The present means by which flood risk is managed globally is predicated on the assumption that history is a good predictor of the future. Be it enforcing regulations within flood zones defined using historical water level records, modelling the cost-benefit ratio of mitigatory actions based on historical flood probabilities, or not considering future risk when permitting new development, ubiquitous flood risk management tools fail to recognise that the nature of floods is changing.
Simple physical reasoning, complex physical modelling, and the recent observational record all suggest that a warming climate is intensifying the hydrological cycle, making extreme precipitation – and thus potentially inland flooding – more severe.1–4 Equally, these sources agree that rising temperatures, leading to oceanic thermal expansion and ice mass loss, induce a rise in global sea levels.5,6 This resultant coastal flooding may be further exacerbated by the low atmospheric pressure and high winds of storms, which themselves may intensify in the future.7
Flood hazard models simulate the physical characteristics of the inundation response to such flood drivers in order to identify potential flood risks. Typical models used for regulatory or commercial applications use historical observations (such as rainfall, river flows, or coastal water levels) as their driving input. Not only does the characterisation of these historical models as ‘present-day’ gradually become more indefensible with the passage of time, they are also instantly outdated if they fail to account for any of the ~1°C temperature rise already experienced during the industrial era, particularly in recent decades.8 Flood risk management requires long term planning. It may be unwise to permit presently low-risk developments to occur in areas where climatic changes in the coming decades may further heighten the flood risk. Investors and mortgage lenders also need to understand an asset’s flood risk through the life of a loan or investment, possibly decades into the future. There is thus a latent need for flood risk assessments in common practice to account for existing and projected climatic non-stationarities.
Academic efforts to model flooding under climate change are in their infancy and so are rarely used for commercial or regulatory applications. Existing models can be broadly characterised as: (i) having spatial resolutions too crude to estimate property-level flood risk;9,10 (ii) unrealistically modelling inundation with simplified volume spreading and storage algorithms;11–13 (iii) lacking crucial local flood adaptation information;14,15 (iv) directly employing precipitation inputs from general circulation models which are too coarse to represent extreme rainfall or resolve tropical cyclones;16,17 (v) focussing on single flood drivers in isolation (e.g. riverine,10 sea level rise,13 storm surge7); and (vi) relatively limited evidence to support an understanding of the fidelity of their model output.18–20
Local-scale studies commonly ameliorate the above concerns relating to modelling accuracy. Flood mapping carried out by the U.S. Federal Emergency Management Agency (FEMA) is often based upon high-precision terrain data, fully surveyed river channels, local gauge information, and a full appraisal of local protection measures. While this represents the current gold standard approach for understanding flood hazard locally, the resource and labour required to replicate these methods at a continental scale is formidable. Consequently, since the start of a national flood mapping programme in 1967, only one-third of U.S. rivers have been modelled by FEMA and only one-quarter of these models have been updated in the last 5 years.21 Furthermore, FEMA models are not mandated to account for climate change and simulate a limited number of flood frequencies, prohibiting a calculation of annualised flood losses. Thus, although policy requirements in the U.S. Water Resources Council’s Principles and Guidelines of 1983 have illustrated the need for considering the future condition in flood risk management for the past four decades,22 the state of the practice has not provided a consistent application of this on a national scale.
The recent work of Bates et al.23 addresses the limitations of existing flood models in the US, fusing the accuracy of local studies with the spatial continuity of large-scale models. They present a hydrodynamic flood model at 30 m spatial resolution accounting for all major flood drivers and built with a well-documented flood protection database. The present and future impact of sea level rise, tropical cyclones, and changing weather patterns are all explicitly represented. Crucially, they benchmark their model against high-quality local flood maps, flood claims information, and, in Wing et al.,24 observations of real flood events. These validation exercises determined the skill of the Bates et al. US-wide flood model to be approaching that of local studies and historical observations (80–90% flood extent similarity), while providing a consistent and comprehensive picture of flood hazard spatially. Bates et al. focus exclusively on the flood hazard; however, their hazard model uniquely enables us to quantify present and future U.S. flood risk – the financial and human implications of the physical phenomenon – with wider scope, scale, and fidelity than prior research.
