US flooding increase will ‘disproportionately’ impact black and low-income groups

Climate change and population growth could drive a 26% rise in US flood risk by 2050 – disproportionately impacting black and low-income groups – new research finds.

The study, published in Nature Climate Change, models property-level changes in flood risk across the US over the next three decades. It finds that in 2020, the US saw an “average annual loss” of $32bn from flooding, but that cost could rise to $41bn by 2050.

The authors find that population growth will be the main driver of increasing flood risk, causing 75% of the rise in “average annual exposure” to flooding by 2050. The impacts of climate change – including rising sea levels, intensifying hurricanes and changing rainfall patterns – will account for 19% of the increased risk.

The study notes that white and low-income communities are currently the most strongly impacted by flooding in the US. However, in the coming decades, black communities – especially in the Deep South – will face the largest increase in flood risk. The results will allow adaptation measures to be targeted towards the most at-risk areas in the US, the paper says.

The lead author of the study tells Carbon Brief that “a study of this granularity has never been deployed at a national scale like this before – and so, in many ways, the results are brand new”. 

The work demonstrates “remarkable teamwork and analysis in evaluating a critical societal problem”, adds a scientist who was not involved in the research.

Rising flood risk

Around 90% of all natural disasters in the US involve some form of flooding. Just one inch of flood water can cause as much as $25,000 in damage to a home – and between 2010-18, floods caused a total of $17bn of damage in the US.

Flood risk is expected to rise over the coming decades. As the climate warms, storm surges, sea level rise and changing rainfall patterns are projected to drive more intense and frequent flood events. Meanwhile, population growth will result in more people living in high-flood-risk areas.

The new study assesses the risk of flooding across the US for the next 30 years. The authors use a high-resolution flood model – which “accounts for all major flood drivers and is built with a well-documented flood protection database” – to determine the present and future impact of sea level rise, tropical cyclones and changing weather patterns on US flooding.

The model is run using RCP4.5 – a moderate-emissions scenario that roughly matches the world’s current trajectory – and a future socioeconomic scenario known as “SSP2”, which assumes that “global population growth is moderate and levels off in the second half of the century”.

The study uses gridded maps of population from the US Environmental Protection Agency to assess the current population exposed to the flood. And to determine how the floods impact infrastructure, the study uses a database called the “National Structure Inventory”.

The study finds that in 2020, the US saw an “average annual loss” of $32bn from flooding, but adds that annual costs could rise to $41bn by 2050 – an increase of around one-quarter above 2020 levels.

In the graphic below, the top two maps show the average annual exposure to flooding in 2020 in dollars (left) and as a percentage of the value of infrastructure in that area (right). The bottom two show the projected increase in risk between 2020-50, as a percentage of the 2020 value (left) and in thousands of dollars (right). Darker colours indicate greater flood risk.

The maps show that flood damage is expected to increase along the US coastlines over the coming decades in particular. This is because increasing hurricane intensity will drive a rise in exposure to flooding as the climate warms – as storm surges add to the already rising sea levels, the authors say. They also note that population density at the coasts is high.

Dr Oliver Wing – an honorary research fellow at the University of Bristol’s School of Geographical Sciences, and lead author of the paper – tells Carbon Brief that the numbers in this study are “actually pretty likely”, because moderate warming and population growth scenarios are used. He adds:

“The other thing is that the timescales are short. We’re not projecting flood risk way out into the future. This is well within our lifetimes – it’s a handful of decades away.”

Flooding impact inequality

To explore the demographics of those most affected by flooding in the US, the paper uses data from the 2019 American Community Survey.

The study finds that given the present-day climate and spread of people, flood risk is mainly concentrated on low income and white communities. In the plot below, the x-axis shows the proportion of a community that is white, while the colour of the bar indicates the proportion of the community that is “impoverished” – where darker green indicates a lower-income community. The y-axis shows the average annual loss from flooding.

