Integrating U.S. Population and Flooding Projections to Evaluate Future Exposure Risks to Vulnerable Populations

This presentation will provide results and discussion from a study that integrates the results of state-of-the-art flood modeling in the United States for two potential future climates in the periods around 2050 and 2090 with projections of future U.S. populations to evaluate potential exposure risks. Results are presented in terms of potential exposure for those of all ages as well as for two age-based vulnerable population subgroups: children age 4 and younger, and older persons age 65 and older.

The flood modeling incorporates results from 29 global climate models with projections for more than 57,000 waterway reaches. Results from three 20-year periods are developed with data from 2001-2020 used to define a baseline period, 2040-2059 used to represent the mid-century around 2050, and 2080-2099 used to represen the end of the century aroudn 2090. Flood exposure is evaluated in terms of county-level results for changes in the future frequency of baseline floods associated with a 1% annual exceedance probability (AEP). The modeling also provides county-level results that summarize the modeled change in flow for floods with a 1% AEP relative to the flow for those same floods in the baseline period.

We find that increases in populations exposed to larger 1% AEP flows are more pronounced under the future climate scenario associated with relatively higher greenhouse gas emissions and more warming (RCP8.5) compared to the scenario where GHG emissions are more controlled and there is relatively less warming (RCP4.5). Particularly striking is the increase in the number of people exposed to future 1% AEP flows that are more than 1.5 times the size of the baseline 1% AEP flow under RCP 8.5. While we find almost no one is exposed to this high level of change in the 1% AEP flow in 2050, roughly 51.3 million people of all ages are projected to experience 1% AEP flows that are 1.5 times, or more, than the flow for 1% AEP floods in the baseline period by 2090. This result includes nearly 5.4 million children age 4 and younger, and roughly 9.3 million persons age 65 and older. This increase is almost entirely attributable to anticipated changes projected for the the Southwest region, particularly in the heavily populated area of Southern California. Also, under both climate scenarios, fewer people are projected to experience 1% AEP flows that are smaller than the baseline in 2090 compared to 2050, this despite increasing populations over time.

Subtracting our estimates of populations projected to experience smaller flows in 2050 from those expected to experience larger flows we find roughly 12 million more children are projected to experience an increase in 1% AEP flow under RCP8.5 than under RCP4.5 along with roughly 27 million persons age 65 and older. By 2090, these values increases to roughly 15 million children age 4 and younger while essentially staying constant at roughly 27 million older adults. Similar results are observed in these periods with respect to the number of persons in these age groups projected to experience baseline 1% AEP floods more frequently compared to less frequently. This indicates exposure to these events could be significantly reduced under a climate scenario with relatively moderated emissions. At the same time, regional variation in the direction and magnitude of the results has important implications for developing future hazard mitigation activities and communications.