Rural belts around cities can reduce urban summer temperatures by up to 0.5°C

The key to cooling 'urban heat islands' may lie in the countryside, according to a new study from scientists at the University of Surrey and Southeast University (China).

Using 20 years of data, researchers showed how nearby rural areas could bring a city's temperature down.

The biggest cooling effects happen where the rural ring around a city extends for at least half the city's diameter.

Professor Shi-Jie Cao, the lead author and visiting professor at the University of Surrey's Global Centre for Clean Air Research (GCARE), said:

"We often focus on how green spaces, wetlands or waterways can cool down cities. Yet, urban land is precious, and these measures can be hard to find space for. We have now shown how land use outside a city can make a big difference to temperatures downtown.

"Our findings allow us to make quite specific recommendations. We found that urban over-heating was mitigated more by joining up patches of rural land, planting more woodland scattered around a city, and by having fewer, bigger lakes rather than lots of little bodies of water.

As warm air rises in a city, it creates a layer of low pressure close to the ground.

This sucks cooler air in from surrounding rural areas. This process is greatly shaped by the size of a city, and the land cover of neighbouring rural areas.

To find out exactly how, scientists compared the areas around 30 Chinese cities between 2000-2020.

Satellite data told them how warm the ground was, and how the land was used.

Professor Prashant Kumar, one of the authors of the study, founding director of GCARE and co-director of Surrey's Institute of Sustainability, said:

"We already suspected that belts of rural land around a city could help cool down the urban centre. Now, thanks to our detailed analysis, we can say which forms of land use lead to the biggest effects.

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How a city is organized can create less-biased citizens

The city you live in could be making you, your family, and your friends more unconsciously racist. Or, your city might make you less racist. It depends on how populous, diverse, and segregated your city is, according to a new study that brings together the math of cities with the psychology of how individuals develop unconscious racial biases.

The study, published in the latest issue of Nature Communications, presents data and a mathematical model of exposure and adaptation in social networks that can help explain why there is more unconscious, or implicit, racial bias in some cities than others.

The authors hope that local communities and governments can use the findings to help create more just and equitable cities.

"What I think is most interesting is the implication that there's a piece of systemic racism that has to do with how people learn and the way cities are organized," says psychologist Andrew Stier, an SFI Complexity Postdoctoral Fellow and lead author of the study.

Cities create dense networks of social interaction between people.

Because of the interactions with many different people, we need to be constantly adapting to new situations and learning, explains SFI External Professor Luís Bettencourt(University of Chicago), a co-leader of SFI's Cities, Scaling and Sustainability project and co-author of the study.

To see how racial biases emerge from how U.S. cities are organized, Stier turned to the enormous database of the Implicit Association Test (IAT). In the popular online test, volunteer participants are given a pairing of White or Black faces with positive or negative words and asked to categorize a single face or word.

If they are faster to categorize things when White/good are paired they have a White-good bias and if they are faster to categorize things when Black/good are paired they have a Black-good bias.

"People may feel they are not prejudiced, but can unconsciously have a preference for one group or another, and this is revealed by these tests," Stier says.

The researchers took the average IAT bias scores from approximately 2.7 million individuals in different geographic areas and linked them to racial demographics and population data from the U.S. Census to build a model that accounts for how individuals learn biases through their social networks.

They found that when these networks are larger, more diverse, and less segregated in cities, implicit racial biases decrease.

The results suggest that there are structural reasons why cities help or deter people from becoming less racially biased.

Perhaps the most pronounced reason is the segregation of different racial groups into different neighborhoods.

Related to that is the lack of more cosmopolitan public spaces where a diverse range of people can experience positive interactions with one another.

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Coastal river deltas threatened by more than climate change

Worldwide, coastal river deltas are home to more than half a billion people, supporting fisheries, agriculture, cities, and fertile ecosystems. In a unique study covering 49 deltas globally, researchers from Lund University and Utrecht University have identified the most critical risks to deltas in the future. The research shows that deltas face multiple risks, and that population growth and poor environmental governance might pose bigger threats than climate change to the sustainability of Asian and African deltas, in particular.

