There is a cynical trick being played by some climate activists to promote misinformation and undercut the assessments of the Intergovermental Panel on Climate Change (IPCC) as they lobby for changes in energy policy.

The trick goes like this:

• Claim, correctly, that some measures of extreme precipitation in some locations have increased on climate time scales and also that the IPCC has attributed that increase to accumulating greenhouse gases, also true.
• Then, imply or state that the documented increase in extreme precipitation is associated with an increase in flooding, including flash floods — including the flood that just happened. This further claim, however, is false.
• Then, assert that changes in energy policy to reduce carbon dioxide emissions can reduce the risks of floods, or even prevent them altogether, by reducing extreme rainfall. This too is false.

At the core of this line of reasoning is the exploitation of an apparent paradox: 

How can it be that extreme precipitation has increased but flooding has not? 

Increasing extreme rainfall with no corresponding increase in flooding is exactly what the best available data shows. There actually is no paradox. 

Both can be true, as I’ll explain in this post.

In our research, we addressed this apparent paradox more than 25 years ago, and the IPCC has much more recently addressed it as well. But before getting to that, let’s look at some examples of false claims by climate activists that increasing extreme precipitation from the accumulation of carbon dioxide in the atmosphere leads to more or more intense flooding — and by extension was in some way a cause of the Texas flood tragedy last week.

In a statement on the Texas flash floods, Frederika Otto of Imperial College London said of the causes of the flood:

“[G]iven the huge amount of scientific evidence on the link between fossil fuel-driven warming and rainfall, we don’t need to wait for an attribution study – extreme downpours like these are exactly what science expects in a rapidly warming world.”

In a commentary on the Texas floods, Andrew Dessler of Texas A&M stated of the flood event:

[E]very weather event we see now carries some influence from climate change. The only question is how big that influence is. Measuring the exact size takes careful attribution studies, but basic physics already tells us the direction: climate change very likely made this event stronger. 

CNN and Bill Nye takes these arguments further, suggesting that changes to energy policy can prevent floods:

How do you prevent “once in a lifetime” flood events like the one in Texas – and beyond – in the past week? @BillNye says the answer is clear: “Stop burning fossil fuels.”

To state the obvious — of course the incredible amount of rainfall that fell in “flash flood alley” in Texas was the proximate factor leading to the flash flood and its deadly consequences. At the same time, to associate a change in climate — the statistics of weather events over many decades — with changes in flooding on the same time scale requires looking at data over many decades and in many places. 

For instance, the figure below, from a paper in the Journal of Hydrology,1 shows a relatively short time series (that is, shorter than climate time scales) of flash flooding in the United States and its regions. The paper finds no significant trends at the national or regional scales, except in the southwest. Over the 20-years covered by the paper there was no increase in flash flooding in the Texas region or the U.S. overall, even as some metrics of extreme precipitation have shown an increase over the same time period.

Renard and colleagues explored the divergence between trends in heavy rainfall and flooding in a 2023 paper in JGR Atmospheres titled, Floods and Heavy Precipitation at the Global Scale: 100-Year Analysis and 180-Year Reconstruction. 

They begin by noting the apparent “disconnect” between well-established research that finds increasing precipitation extremes, but little to no evidence of corresponding changes in flooding:

Floods and heavy precipitation events still hold some mystery despite their disruptive impacts. As an illustration, the latest IPCC report (recently released in 2021) indicates that “the frequency and intensity of heavy precipitation events have increased since the 1950s”, but that at the same time “confidence about peak flow trends over past decades on the global scale is low.” Why this apparent disconnect between floods and heavy precipitation?

Like the IPCC AR6, Renard et al. 2023 conclude that extreme precipitation has increased in some parts of the world, attributed to human-caused changes in the climate system:

Focusing on large-scale studies, there is now growing evidence that heavy precipitation has increased over land since the 1950s (e.g., Dunn et al., 2020; Papalexiou & Montanari, 2019; Q. Sun et al., 2021; Westra et al., 2012). This overall increase is consistent with the larger water-holding capacity of a warmer atmosphere, but regional differences indicate that dynamic changes (e.g., change in storms trajectory) may play a role as well.

And also like the IPCC AR6 they do not find any corresponding or coherent trends in floods:

In contrast, flood changes do not show such a consistent signal. Continental-scale studies generally find a mixture of increasing and decreasing trends, with many regions showing no discernible signal at all (e.g., Berghuijs et al., 2017; Blöschl, Hall et al., 2019; Do et al., 2017; Gudmundsson et al., 2019; Hodgkins et al., 2017; L. Slater et al., 2021).

They explain that the discrepancy may seem counter-intuitive, but actually, the discrepancy is also readily explained:

While the discrepancy between the consistent signal found for precipitation and the lack thereof for floods may appear surprising at first sight, it can be explained by the diversity and the complexity of flood-generating mechanisms (Sharma et al., 2018). For instance, Tramblay et al. (2019) showed that antecedent moisture conditions could resolve an apparent contradiction between increasing heavy precipitation and decreasing floods in Mediterranean France. Brunner et al. (2021) also demonstrated the existence of a catchment-specific threshold below which flood changes do not reflect precipitation changes due to the confounding effect of land surface processes.

