Category: English

Source: The Conversation – USA (3) – By Güneş Murat Tezcür, Professor and Director of the School of Politics and Global Studies, Arizona State University

In July 2025, clashes between the Druze religious minority and Sunni Arabs backed by government-affiliated forces led to hundreds of deaths in Sweida province in southern Syria. Israel later launched dozens of airstrikes in support of the Druze.

This eruption of violence was an eerie reminder of what had unfolded in March 2025 when supporters of the fallen regime led by Bashar Assad, an Alawite, targeted security units. In retaliation, militias affiliated with the newly formed government in Damascus carried out indiscriminate killings of Alawites.

While exact figures remain difficult to verify, more than 1,300 individuals, most of them Alawites, lost their lives. In some cases, entire families were summarily executed.

Although the Syrian government promised an investigation into the atrocities, home invasions, kidnappings of Alawite women and extrajudicial executions of Alawite men continue.

The violence in Sweida also bore a sectarian dimension, pitting members of a religious minority against armed groups aligned with the country’s Sunni majority.

A key difference, however, involved the active Israeli support for the Druze and the U.S. efforts to broker a ceasefire.

Post-Assad Syria has seen promising developments, including the lifting of international sanctions, a resurgence of civil society and the end of diplomatic isolation. There was even a limited rapprochement with the main Kurdish political party controlling northeastern Syria.

The persistent violence targeting the Alawites and, to a more limited extent, the Druze, starkly contrasts with these trends. As a scholar of religious minorities and the Middle East, I argue that the current political situation reflects their historical persecution and marginalization.

History of the Alawites

The Alawites emerged as a distinct religious community in the 10th century in the region of the Latakia coastal mountains, which today make up northwestern Syria.

Although their beliefs have some commonalities with Shiite Islam, the Alawites maintain their own unique religious leadership and rituals. Under the Ottoman regime in the late 19th century, they benefited from reforms such as the expansion of educational opportunities and economic modernization, while gaining geographical and social mobility.

After Hafez Assad, the father of Bashar, came to power in a coup in 1970, he drew upon his Alawite base to reinforce his regime. Consequently, Alawites became disproportionately represented in the officer corps and intelligence services.

Prior to the civil war, which began in 2011, their population was estimated at around 2 million, constituting roughly 10% of Syria’s population. During the civil war, Alawite young men fighting for the regime suffered heavy casualties. However, most Alawites remained in Syria, while Sunni Arabs and Kurds were disproportionately displaced or became refugees.

Among Syria’s minorities, two key factors make the Alawites most vulnerable to mass violence in post-Assad Syria. The first factor is that, like the Druze, Alawites have their own distinct beliefs that deviate from Sunni Islam. Their religious practices and teachings are often described as “esoteric” and remain mostly inaccessible to outsiders.

In my 2024 book “Liminal Minorities: Religious Difference and Mass Violence in Muslim Societies,” I categorize the Alawites and Druze in Syria alongside Yezidis in Iraq, Alevis in Turkey and Baha’is in Iran as “liminal minorities” – religious groups subject to deep-seated stigmas transmitted across generations.

These groups are often treated as heretics who split from Islam and whose beliefs and rituals are deemed beyond the pale of acceptance. For instance, according to Alawite beliefs, Ali, the son-in-law of Prophet Muhammad, is a divine manifestation of God, which challenges the idea of strict monotheism central to Sunni Islam.

From the perspective of Sunni orthodoxy, these groups’ beliefs have been a source of suspicion and disdain. A series of fatwas by prominent Sunni clerics from the 14th to the 19th century declared Alawites heretics.

Resentment against the Alawites

The second factor contributing to the Alawites’ vulnerability is the widespread perception that they were the main beneficiaries of the Assad regime, which engaged in mass murder against its own citizens. Although power remained narrowly concentrated under Assad, many Alawites occupied key positions in the security apparatus as well as the government.

In today’s political landscape where the central government remains weak and its control over various armed groups is limited, religious stigmatization and political resentment create fertile ground for mass violence targeting the Alawites.

The massacres of March 2025 were accompanied by sectarian hate speech, including open calls for the extermination of the Alawites, both in the streets and on social media.

While many Sunni Muslims in Syria also perceive the Druze as heretics, they maintained a greater degree of distance from the Assad regime and were less integrated into its security apparatus.