"We can clearly show that many risks are not linked to climate. While climate change is a global problem, other important risk factors like land subsidence, population density and ineffective governance are local problems. Risks to deltas will only increase over time, so now is the time for governments to take action," says Murray Scown, associate senior lecturer, Lund University Centre for Sustainability Studies, and lead author.

Collapse of delta environments could have huge consequences for global sustainable development.

In the worst-case scenario, deltas could be lost to the sea; other consequences are flooding, salinization of water, which affects agriculture, coastal squeeze, and loss of ecosystems.

The study, published in Global Environmental Change, looked at five different IPCC scenarios for global development in 49 deltas all over the world, including famous deltas such as the Nile, Mekong, and Mississippi, but also more understudied deltas such as the Volta, Zambezi and Irrawaddy deltas.

The research identifies possible risks to deltas stretching 80 years into the future.

The researchers based their analysis on 13 well-known factors affecting risk in deltas and drew upon unique models to identify which of these risks are most likely to endanger different deltas in the future.

Risk factors include increasing population density, urban development, irrigated agriculture, changes to river discharge, land subsidence and relative sea-level rise, limited economic capacity, poor government effectiveness, and low adaptation readiness.

Population density, land subsidence and ineffective governance are high risk factors

The analysis shows that there are some risks that are more critical to deltas than others -- in all of the five future scenarios.

These include land subsidence and relative sea-level rise, population density, ineffective governance, economic capacity, and crop land use.

For some deltas, physical risks are especially pronounced. Land subsidence is, for example, the highest risk factor for the Mekong delta in Vietnam.

Extreme sea levels are among the most concerning risk factors for deltas in China, on the Korean peninsula, and in the Colorado (Mexico) and Rhine (Netherlands) deltas.

In the Nile (Egypt), Niger (Nigeria), and the Ganges (Bangladesh) deltas, it is increasing population density that is of most concern under certain scenarios.

For other deltas, it is the lack of economic capacity and government effectiveness to manage risks, for example in the Irrawaddy (Myanmar) and Congo (Angola and Democratic Republic of the Congo) deltas.

"Analysed all together, we can see that the Asian mega-deltas are at greatest risk, with potentially devastating consequences for millions of people, and for the environment. They are under pressure from population growth, intense agricultural land use, relative sea-level rise, and lagging adaptation readiness," says Murray Scown.

Local and global approaches and a mixture of hard and soft adaptation can mitigate risks

"Instead of sitting back, governments need to think long-term, and put plans in place to reduce or mitigate risks. In the Mekong delta, for example, the Vietnamese government are making strong efforts to restrict future groundwater extraction in the delta to reduce land subsidence and salinization," says Philip Minderhoud, assistant professor at Wageningen University and Research.

The researchers highlight that a mixture of hard ("grey") and soft ("green") adaptation approaches will be required to manage and mitigate delta risks.

They include both hard infrastructures, like sea walls to stop the sea inundating the delta, and soft approaches using nature-based solutions.

One example is the Dutch experience of creating room for the river in the Rhine delta, by lowering floodplains, relocating levees, and using spaces that are allowed to flood for grazing.

Initiatives to build up delta surfaces by allowing rivers to flood and deposit sediment on the delta to maintain elevation above sea level are also promising, notes Frances Dunn, assistant professor at Utrecht University.

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Not so silver lining: Microplastics found in clouds could affect the weather

From the depths of the seas to snow on mountains and even the air above cities, microplastics are turning up increasingly often. Now, in ACS' Environmental Science & Technology Letters, researchers have analyzed microplastics in clouds above mountains. They suggest that these tiny particles could play a role in cloud formation and, in turn, affect weather.

Microplastics -- plastic fragments smaller than five millimeters -- originate from a myriad of items used daily, such as clothing, packaging and car tires. As research in the field evolves, scientists are not only detecting microplastics in the atmosphere but also investigating how they may play a role in cloud formation. For example, a group of researchers recently detected plastic granules, which had water-attracting surfaces, in Japanese mountaintop clouds. So, to learn more, Yan Wang and colleagues set out to look for microplastics in mountain clouds, used computer models to figure out how they could have gotten there, and tested how the particles could have impacted -- and been impacted by -- the clouds.