Conversely, the authors find that over the next three decades, low income and black communities will bear the brunt of the increasing flood risk. They add:

“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.”

Dr Kevin Smiley, from Louisiana State University’s Department of Sociology – who was not involved in the study, but has worked alongside some of its authors – tells Carbon Brief that this finding has important implications for climate justice:

“Research like this shows how our climate justice conundrums will only worsen with inaction and showcase the importance of building meaningful and wide-reaching policy that advocates for more just outcomes for marginalised groups.”

Population growth

The study also separates out the different drivers of rising flood risk. In the plot below, grey indicates the “baseline” flood risk seen in 2020. Meanwhile, orange indicates the additional risk from climate change (including storm surges, sea level rise and changing rainfall patterns), blue indicates the increased risk from population growth and yellow indicates the combined risks of climate change and population growth.

The plot shows that population growth will be the main driver of increasing flood risk in the US over the next three decades – accounting for 75% of the rise in “average annual exposure” to flooding by 2050. The impacts of climate change will account for 19% of the increased risk. The remaining 6% comes from “the intersection of both climate change and population growth” – that is, regions where floods intensify and the population is also growing.

The study finds that the increase in flood risk due to climate change is mostly concentrated on the east coast, with existing Texas and Florida residents expected to see a 50% increase in flood exposure by 2050. Meanwhile, the increase in flood risk due to population growth is expected “in many places where increases due to climate change are minimal”, according to the paper.

Dr Paul Bates, a coauthor on the paper who is also from the University of Bristol – tells Carbon Brief that despite being the main driver of increased flood risk, the socioeconomic scenarios are “more uncertain” than the climate change scenarios. This could be in part because fewer people study socioeconomic projections than climate change projections, he suggests.

In other words, Dr Wing says, “the predominant driver of future flood risk is, comparatively, way under-studied”.

‘Remarkable’ analysis

Bates tells Carbon Brief that the strength of the new study is the high resolution of the models and data that it uses:

“All flooding is very, very local. Whether your house floods or not depends on small scale features – so you could flood and your neighbour might not…All flood models to date have been too coarse to really capture these effects.

“The fact that we’re able to combine this very granular view of flood risk with very granular population data to make these predictions highlights the uniqueness of this analysis – and that allows us to be more confident about our findings.”

Wing adds that “a study of this granularity has never been deployed at a national scale like this before – and so, in many ways, the results are brand new”.

Smiley tells Carbon Brief that the study is “sound” and “sophisticated”, adding that “this study’s carefully carried out national approach gets us closer to a fuller and more accurate picture than we’ve ever had”.

Dr Herbert Longenecker – a postdoctoral researcher in hazards and disasters from the University of Colorado who was not involved in the study – tells Carbon Brief that the study demonstrates “remarkable teamwork and analysis in evaluating a critical societal problem”. He adds:

“This paper further elucidates the rapidly advancing capabilities of geospatial analysis and computing, demonstrating that not only is it possible to perform high accuracy simulations of flooding at continental scale but also that modelling of vulnerabilities and anticipated demographic changes should inform risk assessments if we are to meaningfully reduce risk.”

The study also goes on to discuss policy options. Although climate change will worsen flood risk in the coming decades, the paper finds that future development patterns are expected to be four times more impactful than climate change in elevating national flood losses.

For example, it highlights that “the intensification of development on existing floodplains is relatively severe in the currently sparsely populated central Prairie States and the Deep South”.

Wing tells Carbon Brief that because the projections in this study are for the next few decades, near-term emissions cuts will likely “not have any impact on our results”. Instead, he says, adaptation is needed to reduce future flood risk. He adds:

“We are already experiencing an unacceptably high level of risk – that is down to poor planning, poor decision making and poor policies that have allowed this to happen. All climate change really does is intensify that demand to adapt.

“Land that nobody should be living on is being developed on, and has historically been developed. And the effect of that is much, much greater than climate change… The solutions are conceptually quite straightforward – to not build things in flood hazard zones.”