Nonetheless, in recent months the situation deteriorated rapidly in the Druze heartland. The Druze militias and local Bedouin tribes engaged in heavy fighting in July 2025. Unlike the Alawites, the Druze received direct military assistance from Israel, which has its small but influential Druze population. This further complicates peaceful coexistence among religious groups in post-Assad Syria.

A sober future

Sunni Arab identity is central to the newly formed government in Damascus, which can come at the expense of religious and ethnic pluralism. However, it has incentives to rein in arbitrary violence against the Alawites and Druze. Projecting itself as a source of order and national unity helps the government internationally, both diplomatically and economically.

Internally, however, the new government remains fractured and lacks effective control over vast swaths of territory. While it pays lip service to transitional justice, it is also cautious about being perceived as overly lenient toward individuals associated with the Assad regime and its crimes. Meanwhile, Alawite and Druze demands for regional autonomy continue to stoke popular Sunni resentments and risk triggering further cycles of instability and violence.

I believe that in a post-Assad Syria defined by fractured governance and episodic retribution, the Alawites as well as Druze are likely to face deepening marginalization.

Güneş Murat Tezcür does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

– ref. Understanding the violence against Alawites and Druze in Syria after Assad – https://theconversation.com/understanding-the-violence-against-alawites-and-druze-in-syria-after-assad-255292

Source: The Conversation – USA – By Andrej Prša, Professor of Astrophysics and Planetary Science, Villanova University

Stars are the fundamental building blocks of our universe. Most stars host planets, like our Sun hosts our solar system, and if you look more broadly, groups of stars make up huge structures such as clusters and galaxies. So before astrophysicists can attempt to understand these large-scale structures, we first need to understand basic properties of stars, such as their mass, radius and temperature.

But measuring these basic properties has proved exceedingly difficult. This is because stars are quite literally at astronomical distances. If our Sun were a basketball on the East Coast of the U.S., then the closest star, Proxima, would be an orange in Hawaii. Even the world’s largest telescopes cannot resolve an orange in Hawaii. Measuring radii and masses of stars appears to be out of scientists’ reach.

Enter binary stars. Binaries are systems of two stars revolving around a mutual center of mass. Their motion is governed by Kepler’s harmonic law, which connects three important quantities: the sizes of each orbit, the time it takes for them to orbit, called the orbital period, and the total mass of the system.

I’m an astronomer, and my research team has been working on advancing our theoretical understanding and modeling approaches to binary stars and multiple stellar systems. For the past two decades we’ve also been pioneering the use of artificial intelligence in interpreting observations of these cornerstone celestial objects.

Measuring stellar masses

Astronomers can measure orbital size and period of a binary system easily enough from observations, so with those two pieces they can calculate the total mass of the system. Kepler’s harmonic law acts as a scale to weigh celestial bodies.

Think of a playground seesaw. If the two kids weigh about the same, they’ll have to sit at about the same distance from the midpoint. If, however, one child is bigger, he or she will have to sit closer, and the smaller kid farther from the midpoint.

It’s the same with stars: The more massive the star in a binary pair, the closer to the center it is and the slower it revolves about the center. When astronomers measure the speeds at which the stars move, they can also tell how large the stars’ orbits are, and as a result, what they must weigh.

Measuring stellar radii

Kepler’s harmonic law, unfortunately, tells astronomers nothing about the radii of stars. For those, astronomers rely on another serendipitous feature of Mother Nature.

Binary star orbits are oriented randomly. Sometimes, it happens that a telescope’s line of sight aligns with the plane a binary star system orbits on. This fortuitous alignment means the stars eclipse one another as they revolve about the center. The shapes of these eclipses allow astronomers to find out the stars’ radii using straightforward geometry. These systems are called eclipsing binary stars.

More than half of all Sun-like stars are found in binaries, and eclipsing binaries account for about 1% to 2% of all stars. That may sound low, but the universe is vast, so there are lots and lots of eclipsing systems out there – hundreds of millions in our galaxy alone.

By observing eclipsing binaries, astronomers can measure not only the masses and radii of stars but also how hot and how bright they are.

Complex problems require complex computing

Even with eclipsing binaries, measuring the properties of stars is no easy task. Stars are deformed as they rotate and pull on each other in a binary system. They interact, they irradiate one another, they can have spots and magnetic fields, and they can be tilted this way or that.