Wang and the team first collected 28 samples of liquid from clouds at the top of Mount Tai in eastern China. Then they analyzed the samples and found:

• Low-altitude and denser clouds contained greater amounts of microplastics.
• Particles were made of common polymers, including polyethylene terephthalate, polypropylene, polyethylene, polystyrene and polyamide.
• The microplastics tended to be smaller than 100 micrometers in length, although some were as long as 1,500 micrometers.
• Older, rougher particles had more lead, mercury and oxygen attached to their surfaces, which the researchers suggest could facilitate cloud development.

To investigate where the plastic particles in the clouds originated, Wang and the team developed computer models that approximated how the particles traveled to Mount Tai. These models suggested that airflow from highly populated inland areas, rather than from over the ocean or other nearby mountains, served as the major source of the fragments. In laboratory experiments, the researchers demonstrated that microplastics exposed to cloud-like conditions -- ultraviolet light and filtered cloud-sourced water -- had smaller sizes and rougher surfaces than those exposed to pure water or air. Additionally, particles impacted by the cloud-like conditions had more lead, mercury and oxygen-containing groups. These results suggest that clouds modify microplastics in ways that could enable the particles to affect cloud formation and the fate of airborne metals. The researchers conclude that more work is needed to fully understand how microplastics affect clouds and the weather.

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Over 4% of summer mortality in European cities is attributable to urban heat islands

Over four percent of deaths in cities during the summer months are due to urban heat islands, and one third of these deaths could be prevented by reaching a tree cover of 30%, according to a modelling study published in The Lancet and led by the Barcelona Institute for Global Health (ISGlobal), an institution supported by "la Caixa" Foundation. The study results, obtained with data from 93 European cities, highlight the substantial benefits of planting more trees in cities to attenuate the impact of climate change.

Exposure to heat has been associated with premature mortality, cardiorespiratory disease and hospital admissions. This is particularly true for heat waves, but also occurs with moderately high temperatures in summer. Cities are especially vulnerable to higher temperatures. Less vegetation, higher population density, and impermeable surfaces for buildings and roads, including asphalt, lead to a temperature difference between the city and surrounding areas -- a phenomenon called urban heat island. Given the ongoing global warming and urban growth, this effect is expected to worsen over the next decades.

"Predictions based on current emissions reveal that heat-related illness and death will become a bigger burden to our health services over the next decades," says ISGlobal researcher Tamara Iungman, first author of the study.

An international team led by Mark Nieuwenhuijsen, director of the Urban Planning, Environment and Health Initiative at ISGlobal, estimated mortality rates of residents aged over 20 in 93 European cities (a total of 57 million inhabitants), between June and August 2015, and collected data on daily rural and urban temperatures for each city. The analyses were performed at a high-resolution level (areas of 250m x 250m). First, they estimated the premature mortality by simulating a hypothetical scenario without urban heat island. Second, they estimated the temperature reduction that would be obtained by increasing tree cover to 30% and the associated mortality that could be avoided.

"Our goal is to inform local decision-makers about the benefits of integrating green areas into all neighborhoods in order to promote more sustainable, resilient and healthy urban environments," explains Nieuwenhuijsen.

The protective effect of trees

The results show that, from June to August 2015, cities were on average 1.5oC warmer than the surrounding countryside. In total, 6,700 premature deaths could be attributed to hotter urban temperatures, which represents 4.3% of total mortality during the summer months and 1.8% of year-round mortality. One third of these deaths (2,644) could have been prevented by increasing tree cover up to 30%, thereby reducing temperatures. Overall, cities with the highest excess heat-mortality rates were in Southern and Eastern Europe, with these cities benefiting the most from an increase in tree cover.

The study highlights the substantial benefits of planting more trees in cities, although the authors acknowledge that this can be challenging in some cities due to their design, and that tree planting should be combined with other interventions such as green roofs or other temperature-reducing alternatives.

"Our results also show the need to preserve and maintain the trees that we already have because they are a valuable resource and it takes a long time to grow new trees. It is not only about increasing trees in the city, it is also about how they are distributed," says Nieuwenhuijsen.

