Extreme precipitation from Tropical Cyclones (TC) can cause major inland flooding in both coastal and interior areas in the continental U.S. (CONUS). This can lead to significant economic damage. For example, Hurricane Harvey (2017) caused $136.7 billion in losses, mostly from rain-induced inland flooding1. It ranks as the second most expensive disaster after Hurricane Katrina. Similar flooding disasters were caused by Hurricane Irene (2011), Hurricane Florence (2018), and Hurricane Ida (2021). Tropical Cyclone Precipitation (TCP) is likely to increase in the future because a warming climate will increase TC rain rate and reduce TC translation speed2-4, causing more frequent stalling5. These changes in TCP patterns will elevate inland flooding risk6 and they can be further exacerbated by other factors such as the urban heat island effect7 and possible intensification effect from aerosols released from urban area8.
While the physical risk of TCP is changing, there have also been substantial shifts in regional demographics and economic development and these changes are likely to continue in the future9. Socio-economic trends in TC damages have been quantified for past TCs using inflation-adjusted GDP10-13, and this has also been applied to project future changes14. Past studies have focused on trends in historical hurricane destruction over the CONUS. Some10,15 have concluded that there is no trend in destruction because of the lack of trends in landfalling TC frequency and intensity13. In contrast, Klotzbach et al.11 used a longer period of record to demonstrate that there are increasing trends in hurricane-related economic losses in CONUS. Increased socio-economic exposure is observed globally and regionally16 from TC wind damages11,14,17, as well as other TC-related hazards10. While many discussions have focused on damages caused by TC winds, few18 have focused on impacts from TCP and inland flooding. In a changing climate, local mitigation, adaptation and risk management are of crucial importance to reduce the impact of TCs19. Here we use a long-term, high resolution TCP record (at 0.25°) derived from daily in-situ precipitation observations20, high resolution population data (highest resolution of 1 km)21 and Gross Domestic Product (GDP, 1 km)22 to quantify spatial and temporal changes in physical TCP risk and socioeconomic exposure to TCP in CONUS over 70 years (1949 to 2018). We summarize changes at the local, state, and national levels to inform mitigation and adaptation strategies.
Temporal Trends in TCP
About 3.7 million km2 (30%) of CONUS has consistent TCP records from 1949 to 2018 (Fig 1a) and 52,000 km2 (0.4% of CONUS) have statistically significant increasing trends in annual TCP. More areas have increasing trends in annual maximum event TCP with 144,000 km2 of areas (1.2% of CONUS) having statistically significant trends (Fig 1b). There are also some smaller areas with negative trends (~28,000 km2 for annual TCP and only 1540 km2 for annual maximum event TCP), although none are statistically significant. Most of the areas with increasing annual TCP trends are clustered in the mid-atlantic and south-eastern Atlantic coast (Fig 1a). In comparison, extreme TCP (Fig 1b) has a more expansive spatial pattern with significant increasing trends in the north-eastern Atlantic coast, Gulf coast, and some inland locations. There are different spatial patterns for increases in daily rain rate (SI 2), annual TCP days (SI 3) and durations of TCP events (SI 4) which, together, contribute to the increases in total and extreme TC in Fig 1. Our results generally agree with the spatial patterns shown in previous studies23-25, although our analysis provides more detailed spatial patterns and is based on a longer time series.
Changes in TCP Probability
We have chosen 100 mm and 200 mm as the thresholds to quantify changes in the Return Periods (RP) of extreme TCP events across the CONUS because 100 mm is a general precipitation threshold for generating flooding26 and 200 mm indicates an extreme precipitation event. The RPs are calculated for two overlapping 50-year time windows to evaluate changes between the early period (1949 to 1998) and the late period (1969 to 2018). Only 529,760 km2 of areas in CONUS have < 25 years RP for >100 mm event TCP in the early period (Fig. 2a) and it increases to 904,750 km2 in the late period (Fig 2b) with a relative increase of 70%. The > 200 mm TCP (Fig 2c & 2d) events are rarer but more destructive. We observe they have a ten-fold increase in the areas with < 25 years RP from 11,550 km2 in the early period to 117,810 km2 in the late period. Geographically, locations with a higher risk of extreme TCP are mostly found in coastal areas, but these areas of higher risk have shifted inland during the later period (Fig 2b & d).
