap of linking weather extremes to global warming.  

Apart from the IPCC (Intergovernmental Panel on Climate Change)’s dismissal of such hysterical beliefs, an increasing number of research studies are helping to dispel the notion that a warmer world is necessarily accompanied by more severe weather.

A 2017 Australian study of global flood risk concluded very little evidence exists that worldwide flooding is becoming more prevalent. Despite average rainfall getting heavier as the planet warms, the study authors point out that excessive precipitation is not the only cause of flooding. What is less obvious is that alterations to the catchment area – such as land-use changes, deforestation and the building of dams – also play a major role. 

Yet the study found that the biggest influence on flood trends is not more intense precipitation, changes in forest cover or the presence of dams, but the size of the catchment area. Previous studies had emphasized small catchment areas, as these were thought less likely to have been extensively modified. However, the new study discovered that, while smaller catchments do show a trend in flood risk that’s increasing over time, larger catchments exhibit a decreasing trend.

Globally, larger catchments dominate, so the trend in flood risk is actually decreasing rather than increasing in most parts of the globe, if there’s any trend at all. This is illustrated in the figure below, the data coming from 1,907 different locations over the 40 years from 1966 to 2005. Additional data from other locations and for a longer (93-year) period show the same global trend.

But while the overall trend is decreasing, the local trend in regions where smaller catchments are more common, such as Europe, eastern North America and southern Africa, is toward more flooding. The study authors suggest that the lower flood trend in larger catchment areas is due to the expanding presence of agriculture and urbanization.

Another 2017 study, this time restricted to North America and Europe, found “no compelling evidence for consistent changes over time” in the occurrence of major floods from 1930 to 2010. Like the first study described above, this research included both small and large catchment areas. But the only catchments studied were those with minimal alterations and less than 10% urbanization, so as to focus on any trends driven by climate change.

The second figure below shows the likelihood of a 100-year flood occurring in North America or Europe in any given year, during two slightly different periods toward the end of the 20th century. A 100-year flood is a massive flood that occurs on average only once a century, and has a 1 in 100 or 1% chance of occurring or being exceeded in any given year – although the actual interval between 100-year floods is often less than 100 years.

You can see that for both periods studied, the probability of a 100-year flood in North America or Europe hovers around the 1% (0.01) level or below, implying that 100-year floods were no more or less likely to occur during those intervals than at any time. The straight lines drawn through the data points are meaningless. Similar results were obtained for 50-year floods. 

Although the international study authors concluded that major floods in the Northern Hemisphere between 1931 and 2010 weren’t caused by global warming and were no more likely than expected from chance alone, they did find that floods were influenced by the climate. The strongest influence is the naturally occurring Atlantic Multidecadal Oscillation, an ocean cycle that causes heavier than normal rainfall in Europe and lighter rainfall in North America during its positive phase – leading to an increase in major European floods and a decrease in North American ones.

The illusion that major floods are becoming more common is due in part to the world’s growing population and the appeal, in the more developed countries at least, of living near water. This has led to people building their dream homes in harm’s way on river or coastal floodplains, where rainfall-swollen rivers or storm surges result in intermittent flooding and subsequent devastation. It’s changing human wants rather than climate change that are responsible for disastrous floods.

Next: No Evidence That Climate Change Causes Weather Extremes: (3) Hurricanes