The analyses were done for 2015 because population data were not available for later years, but, as Iungman points out, the study provides valuable information for adapting our cities and making them more resilient to the health impact of climate change. "Here we only looked at the cooling effect of trees, but making cities greener has many other health benefits, including longer life expectancy, fewer mental health problems and better cognitive functioning," she adds.

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Hurricanes expected to linger over Northeast cities, causing greater damage

By the late 21st century, northeastern U.S. cities will see worsening hurricane outcomes, with storms arriving more quickly but slowing down once they've made landfall. As storms linger longer over the East Coast, they will cause greater damage along the heavily populated corridor, according to a new study.

In the new study, climate scientist Andra Garner at Rowan University analyzed more than 35,000 computer-simulated storms. To assess likely storm outcomes in the future, Garner and her collaborators compared where storms formed, how fast they moved and where they ended from the pre-industrial period through the end of the 21st century.

The researchers found that future East Coast hurricanes will likely cause greater damage than storms of the past. The research predicted that a greater number of future hurricanes will form near the East Coast, and those storms will reach the Northeast corridor more quickly. The simulated storms slow to a crawl as they approach the East Coast, allowing them to produce more wind, rain, floods, and related damage in the Northeast region. The longest-lived tropical storms are predicted to be twice as long as storms today.

The study was published in Earth's Future, which publishes interdisciplinary research on the past, present and future of our planet and its inhabitants.

The changes in storm speed will be driven by changes in atmospheric patterns over the Atlantic, prompted by warmer air temperatures. While Garner and her colleagues note that more research remains to be done to fully understand the relationship between a warming climate and changing storm tracks, they noted that potential northward shifts in the region where Northern and Southern Hemisphere trade winds meet or slowing environmental wind speeds could be to blame.

"When you think of a hurricane moving along the East Coast, there are larger scale wind patterns that generally help push them back out to sea," Garner said. "We see those winds slowing down over time." Without those winds, the hurricanes can overstay their welcome on the coast.

Garner, whose previous work focused on the devastating East Coast effects of storms like Hurricane Sandy, particularly in the Mid-Atlantic, said the concern raised by the new study is that more storms capable of producing damage levels similar to Sandy are likely.

And the longer storms linger, the worse they can be, she said.

"Think of Hurricane Harvey in 2017 sitting over Texas, and Hurricane Dorian in 2019 over the Bahamas," she said. "That prolonged exposure can worsen the impacts."

From 2010 to 2020, U.S. coastlines were hit by 19 tropical cyclones that qualified as billion-dollar disasters, generating approximately $480 billion in damages, adjusted for inflation. If storms sit over coasts for longer stretches, that economic damage is likely to increase as well. For the authors, that provides clear economic motivation to stem rising greenhouse gas emissions.

"The work produced yet more evidence of a dire need to cut emissions of greenhouse gases now to stop the climate warming," Garner said.

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Hyena scavenging provides public health and economic benefits to African cities

Hyenas are frequently vilified and often feared. Hemingway once described the hyena as a stinking, foul devourer of the dead, with jaws that crack the bones the lion leaves.

But a new study concludes that spotted hyena scavenging provides significant public health and economic benefits to the African cities they roam.

In a study conducted in and around the Ethiopian city of Mekelle, home to 310,000 people and 120,000 livestock animals, a University of Michigan conservation ecologist and two colleagues found that spotted hyenas annually remove 207 tons of animal carcass waste.

Mekelle is the capital of northern Ethiopia's Tigray region. The carcasses of livestock animals that are slaughtered for food there, or that die naturally, are often dumped at the local landfill or on roadsides, where hyenas feed on the waste. The researchers wanted to know whether hyenas -- by removing this waste from the environment -- might also prevent pathogens from jumping into people and livestock.

They determined that hyena scavenging annually prevents five infections of anthrax and bovine tuberculosis in Mekelle residents and 140 infections in cattle, sheep and goats. This disease-control service potentially saves the city $52,000 annually in treatment costs and livestock losses avoided.

The study, published online Sept. 26 in the Journal of Applied Ecology, is the first to quantify the public health and economic benefits of scavenging by spotted hyenas.