Cities are where people and economic activities are concentrated and so they usually have greater exposure to extreme weather hazards27. Rapid urbanization and elevated risk of extreme weather jointly exacerbate risk28,29. We choose nine large cities along the coast and many have shown elevated TCP risk (Figure 3). Houston has the largest increase, with the 20-year event TCP increased from < 200 mm in the early period to > 400 mm in the late period. The tail of the distribution during the late period increases to > 600 mm as a result of extreme events like Tropical Storm Alison (2001), Hurricanes Ike (2008) and Harvey (2017) in the late period. Other large cities including New Orleans, Mobile, Tampa, Jacksonville, and Raleigh have also experienced substantial increases in TCP risk (Fig. 3b, c, d, e, f). Cities further to the north generally have lower TCP risk and demonstrate variable patterns of change in TCP risk. For example, TCP risk in New York City increased by 50%, while it did not change in Washington, D.C., and it decreased slightly in Boston. This is partly a function of the uncertainties in determining TCP risk in regions that experience fewer TCs.
TC rain rate generally follows the Clausius-Claypron equation and many previous studies already demonstrate that higher global mean atmospheric temperature will possibly lead to more precipitation2,30,31. Our analysis shows substantial increases in TC rain rates over CONUS during the last 70 years (SI 2 & 5) and increased probabilities of higher rain rates in many major cities (SI 6). At the same time, the mean translation speed of TCs decreased from 10.55 kt in the early period to 10.37 kt (SI 7) in the late period (~1.7%). This reduction is most pronounced in the 0 to 20 kt range of the distribution. This agrees with Kossin et al.32, which showed a global shift towards lower TC translation speeds. Therefore, we demonstrate here that recent increases in TCP risk are due to increases in TC rain rate and decreases in TC translation speed.
Changes in socio-economic exposure to TCP
Substantial demographic and economic changes have occurred in the last 70 years in CONUS. The population in the southern U.S. “Sunbelt” has increased since 1950, which can be explained by economic growth, demand for amenities and housing supply33. Meanwhile, these locations are prone to TC-related disasters because of their proximity to the ocean. We created a TCP socio-economic exposure index (TCPEI, see data and methods for details) by combining demographic and economic data with physical TCP risk. The TCPEI quantifies how socio-economic exposure to extreme TCP events has changed between the early period (1949 to 1998) and the late period (1969 to 2018). Overall, there is a general increase in TCPEI from the early period (Fig 4a & c) to the late period (Fig 4b & d). About 55 million people were exposed to > 100 mm TCP in the early period and this increased to 107 million people (92% relative increase) in the late period (Figure 4c & d). For example, the population with > 1TCPEI (the highest category for TCP > 200 mm, Fig 4g) has increased from zero to 4 million. In comparison, the population with > 0 TCPEI (lowest category for TCP > 200 mm) has increased from 4.9 million to 29 million, an ~6-fold increase. This indicates that populated areas are experiencing more substantial increases in exposure to extreme TCP events.
Large changes in TCPEI can be also observed regionally. By averaging the TCPEI within state boundaries, we demonstrate that most states have experienced increases in the median TCPEI, but the magnitude varies from state to state (SI 8). Louisiana, Florida, and North Carolina have the largest absolute increases (0.49 to 0.70 in median) and relative increases (97% to 100%) in population exposure for > 100 mm TCP (SI 8 a & b). Louisiana, Florida, and Texas have the largest increases in population exposure to > 200 mm TCP (0.19 to 0.29 increases in the median with 39% to 58% relative increases, SI 8 c&d). This pattern indicates that states with larger populations are experiencing significantly elevated exposures to extreme TCP events. Florida, Texas and North Carolina all rank high in the relative population growth from 1990 to 2020 (SI 9). Louisiana has a relatively slower population growth so its significant increase in population exposure is mostly due to increases in physical TC risk.
Economic activities are closely associated with population changes and can be substantially disturbed by extreme weather events like TCs. Similar to the population TCPEI, we have found substantial increases in the exposure of economic activities to extreme TCP events (SI 10). There are expansions of areas with high TCPEI in the late period (SI 10b & f) as compared with the early period (SI 10a & e). The > 200 mm TCPEI has a larger relative increase than the > 100 mm TCPEI. For example, the total GDP exposed to > 100 mm TCP increased by 2%, from $5.2 trillion to $5.3 trillion (SI 10c), while the GDP exposed to > 200 mm TCP increased by 124% from $581 billion to $1.3 trillion (SI 10g). Regionally, the majority of states had increases in median GDP TCPEIs (SI 11) and the relative increases are more pronounced for states located in the southern U.S. For example, Florida, Mississippi, Texas, Alabama, and Louisiana rank as the top 5 states highest relative increase (32% to 131%) in the 200 mm TCPEI (SI 11c & d).