Long-abolished discriminatory lending practices in the US are still having an impact on the inequality of climate risks facing urban populations today, according to a new study.
The research, published in Nature Cities, looks at historically “redlined” neighbourhoods – those deemed highly risky for lenders, broadly due to the race and economic profile of those in the area – and compares them to neighbourhoods that were seen as less risky.
The scientists find that, across more than 200 US cities, once-redlined neighbourhoods are at higher risk of heat exposure and flooding.
Even homes just tens of metres apart have different climate risks, they find, with those located on the redlined side of a boundary at higher risk than those living on the other side of the boundary.
The lead author of the study tells Carbon Brief that the work underscores the historical legacy of planning decisions made in the last century, adding that she hopes that current policymakers can better consider the “impact of different planning policies and the unintended consequences”.
One researcher who was not involved in the study tells Carbon Brief that the work makes several significant contributions, but cautions that the authors were “pretty bold” in some of their conclusions.
‘Risky’ investments
“Redlining” refers to a discriminatory historical practice in the US, whereby neighbourhoods were graded as too “risky” for investment based on race, income levels and housing quality. These grades were used as justification for the denial of long-term mortgages and exacerbated existing racial segregation.
One of the most recognisable remnants of redlining is the set of maps produced by the Home Owners’ Loan Corporation (HOLC), established in 1933 as part of US president Franklin D Roosevelt’s “New Deal”. The HOLC refinanced foreclosed mortgages at lower interest rates with the intention of preserving and expanding homeownership.
The HOLC created maps of “riskiness” of investment in an attempt to guarantee that the loans would be paid back and that the burden on the taxpayer would be minimal.
The maps created by the HOLC classified neighbourhoods based on a four-point risk scale, with A-grades – the “best”, or least-risky, investments – outlined in green and D-grades – the most risky, termed “hazardous” – outlined in red, giving rise to the term.
B-graded neighbourhoods, outlined in blue, were termed “still desirable”, while C-graded ones, in yellow, were “declining”.
The HOLC created maps for more than 200 cities across at least 40 states. Other federal agencies and private companies later made their own “risk-assessment” maps, further cementing the practice into policy.
Although redlining was formally outlawed in 1968 by the US Fair Housing Act, the inequalities created and exacerbated by the practice persist in many places to this day, says Dr Arianna Salazar-Miranda, an urban planner and data scientist at Yale University.
Salazar-Miranda, who is the lead author of the new study, tells Carbon Brief:
“There are many social and economic dimensions for which we should be worried about this long-standing legacy of redlining.”
For example, previous research has shown that redlined neighbourhoods have lower rates of homeownership, lower credit scores and lower home values. There are also associations between historically redlined neighbourhoods and prevalence of cancer and asthma, air pollution and proximity to hazardous waste, among other dimensions of health inequality and environmental racism.
Prof Shannon Van Zandt, an urban planner at Texas A&M University, who was a reviewer of the new paper, but not involved in the study itself, tells Carbon Brief:
“Segregation is still so relevant in the experiences of families of colour and, in particular, Black or African American households, because of the very indelible lines that we literally drew [on the map].”
Climate risk
Using maps from 202 cities across the US, Salazar-Miranda and her colleagues examine the risk of heat extremes and flooding for homes in differently graded neighbourhoods. These factors, each graded on a 1-10 scale from least to most hazardous, were developed by the climate research and technology firm First Street.
The heat risk factor combines temperature and humidity to determine a “feels-like” temperature, averaged across the month of July for each location.
The flood risk factor uses flooding factors, such as rainfall and high tide levels, as well as variables that affect water runoff, including elevation and ground permeability. It also incorporates existing community flood defences. The risk is defined by both depth and likelihood of flooding.
Both the heat and flood risk scores also factor in projections of future climate change, including higher temperatures and sea level rise.
The researchers focused specifically on homes within 100 metres of a boundary between two different grades. Salazar-Miranda tells Carbon Brief:
“We’re trying to narrow down on a subset of properties that are very comparable, where they have the same underlying conditions and the only thing that changed is whether they’re on one side of the border or the other.”
The maps below show the digitised redlining map of Baltimore (left), with the colours indicating the different grades and the bold lines depicting boundaries between different grades.
On the right, a zoomed-in portion of the map shows the 100-metre buffer zones drawn around each boundary. Locations of houses are coloured according to which side of the border they fall on – grey for the lower-graded side and black for the higher-graded side.
