There was something wrong with the chimpanzees. For weeks, a community of 205 animals in Uganda’s Kibale national park had been coughing, sneezing and looking generally miserable. But no one could say for sure what ailed them, even as the animals began to die.

Necropsies can help to identify a cause of death, but normally, the bodies of chimps are found long after decomposition has set in, if at all. So when Tony Goldberg, a US wildlife epidemiologist visiting Kibale, got word that an adult female named Stella had been found freshly dead, he knew this was a rare opportunity to look for an answer.

Goldberg and two Ugandan veterinary colleagues drove for two hours to a remote part of the park, then lugged their gear for another hour through the forested terrain to where Stella’s body lay. They lifted the 45kg animal on to a tarpaulin, and got to work. Crouching over the chimp – sweating beneath their full-body protective suits, their goggles fogging in the humid air – they meticulously worked through Stella’s organ systems, collecting samples. Not knowing what had killed her was “unnerving”, Goldberg recalls. “It could have been Ebola.”

As the necropsy progressed, however, Goldberg began to see telltale signs of a familiar disease: fluid buildup in Stella’s chest cavity and around her heart; lung tissue that was dark red, consolidated and marked with lesions. It looked like the chimp had died of severe pneumonia.

Months later, molecular testing revealed the culprit: human metapneumovirus (HMPV), one of a collection of viruses that presents in people as a common cold but is “a well-known killer” in our closest primate relatives, says Goldberg, a researcher at the University of Wisconsin-Madison. More than 12% of the community that Stella belonged to died in the outbreak. Others were lost as a result of being orphaned. “Stella had a baby that was clinging to her body for a while after she died,” Goldberg says. “The baby subsequently died.”

This phenomenon of animals catching diseases from humans, called reverse zoonoses, affects species around the world – from mussels contaminated with hepatitis A virus to tuberculosis transmitted to Asian elephants. But because of their evolutionary closeness to humans, great apes tend to be most vulnerable.

For some great ape populations that live in protected areas, reverse zoonoses are an even bigger threat than habitat loss or poaching. In a group at Kibale, for example, respiratory pathogens such as human rhinovirus C and HMPV have been the leading chimp killers for more than 35 years, accounting for almost 59% of deaths from a known cause.

For conservationists, the phenomenon presents a thorny problem. In many places in Africa, people live in close proximity to great apes. Great ape tourism has also become a central pillar of these endangered species’ conservation: ensuring habitats are preserved and local people are incentivised to support wildlife. But the same industry that funds protection of many apes could also be helping to drive them towards extinction, as close proximity to humans can expose the animals to deadly pathogens.

Tourism is necessary for conservation, says Gladys Kalema-Zikusoka, a wildlife veterinarian and founder of Conservation Through Public Health, a nonprofit group in Entebbe, Uganda. “But it needs to be done carefully, otherwise we won’t have these animals around.”

Some of the first records of reverse zoonoses in great apes were made by British primatologist Jane Goodall. In 1986, Goodall wrote that chimps “quite often” had colds and coughs, and “can contract the same contagious diseases as humans”. But conclusive evidence that chimps were being infected by people didn’t arrive until 2008, when Fabian Leendertz, the director of the Helmholtz Institute for One Health in Greifswald, Germany and his colleagues used molecular tools to show that human viruses were to blame for a decade’s worth of major respiratory disease outbreaks in chimps in Taï national park, Côte d’Ivoire.

Since the paper came out, habitat destruction, human encroachment, the climate crisis and globalisation have only accelerated, and all of Africa’s great ape species are now decreasing. Eastern and western gorillas are both critically endangered, while chimpanzees and bonobos are endangered. The fact that human diseases can take out significant proportions of great ape communities makes the pathogens a dire threat to all four species. “Great ape populations can’t afford these sorts of losses,” Goldberg says. “Their populations are already so small, fragmented and declining that they just don’t have the ability to rebound or adapt.”