Risk assessment requires a quantification of the hazard (local flood intensities and frequencies), the exposure (the location and characteristics of buildings, people, and businesses), and vulnerability (the extent to which hazard intensity impacts exposed entities). For the latter two constituents of risk, we employ detailed information from the U.S. government. The National Structure Inventory (NSI), a database of building locations and characteristics for residential and non-residential structures, was utilised for the representation of exposure. U.S. Army Corps of Engineers (USACE) depth-damage functions are utilised to describe the vulnerability of these buildings to flooding. Combining these three components (hazard, exposure, and vulnerability) yields a step-change in understanding of U.S. flood risk by providing the first national-scale flood risk assessment using property-level residential and non-residential asset data alongside spatially complete hazards maps of multiple frequencies (see Methods for further details). Estimates of annualised flood losses are compared to those recorded historically, and while we do not expect to replicate these precisely owing to uncertainties in those observations and their questionable relevance to present conditions (both in terms of hazard frequency and exposure availability), we use the comparison to demonstrate that the risk model provides sensible quantifications at the U.S. scale (see Supplementary Information).
This analysis reveals that annualised U.S. flood losses are currently $27.4 billion on average and are projected to rise to $37.5 billion by 2050 under the RCP4.5 scenario. This is a 36.7% increase across a typical 30-year mortgage term commencing today: near-term impacts which are essentially locked-in climatically; that is, these projections hold even if dramatic decarbonization is undertaken immediately.
In Fig. 1a, we can see the distribution of AAL by U.S. county. Intuitive hotspots are found in highly populated counties along both coasts, as well as across the Northeast and through Appalachia. Controlling for exposure (i.e., the total value of what could potentially be damaged) in Fig. 1b, hotspots emerge in coastal Louisiana, Appalachia and the inland Northeast, and rural counties of the Pacific Northwest and Northern California. While many of these counties do not have high absolute annual losses, they are proportionally high-risk with AALs greater than 0.2% of exposure (losses expressed as a proportion of total value). Climatic changes alone cause dramatic increases in risk along the east coast in counties which are already high-risk (Fig. 1c and 1d). The intensification of hurricanes on the east coast is particularly evident in risk changes, principally a result of greenhouse gas emissions weakening vertical wind shear in the North Atlantic and permitting hurricanes to intensify more than usual.25 The impact of these projected changes to hurricane behaviour on coastal surges are most keenly felt in Virginia, the Carolinas, and the west coast of Florida, while the contribution of sea level rise to future coastal floods dominates the remaining stretches of the Atlantic and Gulf coasts.23 Intensifying rainfall, both hurricane and non-hurricane, is also expected to drive up risk in inland counties of Florida and the Northeast. Climate risk hotspots are further found in some already-risky western counties in California, Oregon, and Washington. Conspicuous by their absence are risk hotspots along the Mississippi-Missouri, perhaps due to lower asset values and the dominant land-use being agricultural. Furthermore, climate change impacts for large river systems are highly uncertain, while clearer positive signals emerge for short-duration rainfall and sea level rise.
The FEMA Special Flood Hazard Area (SFHA), determined by the nationwide patchwork of local-scale FEMA flood models, is the de facto flood risk zone in the U.S.26 A number of regulations apply to development within SFHAs, as well as the mandatory purchase of flood insurance for those with a federally backed mortgage. Though it was not designed to be a risk communication product, it has become synonymous with that in the public view. Properties located outside the SFHA are commonly misconceived to be risk free, when, in reality, there may simply not be an up-to-date local flood map, they may be at risk of unmapped pluvial (or, indeed, fluvial) floods, or they may be outside of the 100-year flood zone where lower frequency floods can still occur. Additionally, for those located in the 100-year floodplain (at least a 1% chance of inundation each year), their recurrence of flooding can be anywhere from every other year to once every hundred years on average, and these varying probabilities have dramatically different outcomes in the evaluation of risk. The frequency of floods outside the SFHA and its discontinuous spatial coverage has been well documented elsewhere,21,27,28 but here we demonstrate that less than half of the nation’s flood risk is located within the SFHA. Properties within the SFHA are currently subject to AALs of $13.0 billion (47.6%), while AALs outside total $14.4 billion (52.4%). Proportional risk is much higher in the SFHA, with an AAL equal to 0.399% of exposure; roughly 20 times the relative risk of non-SFHA properties (0.021%). This is illustrative of the large number of low- or no-risk buildings outside the SFHA, yet it still remains that the majority of U.S. flood risk is unmapped by FEMA. Climate-induced risk changes in the SFHA are expected to be more intense than elsewhere. Within-SFHA AALs are projected to rise by 42.5% to $18.6 billion (or 0.568% of exposure) by 2050, while the outside-SFHA increase is projected to be 31.5% to $18.9 billion (0.027%).