To study them, astronomers use complex models that have many knobs and switches. As an input, the models take parameters – for example, a star’s shape and size, its orbital properties, or how much light it emits – to predict how an observer would see such an eclipsing binary system.

Computer models take time. Computing model predictions typically takes a few minutes. To be sure that we can trust them, we need to try lots of parameter combinations – typically tens of millions.

This many combinations requires hundreds of millions of minutes of compute time, just to determine basic properties of stars. That amounts to over 200 years of computer time.

Computers linked in a cluster can compute faster, but even using a computer cluster, it takes three or more weeks to “solve,” or determine all the parameters for, a single binary. This challenge explains why there are only about 300 stars for which astronomers have accurate measurements of their fundamental parameters.

The models used to solve these systems have already been heavily optimized and can’t go much faster than they already do. So, researchers need an entirely new approach to reducing computing time.

Using deep learning

One solution my research team has explored involves deep-learning neural networks. The basic idea is simple: We wanted to replace a computationally expensive physical model with a much faster AI-based model.

First, we computed a huge database of predictions about a hypothetical binary star – using the features that astronomers can readily observe – where we varied the hypothetical binary star’s properties. We are talking hundreds of millions of parameter combinations. Then, we compared these results to the actual observations to see which ones best match up. AI and neural networks are ideally suited for this task.

In a nutshell, neural networks are mappings. They map a certain known input to a given output. In our case, they map the properties of eclipsing binaries to the expected predictions. Neural networks emulate the model of a binary but without having to account for all the complexity of the physical model.

We train the neural network by showing it each prediction from our database, along with the set of properties used to generate it. Once fully trained, the neural network will be able to accurately predict what astronomers should observe from the given properties of a binary system.

Compared to a few minutes of runtime for the physical model, a neural network uses artificial intelligence to get the same result within a tiny fraction of a second.

Reaping the benefits

A tiny fraction of a second works out to about a millionfold runtime reduction. This brings the time down from weeks on a supercomputer to mere minutes on a single laptop. It also means that we can analyze hundreds of thousands of binary systems in a couple of weeks on a computer cluster.

This reduction means we can obtain fundamental properties – stellar masses, radii, temperatures and luminosities – for every eclipsing binary star ever observed within a month or two. The big challenge remaining is to show that AI results really give the same results as the physical model.

This task is the crux of my team’s new paper. In it we’ve shown that, indeed, the AI-driven model yields the same results as the physical model across over 99% of parameter combinations. This result means the AI’s performance is robust. Our next step? Deploy the AI on all observed eclipsing binaries.

Best of all? While we applied this methodology to binaries, the basic principle applies to any complex physical model out there. Similar AI models are already speeding up many real-world applications, from weather forecasting to stock market analysis.

Andrej Prša receives funding from the National Aeronautics and Space Administration.

– ref. Binary star systems are complex astronomical objects − a new AI approach could pin down their properties quickly – https://theconversation.com/binary-star-systems-are-complex-astronomical-objects-a-new-ai-approach-could-pin-down-their-properties-quickly-253387

Source: The Conversation – USA – By Alex H. Poole, Associate Professor of Information Science, Drexel University

Carla Hayden, the 14th librarian of Congress, who has held the position since 2016, received an unexpected email on May 8, 2025.

“Carla, on behalf of President Donald J. Trump, I am writing to inform you that your position as the Librarian of Congress is terminated effective immediately. Thank you for your service,” wrote Trent Morse, deputy director of presidential personnel at the White House.

White House Press Secretary Karoline Leavitt later explained that Hayden, who was the first woman, Black person and professionally trained librarian to oversee the Library of Congress, had done “quite concerning things,” on the job, including “putting inappropriate books in the library for children.”

Democratic politicians sharply criticized Hayden’s termination, saying the firing was unjust. It was actually about Trump punishing civil servants “who don’t bend to his every will,” New York Sen. Chuck Schumer said.

An information science scholar, I have written extensively about the history of libraries and archives, including the Library of Congress. To fully understand the role Hayden played for the past nine years, I think it is important to understand what the Library of Congress does, and the overlooked and underappreciated role it has played in American life.