Many of the pathogens cause infections that, in a person, would result in an annoying but mild cold. In great apes, however, these diseases can be deadly, because the animals have no immunity or evolved genetic resistance. Once a chimpanzee or gorilla becomes unwell, there is usually little that can be done to help. There are also no vaccines for most common cold viruses.

What could work, Goldberg realised, was a public-health approach: finding the source of pathogens and preventing them from getting into the populations in the first place.

In 2015, the International Union for Conservation of Nature (IUCN) released guidelines for great ape tourism, recommending that people stay at least 7 metres away from animals, tour groups limit their size, all visitors wear face masks and people who feel unwell be excluded.

But there are obvious reasons why that doesn’t always happen. For one, it relies on honesty from international visitors. “Imagine you’re an American tourist, you’ve gone all the way to Africa, and this is your bucket-list trip,” Goldberg says. “Now you’ve got a stomach ache – and you’re not going to go see the gorillas? Of course you are.”

Tourists often break rules while out in the field, either because of excitement in the moment or wilful disregard. “Some tourists just don’t listen,” says Kalema-Zikusoka. And local guides might or might not correct them. “They don’t want to be rude, and they find it hard to manage tourists.”

Guides may also refrain from reproaching visitors for fear of losing a potential tip. Some guides “get tips that are twice the monthly salary of typical villagers in the area”, Goldberg says. “There are all these perverse incentives.”

One 2020 study that analysed 282 YouTube videos of mountain gorilla tourism found that 40% depicted humans within arm’s reach of gorillas or engaging in physical contact with the animals.

In another 2020 study, Darcey Glasser, then a graduate student at Hunter College of the City University of New York, joined 101 chimp treks at Kibale. Glasser observed tourists coughing during 88% of excursions; sneezing in 65%; and urinating in 37%. “Everyone’s touching everything,” she says.

Glasser presented her findings to wildlife officials in Uganda, who responded encouragingly, she says, adding hand-sanitising stations at the start of trails. In general, however, officials tend to avoid imposing strict rules that they think may impact visitors’ experiences.

Great ape tourism is a key source of revenue for the 13 African countries where it occurs, Leendertz says, so reverse zoonosis is “not always an easy topic”. Officials at the Uganda Wildlife Authority, which oversees the country’s national parks and all tourism activity in them, did not respond to multiple interview requests.

Ecotourism represents one serious disease risk for great apes, but it cannot account for all cases of reverse zoonoses. Some great ape populations never see a tour group – Stella’s community among them – yet still experience deadly outbreaks of human pathogens.

As Goldberg thought about how to tackle this problem, he noticed a perplexing pattern in the list of human pathogens that typically afflict great apes: they’re the infections that, like clockwork, young children catch when they go back to school, and then bring home.

Great apes, it occurred to him, could be catching diseases from adults who go into the forest after catching pathogens from their children. The idea seemed even more plausible when Goldberg realised that adults infected with these “sniffle germs” often show no symptoms, even as they shed copious viral particles.

Goldberg secured a grant for new research, led by Taylor Weary, an epidemiologist who recently graduated from Goldberg’s lab, alongside Patrick Tusiime, health coordinator for the Kasiisi Project, a nonprofit group that supports primary schools around Kibale. They compared monthly nasal swabs from local schoolchildren, parents who worked in the forest, and faecal samples from the chimps.

The findings, which are now in review for publication, confirmed Goldberg’s original hypothesis. Every respiratory pathogen that has caused a chimp outbreak in Kibale was present in children living nearby. Then, during Uganda’s most stringent Covid-19 lockdown between March and September 2020, the researchers observed an “extraordinarily clear” drop in infections across the board, Goldberg says, suggesting that schools are indeed a major source of transmission.

The message, Goldberg says, was clear: “To save the chimps, we have to make kids healthier.”

