On floods and climate change

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U.S. National Climate Assessment draft report: “Certain types of weather events have become more frequent and/or intense, including...in some regions, floods and droughts.” Kevin Trenberth, National Center for Atmospheric Research: “The incidence of floods has a component related to climate change and heavy rains, which are well established to be increasing in the U.S."

The following is a guest post by Climate Nexus (in PDF format here):

National Climate Assessment Accurate:

Floods Intensified by Climate Change

A blog post by Roger Pielke Jr. criticizes the United States Global Change Research Program (USGCRP) for the treatment of flooding in a draft of the National Climate Assessment (NCA). The NCA states: “Certain types of weather events have become more frequent and/or intense, including... in some regions, floods and droughts.” Pielke Jr. argues that the climate-flood connection is weak, and therefore its inclusion is misleading.

The claim:

Roger Pielke Jr. quotes the IPCC Special Report on Extreme Weather (SREX), which references “limited to medium evidence” available to assess climate-driven changes in floods.

He quotes the IPCC in stating, “Observations to date provide no conclusive and general proof as to how climate change affects flood behaviour” and continues, “Given the strength of the science on this subject, the USGCRP must have gone to some effort to mischaracterize it by 180 degrees.”

The context:

- Pielke Jr. ignores trends in sea level rise and rainfall, for which strong climate connections exist, and which directly relate to flooding.  Unlike Pielke, the NCA takes these trends into account.

- The study the NCA cites simply shows that floods have increased in some parts of the country and decreased in others.  The NCA does not use the study to suggest a direct causal link between CO2 and flooding.  The NCA relies on other research to identify mechanisms that may have already increased flooding.

The facts:

Heavy rain leads to floods. Pielke critiques the state of flood research, but sidesteps the strong evidence showing that extreme rainfall events have increased due to human-caused climate change. Simple physics dictates that since climate change increases extreme rainfall, and that rainfall contributes to floods in some cases, there exists a relationship between climate change and flooding.

SREX is accurate in that directly measuring trends in floods is extremely difficult, due to human intervention such as the creation of dams and other infrastructure.  However, focusing on this difficulty is a distraction. Increases in extreme rainfall have been established, and they are projected to increase. Climate-driven sea level rise has also been established, contributing to flooding in coastal areas. Despite difficulties in direct flood measurement, SREX and the NCA both agree that global climate change has worsened conditions that lead to flooding, which have likely worsened flooding already.

Straight from the scientists:

Dr. Kevin Trenberth, Distinguished Senior Scientist, National Center for Atmospheric Research: “The incidence of floods has a component related to climate change and heavy rains, which are well established to be increasing in the US... [A]ny floods that do occur have an element of climate change...owing to thresholds exceeded.”

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Also see this earlier post at Climate Progress:  Flawed USGS Study Still Links Southwestern Drying to Increasing Carbon Dioxide Pollution and Climate Change

Earlier CSW post: Draft U.S. National Climate Assessment report released for public review

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One Response to On floods and climate change

  1. Forest says:

    For every degree C rise in temperature the atmosphere increases its moisture content by 7% stemming from added oceanic evaporation.

    Continuing to increase the green house gas carbon dioxide to our atmosphere results in temperature rise. How can human-induced climate change not lead to areas of the globe where the extra water is suddenly released, resulting in an increase in flooding?

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