The Library of Congress’ work

The Library of Congress is an agency that was first established, by an act of Congress, in 1800. The act provided for “the purchase of such books as may be necessary for the use of Congress at the said city of Washington, and for fitting up a suitable apartment for containing them.” Its chief librarian is appointed by the president and confirmed by the Senate.

The library has six buildings in Washington that hold a print and online collection of nearly 26 million books, as well as more than 136 million other items, including manuscripts, maps, sheet music and prints and photographs.

It also houses historic documents, like Thomas Jefferson’s rough draft of the Declaration of Independence and James Madison’s notes on the 1787 Constitutional Convention.

The library is the property of the American people. Anyone over the age of 16 with a government-issued photo identification can enter its buildings and read or view its materials on-site. The Library of Congress was partially designed as a research institution to suit the needs of members of Congress, and only Congress members can borrow items from the library and take them home.

The Library of Congress has an annual budget of about US$900 million, with a staff of 3,263. In 2024, the library’s staff helped acquire 1,437,832 million new items, issue nearly 69,000 library cards and answer more than 764,000 reference requests, among other tasks.

The library’s deep roots

The library has evolved alongside the U.S. itself. Five years before the Constitutional Convention of 1787, future president James Madison called for a library to provide materials to help inform Congress and its members. In 1800, President John Adams signed a bill that established the institution, which began with a $5,000 government appropriation, equivalent to more than $127,000 today.

The library’s first collection included 152 works in 740 volumes imported from England. It occupied a space in a Washington Senate office that measured just 22 feet by 34 feet.

The British army torched the infant library and its collection that had grown to 3,000 books in 1814, during the War of 1812. In response, former president Thomas Jefferson sold his personal collection of 6,479 books to the library, which he called “unquestionably the choicest collection of books in the U.S.”

Tragedy struck again in 1851, with a fire that incinerated two-thirds of the library’s 55,000 volumes, including most of Jefferson’s personal collection.

The organization rebounded in the next few years, as it purchased the 40,000-volume Smithsonian library in 1866, among other new acquisitions.

Ainsworth Spofford, the sixth librarian of Congress, boosted the library’s national image in the late 1800s when he tried to centralize the country’s patchwork copyright system.

Spofford also successfully lobbied Congress to pass the Copyright Act of 1870, which stipulated that any party registering a work for copyright needed to deposit two copies of that work with the library.

A growing place in American life

As its collections burgeoned in both scale and scope in the latter part of the 19th century, the library assumed an increasingly visible role and became known by some as “the nation’s library.” By 1900, it had nearly 1 million printed books and other materials.

The opening of a new library building in 1897, offering services to blind people with a designated reading room containing 500 raised character – or braille – books and music items, epitomized the library’s new status.

President Theodore Roosevelt said in 1901 that the library was “the one national library of the United States” and that was “a unique opportunity to render to the libraries of this country – to American scholarship – service of the highest importance.”

The library’s work, and global approach, continued to grow during the 20th century.

By the late 1900s, the library held materials in more than 450 languages.

It continued to add remarkable items to its collection, including a Gutenberg Bible, the first book printed in Europe from movable metal type, a kind of printing technology, in 1455.

Documenting the evolution of democracy, the library also assumed stewardship of 23 presidents’ official papers, from George Washington to Calvin Coolidge, during this time frame.

A public service

While primarily designated a research institution for Congress, the library has also catered to a diverse range of patrons, including by mail and telephone.

As one Science Digest writer noted in 1960, reference staff members fielded questions ranging from “What was the color of a mastodon’s eye?” to “How many words are there in the English language?” and “Could you suggest a name for twins?”

The library’s register of copyrights received similarly diverse and even humorous inquiries. One older woman seeking to publish her poetry wrote in 1954 to request “a poetic license” to ensure her work conformed to the law.

In the late 20th century, the library focused on a new democratic national and international mission, as it embraced a new role. Daniel Boorstin, the librarian from 1975 to 1987, termed that role a “multimedia encyclopedia.”

A congressional resolution marking the Library of Congress’s bicentennial in 2000 noted that it was “the largest and most inclusive library in human history,” as it digitized its collections to extend its reach still further with the growth of the internet.

As the library marks its 225th year, it continues to represent, as David Mearns, chief of the library’s manuscript division, said in 1947, “the American story.”