"Yes, the effects are modest. Even so, this study totally upends the traditional narrative around hyenas -- that they are a nuisance and should be removed," said U-M's Neil Carter, senior author of the study and an assistant professor at the School for Environment and Sustainability. "This is an important contribution to a growing body of work that highlights the benefits of predators and scavengers, rather than focusing only on their costs to humanity."

The authors emphasize that these sanitation and disease-control services are particularly valuable in low-income and rural areas. In Mekelle, the scavenging behavior of hyenas advances three U.N. Sustainable Development Goals: ensuring good health and well-being, providing clean water and sanitation, and promoting terrestrial biodiversity.

To estimate the benefits of spotted hyenas, the researchers examined their feeding habits and the population size around Mekelle. During the daytime, the Mekelle hyenas rest at sites outside the city to avoid human disturbances; they come into the city at night to feed.

The field data were collected and analyzed by study lead author Chinmay Sonawane, a former student of Carter's who is now a doctoral student at Stanford University.

Those data were integrated into a disease transmission model used to predict the number of anthrax and bovine tuberculosis infections arising in humans and livestock from infected carcass waste, as well as the costs associated with treating those infections and losing livestock. The researchers compared these public health and economic outcomes under two scenarios: hyenas present and hyenas absent.

"In comparison to the large cattle enterprises in the Global North, the benefits that we found are quite small," Sonawane said. "However, losing even a single cow to anthrax or bovine tuberculosis can cause significant financial stress to cattle owners in Ethiopia, who tend to own much smaller herds of cattle than cattle owners in other parts of the world.

"In addition, we only estimated the benefits accrued from hyenas within 5 kilometers of the city. It is very, very likely that hyenas much further than 5 kilometers are coming into Mekelle and consuming even more waste."

For Sonawane, the fieldwork involved spending nights at the Mekelle landfill and recording the number and type of animal carcasses (mostly cattle, horses, chickens, sheep and goats) available to the scavengers.

He counted the hyenas and watched them feed, using night-vision binoculars that operate at infrared wavelengths. The black-and-white photos and video clips he captured are haunting: the eyes of the hyenas glow with an unearthly fire.

The study authors determined that an individual Mekelle hyena annually scavenges about 2,100 pounds of carcass waste around the city. For hyenas feeding at the landfill, horse carcasses accounted for about 80% of that total, while poultry waste was the most frequently consumed type of animal carcass waste.

The researchers estimated a population size of 210 hyenas around Mekelle using "call-in surveys." This field method involved broadcasting prerecorded hyena distress calls with a 45-watt megaphone from the top of a vehicle at five locations around the city. During each hour-long survey, the researchers counted the number of hyenas attracted to the sounds and visible with the night-vision binoculars.

Given the consumption rate and the estimated population size, the Mekelle hyenas were expected to annually remove 207 metric tons of animal carcass waste, which is 4.2% of the total animal carcass waste thrown away by residents and businesses.

Waste generation in Mekelle is expected to increase with the exponentially growing human population there, Carter and his colleagues wrote: "Given the financial constraints on waste collection and disposal services, sanitary conditions are expected to deteriorate further." Hyena scavenging may, therefore, be even more valuable in the future.

The authors note that the benefits of hyena presence must be weighed against the costs. A 2011 study reported 10 nonfatal hyena attacks on humans annually in Mekelle, along with an estimated economic loss of $2,928 from 33 fatal attacks on cattle and small ruminants annually.

To preserve the public health and financial benefits provided by hyenas into the future, nonlethal management actions that minimize risks and enable hyenas to coexist with humans are necessary, according to the authors.

"By maintaining the hyenas' access to waste, human-hyena conflict can be mitigated in Mekelle, and the sanitation and disease control benefits provided by hyena scavenging can be preserved or even increased," they wrote.

But excluding hyenas from scavenging animal carcass waste may compel them to hunt livestock and could increase human-hyena conflict, as a previous study in Mekelle has shown.

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Cities are making mammals bigger

A new study shows urbanization is causing many mammal species to grow bigger, possibly because of readily available food in places packed with people.