Flood risk is not borne equally by all. We use census-tract level data from the 2019 American Community Survey to assess the demographic characteristics of flood risk across the US. Normalising for exposure (to understand risk as a fraction of the total that could be damaged), we consistently see that present-day flood risk is concentrated in both the most White and the most impoverished communities across the nation (Fig. 2a). When grouped into ordinal quintiles (bins containing 20% of U.S. census tracts), the data indicate a persistent increase in relative AAL with increasing poverty rate and the proportion of the population that is White. The flood risk of the top 20% proportionally White and impoverished census tracts (>90% White, >22% in poverty) is roughly 10 times higher than those which fall into the least White and the least impoverished quintiles (<30% White, <5% in poverty). The spatial distribution of these census tract groups and their relative risk is shown in Fig. 3a and 3b. More White and impoverished communities with high relative risk are noticeably concentrated in Appalachia (West Virginia and Kentucky; Fig. 3ai), covering the high-risk counties highlighted in Fig. 2b. Rural and small-town Pennsylvania, other communities in the Ohio River valley (Fig. 3aiv), as well as northern New England (Fig. 3aii) and Oregon lie at the nexus of high-risk, high-poverty, high-White proportion. The relative risk of the opposite group – census tracts with the smallest White population proportions and lowest poverty rates – is shown in Fig. 3b. These low-risk communities are predominantly urban, with clusters on both coasts of the US. Suburbs of Washington, D.C. (Fig. 3bi), and a stretch of tracts from Philadelphia, PA through New York City, NY (Fig. 3bii) generally have low relative flood risk, with AALs less than 0.01% of exposure. In California, communities in Los Angeles (Fig. 3biii), San Francisco (Fig. 3biv), and a handful in the Central Valley are also generally more affluent, less White, and lower risk. Pockets of urban centres in the Deep South also share these traits: Montgomery and Birmingham, AL; Atlanta, GA; Tallahassee, FL; and Houston, San Antonio, and Dallas, TX.
Meanwhile, expected changes in flood risk up to 2050 show largely the opposite trend in demography compared to who bears present-day risk. The sensitivity of flood risk to climate change is concentrated in Black communities across the US. Fig. 2b, with census tracts again grouped into equal-count quintiles of Black population proportion, illustrates that the more Black an area is, the larger its expected increase in flood risk due to climate change. The top 20% proportionally Black census tracts (>20% Black) are expected to see flood risk increase at double the rate of the bottom 20% (<1% Black) of Black census tracts. Fig. 3c illustrates that areas with high Black population proportions are clearly concentrated across the Deep South, in the very locations where climate change is expected to intensify flood risk (Fig. 2c). Urban and rural areas alike from Texas through Florida to Virginia contain predominantly Black communities projected to see at least a 20% increase in flood risk over the next 30 years. Indeed, virtually every high-Black population proportion census tract in urban areas of Florida, Alabama, Georgia, Louisiana, Mississippi, and the Carolinas (Fig. 3ci and 3civ) bear outsized climate risks. The same can be said for Black communities in Detroit, MI; Cincinnati, Dayton, Columbus, and Cleveland, OH (Fig. 3cii); as well as those around the Chesapeake Bay (Fig. 3ciii). In contrast, most census tracts with the lowest Black population proportions see very little increase in climate-induced flood risk (Fig. 3d): particularly in the predominantly White central U.S. and Midwest (Fig. 3di and 3dii) and the low-Black populations of the arid Southwest (Fig. 3div). Some communities with a low proportion of Blacks in the Northwest see heightened future risk proportionally, but these are mostly small in magnitude due to their current low risk (Fig. 3diii). Present and future trends in the flood risk of other demographic groups are less clear and consistent and are shown in Supplementary Figs. 1-9.
Climate will not be the only thing changing over the next 30 years. The U.S. population is expected to continue to grow, and so, accordingly, is development. We use gridded maps of population from the U.S. Environmental Protection Agency (EPA) to calculate the current population exposed to floods, and their gridded projections of 2050 populations under the SSP2 scenario to analyse the relative contributions of climate change and population growth to future U.S. flood risk (Fig. 4a). The average annual exposure (AAE) of the current U.S. population to flooding is 3.63 million (1.18%). Climate change is projected to increase the AAE of present populations to 4.31 million (1.41%); an increase of 18.6%. Meanwhile, population growth alone in a static climate (i.e., no future changes in flood hazard) would result in a 72.6% increase to 6.27 million AAE by 2050. This corresponds to 1.60% of 2050’s projected population, indicating that future development is projected to disproportionately intensify in hazardous areas (given the present-day proportion is 1.18%). Absent of policies to direct new development into safer areas, the contribution of population growth to future U.S. flood risk dwarfs that of climatic changes. Population growth alone accounts for 74.7% of the increase in AAE to 2050, while climate change represents 19.1% of the change. There is a remaining 6.2% (yellow in Fig. 4a) of 2050’s projected AAE which represents the intersection of both climate change and population growth. Conceptually, this is due to floods intensifying in places where populations are also increasing – and so the compound intensification of both hazard and exposure is required to capture the increased total risk. AAE of the U.S. population to floods in 2050 is projected to be 7.16 million (1.83%), a 97.3% increase from the present-day.