A library for all

Following Hayden’s dismissal, Trump appointed Deputy Attorney General Todd Blanche, his former personal lawyer, as acting librarian of Congress.

Hayden has contended that her dismissal, which occurred alongside other firings of top civil servants, including the national archivist, represents a broad threat to people’s right to easily access free information.

“Democracies are not to be taken for granted,” Hayden said in June. She explained in an interview with CBS that she never had a problem with a presidential administration and is not sure why she was dismissed.

“And the institutions that support democracy should not be taken for granted,” Hayden added.

In her final annual report as librarian, Hayden characterized the institution as “truly, a library for all.” So far, even without her leadership, it remains just that.

Alex H. Poole does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

– ref. Trump has fired the head of the Library of Congress, but the 225-year-old institution remains a ‘library for all’ – so far – https://theconversation.com/trump-has-fired-the-head-of-the-library-of-congress-but-the-225-year-old-institution-remains-a-library-for-all-so-far-257508

Source: The Conversation – USA (2) – By Dana Zartner, Professor of International Studies, University of San Francisco

While the dangerous effects of climate change continue to worsen, legal efforts to address a range of environmental issues are also on the rise.

Headlines across the globe tout many of these legal actions: South Korea’s Climate Law Violates Rights of Future Generations; Ukraine is Ground Zero in Battle for Ecocide Law; Paris Wants to Grant the River Seine Legal Personhood; and Montana Court Rules Children Have the Right to a Healthy Environment, to name a few recent examples.

As an environmental lawyer, I see that most of these suits use one of five legal strategies that have been developed over the past couple of decades. These approaches vary in terms of who is filing the lawsuit, against whom, and whether the underlying legal perspective is based on protecting human rights or the rights of the environment itself. But they all share an innovative approach to protect all life on this planet.

1. Right to a healthy environment

In 2022, the United Nations declared that humans have “the right to a clean, healthy and sustainable environment … essential to protecting human life, well-being and dignity.” More than 150 countries have similar declarations in their constitutions or laws, often alongside protections for other human rights, such as those to education and medical care.

These rights are held by humans, so people can sue for alleged violations. Typically they sue one or more government agencies, whose responsibility it is to protect human rights.

One recent case using this approach was Held v. Montana, in which a group of young people in 2024 won a lawsuit against the state of Montana for violating the state constitution’s right to a “clean and healthful environment.” The state Supreme Court agreed with the plaintiffs and struck down a law barring the consideration of climate effects when evaluating proposals for fossil fuel extraction. Similar cases have been heard in the U.S. and other countries around the world.

2. The rights of future generations

A legal concept called “intergenerational equity” is the idea that present generations must “responsibly use and conserve natural resources for the benefit of future generations.” First codified in international law in the 1972 Stockholm Declaration, the principle has been gaining popularity in recent decades. International organizations and national governments have enshrined this principle in law.

Focused on humans’ rights, these laws allow people and groups to bring claims, usually against governments, for allowing activities that are altering the environment in ways that will harm future generations. One well-known case that relied on this legal principle is Future Generations v. Ministry of the Environment and Others, in which a Colombian court in 2018 agreed with young people who had sued, finding that the Colombian government’s allowance of “rampant deforestation in the Amazon” violated the pact of intergenerational equity.

3. Government responsibility

Another human-centered approach is the public trust doctrine, which establishes “that certain natural and cultural resources are preserved for public use” and that governments have a responsibility to protect them for everyone’s benefit.

While the concept of “public trust” has long existed in the law, recently it has been used to bring suit against governments for their failure to address climate change and other environmental degradation. In Urgenda Foundation v. the State of the Netherlands, a Dutch court held in 2019 that the government has a responsibility to mitigate the effects of climate change due to the “severity of the consequences of climate change and the great risk of climate change occurring.” Since the decision, the Dutch government has sought to reduce emissions by phasing out the use of coal, increasing reliance on renewable energy and aiming to achieve carbon neutrality by 2050.

Government responsibility for the public trust was also a basis of the Juliana v. U.S. case, where a group of young people sued the U.S. government for breaching the public trust by not doing enough to curb greenhouse gas emissions. The U.S. Supreme Court ultimately declined to hear an appeal of a lower court’s ruling, but the lack of a specific ruling by the nation’s highest court has given continued hope to new cases, which continue to be filed based on the same principle.