Geographical and climatic features, such as elevation and amount of rainfall, did not vary significantly across the boundaries because the researchers were only looking at homes close to a grade boundary.
They find that, aggregated across all cities, D-graded neighbourhoods have a flood risk factor that is 0.245 points higher than A-graded ones – more than three times higher than the additional risk of a C-graded neighbourhood.
The heat risk effect is smaller, but still significant, with D-graded neighbourhoods scoring 0.033 points higher than A-graded neighbourhoods.
The chart below shows the flood and heat exposure risks for neighbourhoods graded B, C and D, relative to the average risk for A-graded neighbourhoods. While both risk factors increase as the grade decreases, the effect is much more pronounced for flood risk.
They also find that flood risk factor increases by 0.1 points, or about 5.5% on average, for homes that are on the lower-graded side of a border, as compared to homes on the higher-graded sides. For the heat risk factor, this figure is 0.011 points.
Although the absolute change in the heat risk factor is relatively small, Salazar-Miranda tells Carbon Brief that these “very small changes…can really harm your health”. She adds:
“It really depends on your pre-determinants of health – how healthy you are, how well you eat, whether you have diabetes or an underlying health condition. And we know that these are particularly worse in disadvantaged communities.”
Doing the analysis on a parcel-scale – namely, house-by-house – is one of the most significant contributions of the new work, says Prof Vivek Shandas, a professor of geography focusing on urban climate at Portland State University in Oregon, who was not involved in the new research. However, Shandas adds:
“There’s a lot that happens across 200 or 100 metres in a city…If we’re doing parcel-scale assessments, we need to get parcel-scale understanding of movement of water and the way that heat is distributed.”
‘Environmental capital’
The researchers then investigate a potential mechanism for how historical redlining could still be impacting vulnerability to current and future climate risks.
They propose that lower-graded neighbourhoods had less investment in what they call “environmental capital”, such as trees, public parks and drainage systems.
This, they say, could be due to a combination of factors: lower property values in the neighbourhoods reduces the communities’ tax income that could be invested in such projects; places with high levels of income inequality tend to have lower community engagement; and low homeownership rates can lead to reduced community investment in public goods, such as parks.
As a proxy for environmental capital investment, the authors look at four measurable factors of environmental quality: tree canopy, street-level vegetation, ground-surface perviousness and home foundation height. Tree cover and street-level vegetation can both mitigate heat risk by providing shade and inducing a cooling effect. More pervious ground surfaces allow more drainage, while higher foundations can decrease an individual home’s risk of flooding.
They find that for each measure of environmental quality, lower-graded neighbourhoods score progressively worse than higher-graded ones, as seen in the chart below.
Houses in D-graded neighbourhoods are, on average, nearly 5.7 percentage points less pervious and have 3.4 percentage points less tree cover than those in A-graded areas.
Similarly, homes on the lower-graded side of a border have lower perviousness and foundations closer to the ground level than homes on the higher-graded side, by 1.9 and 2 percentage points, respectively. Tree canopy and street-level vegetation differ between the two by 1.03 and 1.2 percentage points.
Shandas tells Carbon Brief that introducing the idea of capital into this type of analysis is “really interesting”, but the claims the authors make about their proposed mechanism are “pretty bold”. He adds:
“Each city is so unique…We can find an association, but getting a mechanism has to be [on] a case-by-case basis.”
Van Zandt adds that the redlined maps are a “good proxy”, but not necessarily the driver of inequity. The important part, she says, is “that we identified neighbourhoods that banks should not invest in – and that those patterns persist to today”.
Lived experience
Given the disparities identified in the work, Salazar-Miranda says she hopes that policymakers can incorporate this type of information into funding and other policy decisions. As an added benefit, she says, many of the investments in environmental capital – such as additional green spaces – can improve mental and physical well-being. She adds:
“One of the conversations that would be interesting, from a policy point of view, is how do we bring the types of resources to these communities that can be helpful in mitigating these environmental risks, but also from a social point of view.”
While the findings themselves are not surprising, “it’s great to have systematic assessments” and scientific evidence to back up people’s firsthand knowledge, Shandas says. He tells Carbon Brief:
“Historically disinvested parts of cities tend to be at the frontline of extreme climate events – including flooding and heat. I know the communities that live in the cities that I [have worked with] regularly have brought this up for many, many years.
“The most significant part of this study is that it’s corroborating what the lived experiences of communities have been for quite some time.”