The finding runs counter to many scientists' hypothesis that cities would trigger mammals to get smaller over time. Buildings and roads trap and re-emit a greater degree of heat than green landscapes, causing cities to have higher temperatures than their surroundings, a phenomenon known as the urban heat island effect. Animals in warmer climates tend to be smaller than the same species in colder environments, a classic biological principle called Bergmann's Rule.

But Florida Museum of Natural History researchers discovered an unexpected pattern when they analyzed nearly 140,500 measurements of body length and mass from more than 100 North American mammal species collected over 80 years: City-dwelling mammals are both longer and heftier than their rural counterparts.

"In theory, animals in cities should be getting smaller because of these heat island effects, but we didn't find evidence for this happening in mammals," said study lead author Maggie Hantak, a Florida Museum postdoctoral researcher. "This paper is a good argument for why we can't assume Bergmann's Rule or climate alone is important in determining the size of animals."

Hantak and her collaborators created a model that examined how climate and the density of people living in a given area -- a proxy for urbanization -- influence the size of mammals. As temperatures dropped, both body length and mass increased in most mammal species studied, evidence of Bergmann's Rule at work, but the trend was stronger in areas with more people.

Surprisingly, mammals in cities generally grew larger regardless of temperature, suggesting urbanization rivals or exceeds climate in driving mammal body size, said Robert Guralnick, Florida Museum curator of biodiversity informatics.

"That wasn't what we expected to find at all," he said. "But urbanization represents this new disturbance of the natural landscape that didn't exist thousands of years ago. It's important to recognize that it's having a huge impact."

About a decade ago, scientists began to raise the alarm that warmer temperatures brought by climate change are causing many animal species to grow smaller over time. While many of the consequences of shifting body size are unknown, researchers cautioned that smaller animals may have smaller or fewer offspring, creating a feedback loop, and shrinking prey could also put pressure on meat-eaters to find more resources.

Guralnick and Hantak said they hope their findings will lead more researchers to add urbanization to their analyses of changing body size.

"When we think about what's going to happen to mammalian body size over the next 100 years, a lot of people frame that as global warming causing animals to get smaller," Guralnick said. "What if that isn't the biggest effect? What if it's that urbanization is going to lead to fatter mammals?"

Not all animals respond to human-induced environmental changes in the same way, Hantak added. The researchers also investigated how the effects of climate and urbanization may be tempered or amplified by the behavior and habits of certain species.

They found animals that use hibernation or torpor, a temporary way of slowing metabolic rate and dropping body temperature, shrank more dramatically in response to increases in temperature than animals without these traits. The finding could have important implications for conservation efforts, Hantak said.

"We thought species that use torpor or hibernation would be able to hide from the effects of unfavorable temperatures, but it seems they're actually more sensitive," she said.

While cities radically transform the landscape, they provide animals with new opportunities as well as threats, Guralnick said. The abundance of food, water and shelter and relative lack of predators in cities may help certain species succeed in comparison with their neighbors in rural areas. The results of the 2020 U.S. Census show that almost all human population growth over the past decade has occurred in the nation's metro areas. As urbanization ramps up, animals could be divided into "winners and losers," and mammal distributions may shift, he said.

"Animals that like living in urban environments could have a selective advantage while other species may lose out because of the continued fragmentation of landscapes," Guralnick said. "This is relevant to how we think about managing suburban and urban areas and our wildlands in 100 years."

While bigger is often better biologically, the long-term consequences to urban mammals of eating a diet of human food waste have yet to be determined, Hantak said.

"When you change size, it could change your whole lifestyle," she said.

Hantak and her collaborators were able to conduct the study thanks to thousands of measurements collected by natural historians in the field and museums. The research team used information from three databases: VertNet, the National Science Foundation's National Ecological Observatory Network (NEON) and the North American Census of Small Mammals (NASCM). Cumulatively, this data offers a broadscale view of how increasing urbanization is impacting mammals with very different life histories, from wolves, bobcats and deer to bats, shrews and rodents, Guralnick said.

"Museum collections have the power to tell us stories about the natural world," he said. "Because we have these collections, we can ask questions about what mammals looked like before humans dominated the landscape. Digitizing specimen data unlocks these resources so that everyone can make discoveries about our planet."

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