4. Rights of nature

The rights of nature is one of the fastest-growing environmental legal strategies of the past decade. Since Ecuador recognized the rights of Pachamama, the Quechua name for Mother Earth, in its Constitution in 2008, more than 500 laws on the rights of nature have been enacted around the world.

The principle recognizes the legal rights of natural entities, such as rivers, mountains, ecosystems or even something as specific as wild rice. The laws that grant these rights don’t focus on humans but rather nature itself, often including language that the natural entity has the right to “exist and persist.”

The laws then provide a mechanism for the natural entity – whether through a specific group assigned legal guardianship or other community efforts – to protect itself by filing lawsuits in court. In the 2018 Colombian case, the court found that the Amazon ecosystem has rights, which must be respected and protected.

Similarly, in Bangladesh in 2019 the courts recognized the rights of all the country’s rivers, requiring, among other things, a halt on damaging development along the rivers that block their natural flow. The court also created a commission to serve as legal guardians of the country’s rivers.

5. Defining a new crime: Ecocide

In 2024, the governments of Vanuatu, Fiji and Samoa formally proposed that the international community recognize a new crime under international law. Called “ecocide,” the principle takes a nature-focused approach and includes any unlawful act committed with “the knowledge that there is a substantial likelihood of severe and either widespread or long-term damage to the environment.”

Put another way, what genocide is to humans, ecocide is to nature. It is being proposed as an addition to the 2002 Rome Statute, which created the International Criminal Court to prosecute war crimes, genocide and crimes against humanity.

While the idea is relatively new, in addition to the international efforts, several countries have incorporated ecocide into their laws – including Vietnam, France, Chile and Ukraine. A Ukrainian prosecutor is currently investigating the June 2023 destruction of a dam in a Russian-occupied area of the country as a potential crime of ecocide, because of the widespread flooding and habitat destruction that resulted.

The European Union has also incorporated ecocide into its Environmental Crime Directive, which applies to all EU member countries, providing them with a mechanism to hear ecocide claims in their national courts.

Using these ideas

Each of these legal concepts has the potential to increase protection for the environment – and the people who live in it. But determining which strategy has the greatest chance of success depends on the details of the existing law and legal system in each community.

All of these legal strategies have a role in the fight to protect and preserve the environment as an integral, interdependent living thing that is vitally important to us as humans but also in its own right.

Dana Zartner is a volunteer with the Earth Law Center assisting with the editing of toolkits and guides, but has not worked on any of its lawsuits.

– ref. How the nature of environmental law is changing in defense of the planet and the climate – https://theconversation.com/how-the-nature-of-environmental-law-is-changing-in-defense-of-the-planet-and-the-climate-258982

Source: The Conversation – USA (2) – By Ian Smith, Research Scientist in Earth & Environment, Boston University

When summer turns up the heat, cities can start to feel like an oven, as buildings and pavement trap the sun’s warmth and vehicles and air conditioners release more heat into the air.

The temperature in an urban neighborhood with few trees can be more than 10 degrees Fahrenheit (5.5 Celsius) higher than in nearby suburbs. That means air conditioning works harder, straining the electrical grid and leaving communities vulnerable to power outages.

There are some proven steps that cities can take to help cool the air – planting trees that provide shade and moisture, for example, or creating cool roofs that reflect solar energy away from the neighborhood rather than absorbing it.

But do these steps pay off everywhere?

We study heat risk in cities as urban ecologists and have been exploring the impact of tree-planting and reflective roofs in different cities and different neighborhoods across cities. What we’re learning can help cities and homeowners be more targeted in their efforts to beat the heat.

The wonder of trees

Urban trees offer a natural defense against rising temperatures. They cast shade and release water vapor through their leaves, a process akin to human sweating. That cools the surrounding air and reduces afternoon heat.

Adding trees to city streets, parks and residential yards can make a meaningful difference in how hot a neighborhood feels, with blocks that have tree canopies nearly 3 F (1.7 C) cooler than blocks without trees.

But planting trees isn’t always simple.

In hot, dry cities, trees often require irrigation to survive, which can strain already limited water resources. Trees must survive for decades to grow large enough to provide shade and release enough water vapor to reduce air temperatures.

Annual maintenance costs – about US$900 per tree per year in Boston – can surpass the initial planting investment.