Van Zandt adds:
“It’s not a historical study. It’s a study of what’s happening today and what’s going to continue to happen in the future.”
The post Discriminatory ‘redlining’ increases climate risk in disadvantaged US neighbourhoods appeared first on Carbon Brief.
Discriminatory ‘redlining’ increases climate risk in disadvantaged US neighbourhoods
Drought and heatwaves occurring together – known as “compound” events – have “surged” across the world since the early 2000s, a new study shows.
Compound drought and heat events (CDHEs) can have devastating effects, creating the ideal conditions for intense wildfires, such as Australia’s “Black Summer” of 2019-20 where bushfires burned 24m hectares and killed 33 people.
The research, published in Science Advances, finds that the increase in CDHEs is predominantly being driven by events that start with a heatwave.
The global area affected by such “heatwave-led” compound events has more than doubled between 1980-2001 and 2002-23, the study says.
The rapid increase in these events over the last 23 years cannot be explained solely by global warming, the authors note.
Since the late 1990s, feedbacks between the land and the atmosphere have become stronger, making heatwaves more likely to trigger drought conditions, they explain.
One of the study authors tells Carbon Brief that societies must pay greater attention to compound events, which can “cause severe impacts on ecosystems, agriculture and society”.
Compound events
CDHEs are extreme weather events where drought and heatwave conditions occur simultaneously – or shortly after each other – in the same region.
These events are often triggered by large-scale weather patterns, such as “blocking” highs, which can produce “prolonged” hot and dry conditions, according to the study.
Prof Sang-Wook Yeh is one of the study authors and a professor at the Ewha Womans University in South Korea. He tells Carbon Brief:
“When heatwaves and droughts occur together, the two hazards reinforce each other through land-atmosphere interactions. This amplifies surface heating and soil moisture deficits, making compound events more intense and damaging than single hazards.”
CDHEs can begin with either a heatwave or a drought.
The sequence of these extremes is important, the study says, as they have different drivers and impacts.
For example, in a CDHE where the heatwave was the precursor, increased direct sunshine causes more moisture loss from soils and plants, leading to a drought.
Conversely, in an event where the drought was the precursor, the lack of soil moisture means that less of the sun’s energy goes into evaporation and more goes into warming the Earth’s surface. This produces favourable conditions for heatwaves.
The study shows that the majority of CDHEs globally start out as a drought.
In recent years, there has been increasing focus on these events due to the devastating impact they have on agriculture, ecosystems and public health.
In Russia in the summer of 2010, a compound drought-heatwave event – and the associated wildfires – caused the death of nearly 55,000 people, the study notes.
The record-breaking Pacific north-west “heat dome” in 2021 triggered extreme drought conditions that caused “significant declines” in wheat yields, as well as in barley, canola and fruit production in British Columbia and Alberta, Canada, says the study.
Increasing events
To assess how CDHEs are changing, the researchers use daily reanalysis data to identify droughts and heatwaves events. (Reanalysis data combines past observations with climate models to create a historical climate record.) Then, using an algorithm, they analyse how these events overlap in both time and space.
The study covers the period from 1980 to 2023 and the world’s land surface, excluding polar regions where CDHEs are rare.
The research finds that the area of land affected by CDHEs has “increased substantially” since the early 2000s.
Heatwave-led events have been the main contributor to this increase, the study says, with their spatial extent rising 110% between 1980-2001 and 2002-23, compared to a 59% increase for drought-led events.
The map below shows the global distribution of CDHEs over 1980-2023. The charts show the percentage of the land surface affected by a heatwave-led CDHE (red) or a drought-led CDHE (yellow) in a given year (left) and relative increase in each CDHE type (right).
The study finds that CDHEs have occurred most frequently in northern South America, the southern US, eastern Europe, central Africa and south Asia.
Threshold passed
The authors explain that the increase in heatwave-led CDHEs is related to rising global temperatures, but that this does not tell the whole story.
In the earlier 22-year period of 1980-2001, the study finds that the spatial extent of heatwave-led CDHEs rises by 1.6% per 1C of global temperature rise. For the more-recent period of 2022-23, this increases “nearly eightfold” to 13.1%.
The change suggests that the rapid increase in the heatwave-led CDHEs occurred after the global average temperature “surpasse[d] a certain temperature threshold”, the paper says.
This threshold is an absolute global average temperature of 14.3C, the authors estimate (based on an 11-year average), which the world passed around the year 2000.
