Climate Change is Causing Floods

Flash Flood Waters after the storm flows through the Canyonlands Needle District Utah USA 123rf.com

Pro: Disastrous Floods are Increasing

Floods in inland areas are the most common type of natural disaster in the United States, and one of the most harmful to people and property. In 2017 alone 25 people lost their lives trapped in floods, and more than 3 billion dollars were lost in property damages and ruined crops.

Global warming is shifting rainfall patterns, making heavy rain more frequent in many areas of the country. With human alteration of the land—like the engineering of rivers, the destruction of natural protective systems, and increased construction on floodplains—many parts of the United States are at greater risk of experiencing destructive and costly floods. 

https://www.ucsusa.org/resources/climate-change-extreme-precipitation-and-flooding

However, as the [UN’s] IPCC ([United Nations] Intergovernmental Panel on Climate Change) noted in its special report on extremes, it is increasingly clear that climate change “has detectably influenced” several of the water-related variables that contribute to floods, such as rainfall and snowmelt. In other words, while our warming world may not induce floods directly, it exacerbates many of the factors that do. According to the Climate Science Special Report (issued as part of the Fourth National Climate Assessment, which reports on climate change in America), more flooding in the United States is occurring in the Mississippi River Valley, Midwest, and Northeast, while U.S. coastal flooding has doubled in a matter of decades.

A warmer atmosphere holds and subsequently dumps more water. As the country has heated up an average of 1.8 degrees Fahrenheit since 1901, it has also become about 4 percent wetter, with the eastern half of the United States growing soggiest. In the Northeast, the most extreme storms generate approximately 27 percent more moisture than they did a century ago. Basically, because of global warming, when it rains, it pours more. Such was the finding of a study by the National Oceanic and Atmospheric Administration (NOAA) examining the record-breaking rainfall that landed on Louisiana in 2016, causing devastating flooding. The study determined that these rains were at least 40 percent more likely and 10 percent more intense because of climate change.

https://www.nrdc.org/stories/flooding-and-climate-change-everything-you-need-know

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Con: Flooding is Not Increasing

The U.N. Intergovernmental Panel on Climate Change (IPCC) admits having “low confidence” in any climate change impact regarding the frequency or severity of floods. There has been no evidence of increasing flooding frequency or severity as the climate modestly warms.

The U.N. IPCC admits having “low confidence” in even the “sign” of any changes—in other words, it is just as likely that climate change is making floods less frequent and less severe.  Even if more flooding occurs in the future, any asserted increase in heavy precipitation would likely reduce drought frequency and severity. This is very important because drought is generally a greater climate concern than abundant precipitation.

Occasional heavy precipitation events and floods have always occurred and always will. The U.N. Intergovernmental Panel on Climate Change reports low confidence in any climate change impact on floods, and even acknowledges that climate change is as likely to have reduced flooding frequency and severity as it has been to make them more common. When alarmists point to a particular flooding event and claim climate change is to blame, the assertion defies objective data and even the U.N. IPCC. Also, as shown in the National Oceanic and Atmospheric Administration (NOAA) chart below, the documented decline in drought in recent decades would more than offset any asserted future harms of increased flooding, if they were to occur. Less frequent drought means more abundant crop production and more abundant water resources.

It would require several hundred years of detailed flood data both before and after 1850 to accurately determine whether the natural trends and fluctuations of flooding around the globe have been altered by the 1.3 parts per ten thousand increase in atmospheric carbon dioxide since 1850.  This is the period during which human activity contributed a portion of the increase of CO2 in the atmosphere, most of which occurred since 1940.  This is particularly the case considering the likely impact on flooding of ocean circulations such as El Niño, La Niña, the Pacific Decadal Oscillation (PDO) and the Atlantic Multi-Decadal Oscillation, among others.  To understand how these gigantic multi-decadal events affect the frequency and severity of global flooding would require hundreds of years of detailed data.  Without such sufficient data, no scientific comparisons and conclusions concerning the impact of the tiny increase in carbon dioxide in the atmosphere would be statistically meaningful.

Despite the lack of long-term historical data, short term data contradicts theories of increased flooding due to the tiny increase in atmospheric CO2 levels.

Figure 1: U.S. Wet-Dry Extremes

Figure 1: Percentage of United States experiencing “very wet” (in green) and “very dry” (in yellow) conditions. Source: National Oceanic and Atmospheric Administration, https://www.ncdc.noaa.gov/temp-and-precip/uspa/wet-dry/0.

In a study on the climate impact on flooding for the USA and Europe, published in the Journal of Hydrology, Volume 552, September 2017, Pages 704-717, the study found:

  • ‘The number of significant trends was about the number expected due to chance alone.’

  • ‘Changes in the frequency of major floods are dominated by multidecadal variability.’

  • ‘The Intergovernmental Panel on Climate Change (IPCC) concluded (Hartmann et al., 2013) that globally there is no clear and widespread evidence of changes in flood magnitude or frequency in observed flood records.’

  • ‘The results of this study, for North America and Europe, provide a firmer foundation and support the conclusion of the IPCC that compelling evidence for increased flooding at a global scale is lacking.’

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