If “Extreme” Precipitation is Up, Why Aren’t Floods?

PD00-floodsA new paper in Water Resources Research discusses an apparent paradox:

Extreme precipitation events, which fall into the 99th percentile of daily events, have increased across the contiguous United States since the 1950s in response to rising temperatures. But despite assertions by the climate community that increasing precipitation extremes inevitably lead to higher flood magnitudes, multiple studies have demonstrated that this has not been the case.

In this post I discuss this apparent paradox, which we wrote about 20 years ago, and its relationship to common misrepresentations of climate science. More after the jump.

The new paper is:

Sharma, A., Wasko, C., & Lettenmaier, D. P. (2018). If precipitation extremes are increasing, why aren’t floods?. Water Resources Research, 54:8545-8551.

Many people believe that flooding has been on the increase. It hasn’t. One reason for this confusion may be the fact that “extreme” precipitation has increased in many places. I put “extreme” in scare quotes because many scientific definitions of “extreme” are statistical and are not necessarily closely associated with flooding or flood damage (see Pielke and Downton 2000 linked below). However, another reason for confusion is that some enthusiastic advocates for climate action willfully misrepresent the science of flooding and precipitation. More on that at the conclusion to this post.

Back to Sharma et al. who conclude that the relationship between extreme precipitation and flooding is complicated.

There is a clear dichotomy between observed increases in precipitation extremes and the lack of corresponding increases in floods, with reduced flood magnitudes observed in many cases. Despite the conceptual arguments we’ve made, there remains a good deal of uncertainty in the relationships between changes in precipitation and flood magnitude across the spectrum of catchment, storm, and antecedent hydrologic conditions. Although changes in flood magnitude are unlikely to be explainable by precipitation changes alone, this has largely been the focus to date in the climate literature.

They identify five aspects of this complexity:

  1. Changes to antecedent hydrologic conditions and their impact on flood response;
  2. Changes in the proportion and persistence of storms arising from different causative mechanisms, such as an increased proportion and frequency of convective extremes;
  3. Interaction among catchment size and geometry and changing storm characteristics including extent, intensity, and duration;
  4. Snow cover and snow volume changes and their changing contributions to flood extremes in a warmer climate;
  5. The role of land cover change (especially, but not only, urbanization) and the interaction of land cover change with climatic factors.

Twenty years ago we identified the paradox of increasing extreme precipitation but a corresponding lack of increase in floods.

Pielke, Jr., R. 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, 80:1435-1436. (PDF)

We wrote:

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?

We drew on our major study looking at the relationship of various metrics of precipitation and flood damage to suggest an answer to this question (its complicated!).

Pielke and Downton [2000] relate trends in various measures of precipitation with trends in flood damage in the United States. The study finds that the increase in precipitation (however measured) is insufficient to explain increasing flood damages or variability in flood damages. The study strongly suggests that societal factors—growth in population and wealth—are partly responsible for the observed trend in flood damages. The analysis shows that a relatively small fraction of the increase in damages can be associated with the small increasing trends in precipitation. Indeed, after adjusting damages for the change in national wealth, there is no significant trend in damages. This would tend to support the assertion by Lins and Slack (1999) that increasing precipitation is not inconsistent with an absence of upward trends in extreme streamflow. In other words, there is no paradox.

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

In Pielke and Downton (2000) we concluded that the relationship of precipitation and floods (and then flood damage) varies by region. Importantly, we did find that some metrics of precipitation are indeed related to flood damage. However, the metrics that were most closely related to damage varied by location and by metric of precipitation. For instance, extreme precipitation over many days was more likely to result in flooding and flood damage in the US Midwest but flash floods over short time periods were more likely to lead to flooding and flood damage in the US Southwest.

Thus, any single measure of precipitation would be unlikely to explain much about flooding or flood damage. Hence, it is perfectly reasonable that some “extreme” precipitation metrics would show an increase, yet flooding or flood damage would not show a corresponding increase, and might even show no trends or a decrease. The figure at the top of this post comes from our paper and presents some of this complexity.

We concluded:

[W]hether a given increase in precipitation leads to increased hydrologic flooding will depend on its geographical distribution and timing, and whether population growth leads to increased flood damage depends on whether and how the growth occurs within the flood plain. In addition, our analysis provides supporting evidence for the notion that increasing U.S. precipitation documented by Karl and colleagues has not led to a corresponding increase in the highest levels of streamflow documented by Lins and Slack. . . In conclusion, the relationship of climate, hydrology, and society in producing damaging floods is complex
and not fully understood.

The lack of trends in flooding — in the US or globally — is well documented and appears in the assessments of the IPCC and US National Climate Assessment. Yet despite this robust understanding, claims persist by many journalists and leading climate scientists that flooding has increased. Such false claims are made even worse when the (nonexistent) increases in flooding are attributed to an increase in greenhouse gases. A third misrepresentation occurs when the phantom trends flooding are then linked to increasing precipitation, which has an actual trend in some place that has been attributed to some degree to human influences (per IPCC).

As I have written about for years, the misrepresentation of the detection and attribution of trends in extreme weather events is common by advocates for climate action, many of whom certainly know better. My pointing this out has led to considerable career repercussions, but I’ll continue to do so because I’m on the right side of the evidence and have been for a long time. Good science wins out in the end, even if it takes 20 years.

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