Most challenging of all, dense urban neighborhoods where heat is most intense are often too packed with buildings and roads to grow more trees.

How cool roofs can help on hot days

Another option is “cool roofs.” Coating rooftops with reflective paint or using light-colored materials allows buildings to reflect more sunlight back into the atmosphere rather than absorbing it as heat.

These roofs can lower the temperature inside an apartment building without air conditioning by about 2 to 6 F (1 to 3.3 C), and can cut peak cooling demand by as much as 27% in air-conditioned buildings, one study found. They can also provide immediate relief by reducing outdoor temperatures in densely populated areas. The maintenance costs are also lower than expanding urban forests.

However, like trees, cool roofs come with limits. Cool roofs work better on flat roofs than sloped roofs with shingles, as flat roofs are often covered by heat-trapping rubber and are exposed to more direct sunlight over the course of an afternoon.

Cities also have a finite number of rooftops that can be retrofitted. And in cities that already have many light-colored roofs, a few more might help lower cooling costs in those buildings, but they won’t do much more for the neighborhood.

By weighing the trade-offs of both strategies, cities can design location-specific plans to beat the heat.

Choosing the right mix of cooling solutions

Many cities around the world have taken steps to adapt to extreme heat, with tree planting and cool roof programs that implement reflectivity requirements or incentivize cool roof adoption.

In Detroit, nonprofit organizations have planted more than 166,000 trees since 1989. In Los Angeles, building codes now require new residential roofs to meet specific reflectivity standards.

In a recent study, we analyzed Boston’s potential to lower heat in vulnerable neighborhoods across the city. The results demonstrate how a balanced, budget-conscious strategy could deliver significant cooling benefits.

For example, we found that planting trees can cool the air 35% more than installing cool roofs in places where trees can actually be planted.

However, many of the best places for new trees in Boston aren’t in the neighborhoods that need help. In these neighborhoods, we found that reflective roofs were the better choice.

By investing less than 1% of the city’s annual operating budget, about US$34 million, in 2,500 new trees and 3,000 cool roofs targeting the most at-risk areas, we found that Boston could reduce heat exposure for nearly 80,000 residents. The results would reduce summertime afternoon air temperatures by over 1 F (0.6 C) in those neighborhoods.

While that reduction might seem modest, reductions of this magnitude have been found to dramatically reduce heat-related illness and death, increase labor productivity and reduce energy costs associated with building cooling.

Not every city will benefit from the same mix. Boston’s urban landscape includes many flat, black rooftops that reflect only about 12% of sunlight, making cool roofs that reflect over 65% of sunlight an especially effective intervention. Boston also has a relatively moist growing season that supports a thriving urban tree canopy, making both solutions viable.

In places with fewer flat, dark rooftops suitable for cool roof conversion, tree planting may offer more value. Conversely, in cities with little room left for new trees or where extreme heat and drought limit tree survival, cool roofs may be the better bet.

Phoenix, for example, already has many light-colored roofs. Trees might be an option there, but they will require irrigation.

Getting the solutions where people need them

Adding shade along sidewalks can do double-duty by giving pedestrians a place to get out of the sun and cooling buildings. In New York City, for example, street trees account for an estimated 25% of the entire urban forest.

Cool roofs can be more difficult for a government to implement because they require working with building owners. That often means cities need to provide incentives. Louisville, Kentucky, for example, offers rebates of up to $2,000 for homeowners who install reflective roofing materials, and up to $5,000 for commercial businesses with flat roofs that use reflective coatings.

Efforts like these can help spread cool roof benefits across densely populated neighborhoods that need cooling help most.

As climate change drives more frequent and intense urban heat, cities have powerful tools for lowering the temperature. With some attention to what already exists and what’s feasible, they can find the right budget-conscious strategy that will deliver cooling benefits for everyone.

Lucy Hutyra has received funding from the U.S. federal government and foundations including the World Resources Institute and Burroughs Wellcome Fund for her scholarship on urban climate and mitigation strategies. She was a recipient of a 2023 MacArthur Fellowship for her work in this area.

Ian Smith does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

– ref. 2 ways cities can beat the heat: Which is best, urban trees or cool roofs? – https://theconversation.com/2-ways-cities-can-beat-the-heat-which-is-best-urban-trees-or-cool-roofs-260188