Fig. 4b, like Fig. 1a, shows that concentrations of population AAE generally fall within populous U.S. states. Populations of 547k, 345k, 247k, and 247k in Florida, California, New York, and Texas respectively are expected to be impacted by flooding every year, on average, under current conditions. Fig. 4c shows that, proportionally, West Virginia, Vermont, Florida, and Louisiana have the highest AAEs: they can expect over 2% of their populations to be impacted by flooding every year currently (mirroring Fig. 1b). AAE increases due to climate change are generally found across the east coast, with existing Texas and Florida residents seeing a roughly 50% increase in flood exposure by 2050 (Fig. 4d). Interestingly, AAE increases due to population growth occur in many places where increases due to climate change are minimal (Fig. 4e). Intensification of development on existing floodplains is relatively severe in the currently sparsely populated central Prairie States and the Deep South. The consequence is a more widespread increase in flood risk to 2050 than Fig. 1 suggests: states with little climate risk may still see large increases in flood risk unless future development patterns are managed appropriately (Fig. 4g). Areas where the compound effect of climate change and population growth is significant are scattered across the nation: over 10% of the risk increase to 2050 is compound in West Virginia, Louisiana, Idaho, and Mississippi (Fig. 4f).
With future development patterns projected to be 4x more impactful than climate change in elevating national flood losses, the importance of improved flood risk management in the U.S. is clear. More aggressive local land use controls restricting new developments in the highest risk areas, coupled with stronger building codes, could help lower the growth in flood losses that is currently projected to accompany expanding populations. Such regulations imposed on future development will also need to be accompanied by investments in both relocation and retrofits for existing construction in areas where flood risk is high and/or growing. The federal government has several programs that currently fund such efforts, although not at levels that will be required to fully adapt to increasing risk.29 Further, several of these programs have been criticized for privileging more affluent and White communities.30,31 Equity-centred reform in light of climate change is needed for U.S. disaster policy; a call given greater emphasis by the demographic make-up of present and future bearers of U.S. flood risk shown here.
An important conclusion from Bates et al. should not go neglected, however. They found that when considering flood hazard projections derived from only the central 50% of climate model ensemble members (i.e., ignoring outlier simulations), the variability between models representing the present-day is over double the magnitude of the change signal to 2050. In simpler terms, increased flooding due to climate change is within the error of present-day flood models. Furthermore, these projections assume no further adaptation to present and future flood risks take place. Existing protection measures maintain their integrity up to their original design standard, but no further defences are projected to account for increasing flood hazard or the proliferation of flood-exposed developments. While future work will examine the efficacy of targeted adaptation measures in drawing down the flood risks we project here, it is fair to assume that some level of adaptation will be put in place to protect new development. That being said, the ability to understand future risk as the process of development takes place is essential to reducing risk in future environmental conditions.
The threat that floods presently pose – both direct and indirect, tangible and intangible – is evidence enough that there is a dearth of flood resilience in the U.S., regardless of what the future holds in terms of climate and demographic change. Layered on top of this already critical problem is the large increase in risk that we project a warming world would portend. These impacts are so near-term that climate mitigation (i.e., decarbonization) is futile, meaning we can only adapt to this increasing risk in areas currently developed. We thus have to adapt to both the ‘now’ and to the future. Mitigation will largely determine how much worse flood hazard will get in the latter half of this century. The lack of quality publicly available flood risk information has meant risky developments have proliferated across the U.S.; planning and investment decisions by the public, governments, and corporations rarely consider flood risk adequately.32 The current state of the science means there is no longer an excuse for this to continue. It is critical that information on changing risks be made widely available and transparent in order to fully inform housing and mortgage markets to guide capital away from the riskiest areas. The findings of this paper provide important insights for communities and the federal government in designing future flood risk management interventions and in allocating federal dollars more effectively. Models such as these can and should inform zoning regulations to prevent anticipated future developments making largely inevitable hazard changes unnecessarily inflate risk. Adaptation policies can be targeted towards locations with disproportionate risk, or where risk is expected to increase, using these data. Furthermore, public consciousness surrounding present and future flood risks must improve to spur individual and collective risk reduction. To that end, these flood data have been released on FloodFactor.com to ensure every American resident has access to high-quality flood information.