In 1999, Mary Downton and I came to exactly the same conclusion in a paper in the Bulletin of the American Meteorological Society titled, U.S. Trends in Streamflow and Precipitation: Using Societal Impact Data to Address an Apparent Paradox.² Here is how we described the apparent paradox:

Recently, Lins and Slack (1999) published a paper showing that in the United States in the twentieth century, there have not been significant trends up or down in the highest levels of streamflow. This follows a series of papers showing that over the same period “extreme” precipitation in the United States has increased (e.g., Karl and Knight 1998a; Karl et al. 1995). The differences in the two sets of findings have led some to suggest the existence of an apparent paradox: How can it be that on a national scale extreme rainfall is increasing while peak streamflow is not? Resolving the paradox is important for policy debate because the impacts of an enhanced hydrological cycle are an area of speculation under the Intergovernmental Panel on Climate Change (Houghton et al. 1996).

We argued that there was in fact no paradox and of the divergent trends in extreme precipitation and streamflow, “it would be physically possible that the two sets of analyses are complementary.”

In 2021, the IPCC AR6 was very clear on this point (emphasis added):

Attributing changes in heavy precipitation to anthropogenic activities (Section 11.4.4) cannot be readily translated to attributing changes in floods to human activities, because precipitation is only one of the multiple factors, albeit an important one, that affect floods.

The IPCC AR6 explained in detail why it is inappropriate to jump from trends in heavy precipitation to trends in flooding (emphasis added):

[H]eavier rainfall does not always lead to greater flooding. This is because flooding also depends upon the type of river basin, the surface landscape, the extent and duration of the rainfall, and how wet the ground is before the rainfall event (FAQ 8.2, Figure1). Some regions will experience adrying in the soil as the climate warms, particularly in subtropical climates, which could make floods from arainfall event less probable because the ground can potentially soak up more of the rain. On the other hand, less frequent but more intense downpours can lead to dry, hard ground that is less able to soak up heavy rainfall when it does occur, resulting in more runoff into lakes, rivers and hollows. Earlier spring snowmelt combined with more precipitation falling as rain rather than snow can trigger flood events in cold regions. Reduced winter snow cover can, in contrast, decrease the chance of flooding arising from the combination of rainfall and rapid snowmelt. Rapid melting of glaciers and snow in awarming climate is already increasing river flow in some regions, but as the volumes of ice diminish, flows will peak and then decline in the future. Flooding is also affected by changes in the management of the land and river systems. For example, clearing forests for agriculture or building cities can make rainwater flow more rapidly into rivers or low-lying areas. On the other hand, increased extraction of water from rivers can reduce water levels and the likelihood of flooding.

In another paper with Mary Downton — in the Journal of Climate³ — we looked at how different metrics of precipitation were related to different metrics of flood damage. We found weak relationships that varied contextually, meaning that there was no single measure of “extreme precipitation” that was most closely associated with economic losses for all places. Where the rain fell mattered a lot — a flood in Phoenix will be very different than a flood in Grand Forks.

The figure below, from that paper, illustrates why it is not at all straightforward to go from increasing precipitation to increasing streamflow to increasing damage.

Contextuality matters. While many metrics of extreme precipitation have been studied, the IPCC AR6 focused on what is called “annual maximum daily precipitation”⁴ which is the most precipitation recorded at a site during one day (24 hours), across the calendar year. 

However, most flash flooding occurs on “sub-daily” time scales. For instance, Ahmadalipour and Moradkhani 2019 find that the median duration of a flash flood event is 2 to 3 hours. Of these shorter time scales of extreme precipitation, the IPCC AR6 concludes:

There is very low confidence about changes in sub-daily extreme precipitation due to the limited number of studies and available data.

The phrase “extreme precipitation” is often used imprecisely to mean “lots of rain,” but it turns out that its precise definition matters when seeking to associate trends in heavy rainfall with societal impacts that we care about. 

To summarize:

• Some climate scientists and journalists are promoting misinformation by claiming that flooding has increased because of increasing extreme precipitation;
• It is true that increasing trends have been detected in some metrics of extreme precipitation, and also that the IPCC has attributed these trends to accumulating greenhouse gases in the atmosphere;
• However, metrics of flooding, including flash flooding, have not shown corresponding increases. In fact, the IPCC has not detected an increase in flooding at the global scale, and across regions the IPCC finds various increasing and decreasing trends;
• Thus, these activists are taking a consensus finding of the IPCC (heavier rainfall) and using it to undercut another consensus finding of the IPCC (no increase in flooding). That’s a good way to destroy trust in climate science overall;
• Some have even gone so far to suggest cynically that energy policy can be used as a control knob to limit or even prevent flooding. It can not.

Reducing the impacts of flooding — both property damage and loss of life — depends on what happens in particular floodplains. Local decisions will determine local impacts, not global energy policy.

The tragedy in Texas last week should not have happened. Nor should its exploitation by those seeking to advance a political agenda focused on climate change. 

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1 Ahmadalipour, A., & Moradkhani, H. (2019). A data-driven analysis of flash flood hazard, fatalities, and damages over the CONUS during 1996–2017. Journal of Hydrology, 578, 124106.

2 Pielke, Jr., R. A., and M. Downton, 1999: U.S. Trends in Streamflow and Precipitation: Using Societal Impact Data to Address an Apparent Paradox. Bulletin of the American Meteorological Society, 80(7), 1435-1436.

3 Pielke Jr, R. A., & Downton, M. W. (2000). Precipitation and damaging floods: Trends in the United States, 1932–97. Journal of Climate, 13(20), 3625-3637.

4 The IPCC AR6 also discussed annual 5 day maximum prediction and several other metrics. The IPCC finds the detection of an increase in extreme precipitation to be a robust conclusion and aligned with theoretical expectations.