Investigating the recent surge in heatwave-leading CDHEs further, the researchers find a “regime shift” in land-atmosphere dynamics “toward a persistently intensified state after the late 1990s”.
In other words, the way that drier soils drive higher surface temperatures, and vice versa, is becoming stronger, resulting in more heatwave-led compound events.
Daily data
The research has some advantages over other previous studies, Yeh says. For instance, the new work uses daily estimations of CDHEs, compared to monthly data used in past research. This is “important for capturing the detailed occurrence” of these events, says Yeh.
He adds that another advantage of their study is that it distinguishes the sequence of droughts and heatwaves, which allows them to “better understand the differences” in the characteristics of CDHEs.
Dr Meryem Tanarhte is a climate scientist at the University Hassan II in Morocco, and Dr Ruth Cerezo Mota is a climatologist and a researcher at the National Autonomous University of Mexico. Both scientists, who were not involved in the study, agree that the daily estimations give a clearer picture of how CDHEs are changing.
Cerezo-Mota adds that another major contribution of the study is its global focus. She tells Carbon Brief that in some regions, such as Mexico and Africa, there is a lack of studies on CDHEs:
“Not because the events do not occur, but perhaps because [these regions] do not have all the data or the expertise to do so.”
However, she notes that the reanalysis data used by the study does have limitations with how it represents rainfall in some parts of the world.
Compound impacts
The study notes that if CDHEs continue to intensify – particularly events where heatwaves are the precursors – they could drive declining crop productivity, increased wildfire frequency and severe public health crises.
These impacts could be “much more rapid and severe as global warming continues”, Yeh tells Carbon Brief.
Tanarhte notes that these events can be forecasted up to 10 days ahead in many regions. Furthermore, she says, the strongest impacts can be prevented “through preparedness and adaptation”, including through “water management for agriculture, heatwave mitigation measures and wildfire mitigation”.
The study recommends reassessing current risk management strategies for these compound events. It also suggests incorporating the sequences of drought and heatwaves into compound event analysis frameworks “to enhance climate risk management”.
Cerezo-Mota says that it is clear that the world needs to be prepared for the increased occurrence of these events. She tells Carbon Brief:
“These [risk assessments and strategies] need to be carried out at the local level to understand the complexities of each region.”
The post Heatwaves driving recent ‘surge’ in compound drought and heat extremes appeared first on Carbon Brief.
Heatwaves driving recent ‘surge’ in compound drought and heat extremes
Greenhouse Gases
DeBriefed 6 March 2026: Iran energy crisis | China climate plan | Bristol’s ‘pioneering’ wind turbine
Welcome to Carbon Brief’s DeBriefed.
An essential guide to the week’s key developments relating to climate change.
This week
Energy crisis
ENERGY SPIKE: US-Israeli attacks on Iran and subsequent counterattacks across the Middle East have sent energy prices “soaring”, according to Reuters. The newswire reported that the region “accounts for just under a third of global oil production and almost a fifth of gas”. The Guardian noted that shipping traffic through the strait of Hormuz, which normally ferries 20% of the world’s oil, “all but ground to a halt”. The Financial Times reported that attacks by Iran on Middle East energy facilities – notably in Qatar – triggered the “biggest rise in gas prices since Russia’s full-scale invasion of Ukraine”.
‘RISK’ AND ‘BENEFITS’: Bloomberg reported on increases in diesel prices in Europe and the US, speculating that rising fuel costs could be “a risk for president Donald Trump”. US gas producers are “poised to benefit from the big disruption in global supply”, according to CNBC. Indian government sources told the Economic Times that Russia is prepared to “fulfil India’s energy demands”. China Daily quoted experts who said “China’s energy security remains fundamentally unshaken”, thanks to “emergency stockpiles and a wide array of import channels”.
‘ESSENTIAL’ RENEWABLES: Energy analysts said governments should cut their fossil-fuel reliance by investing in renewables, “rather than just seeking non-Gulf oil and gas suppliers”, reported Climate Home News. This message was echoed by UK business secretary Peter Kyle, who said “doubling down on renewables” was “essential” amid “regional instability”, according to the Daily Telegraph.
China’s climate plan
PEAK COAL?: China has set out its next “five-year plan” at the annual “two sessions” meeting of the National People’s Congress, including its climate strategy out to 2030, according to the Hong Kong-based South China Morning Post. The plan called for China to cut its carbon emis