One big takeaway from the findings was that the current model is inadequate to reduce the risk of reverse zoonoses in Kibale’s chimpanzees, and probably in great apes in Africa as a whole. It hinges on stopping symptomatic people from going into the forest, but infected adults are usually asymptomatic. Forbidding guides and trackers from working whenever their children are ill isn’t a solution, Goldberg says: kids “are sick all the time”.

Banning tourism also wouldn’t work. Parks depend on visitor fees to pay salaries, maintain local support for conservation and justify the cost of setting land aside for wildlife. “When I was growing up, the perception was that chimps are bad,” says Tusiime, who was born in a rural village near Kibale. “Now there’s a shift to a positive attitude towards chimpanzees because they bring in tourists, they bring in revenue.”

Focusing on making children living near great apes healthier, then, could be the best bet for keeping human diseases out of great ape populations. Programmes have already been launched to reduce transmission among local children, teaching handwashing and other hygiene measures.

Scientists also believe that enforcement of existing biosecurity rules could go a long way toward reducing transmission – but that will require focused commitment from African governments and tourism providers, says Cristina Gomes, a wildlife conservationist at Florida International University in Miami who helped launch a working group to identify new strategies. One idea is to entitle guides working with chimps to paid sick days – a luxury most do not have. Another suggestion is to certify companies that follow best practices, justifying a slightly higher fee for their services.

Common cold viruses cannot be eradicated, and people and great apes won’t be staying apart anytime soon. Goldberg says outbreaks of respiratory disease were documented in chimps in at least five locations throughout sub-Saharan Africa in 2023 alone.

The hope, however, is that these will become rarer as scientists, officials, rural residents and tourists gain a deeper understanding of the problem. “Behavioural change takes time, but if you’re committed, it eventually happens,” Tusiime says. “So we need to start now.”

A version of this report was previously published in Nature

You might visit Amsterdam for its canals, and who could blame you, really. But the truly interesting waterways aren’t under your feet – they’re above your head.

Beautiful green roofs have popped up all over the world: specially selected plants growing on structures designed to manage the extra weight of biomass. Amsterdam has taken that one step further with blue-green roofs, specially designed to capture rainwater. One project, the resilience network of smart, innovative, climate-adaptive rooftops (Resilio), has covered more than 9,000 sq metres (100,000 sq ft) of Amsterdam’s roofs, including 8,000 sq metres on social housing complexes. Citywide, the blue-green roof coverage is even bigger, estimated at more than 45,000 sq metres.

The “sponge city” concept is becoming increasingly popular. Planners deploy more green spaces that soak up downpours that are getting heavier as the world warms. That simultaneously reduces flooding and recharges the underlying layer of absorbent rock, which can then be tapped into in times of need. Whereas cities used to be designed to divert rainwater away as quickly as possible, increasingly, they are exploiting that resource.

A big challenge with sponge cities is that so much of an urban area is rooftops. Green roofs will soak up some rainwater to hydrate the plants there, but blue-green roofs go a step further, with infrastructure that gathers the liquid, stores it and dispenses it to the building’s residents for watering plants and flushing toilets.

The system works in layers. At the surface, you have plants: some combination of mosses, shrubs, grasses, ferns, herbs and sedum, a hardy genus that’s a staple of green roofs. (While plants need sunlight to survive, on a roof, they can be bombarded with too much light. It can also get hot and windy up there.) The plants are rooted in soil, providing nutrients and support.

Below that is a filter layer, which keeps the soil from getting into the next layer: a lightweight crate system that stores the water. Finally, below that are additional layers to keep water and plant roots from infiltrating the actual roof. “You have, in fact, a flat rain barrel on top of your roof,” says Kasper Spaan, policy developer for climate adaptation at Waternet, Amsterdam’s public water management organisation, which is participating in Resilio.

The water levels in the blue-green roof are managed by a smart valve. If the forecast says a storm is coming, the system will release stored water from the roof ahead of time. That way, when a downpour comes, the roof refills, meaning less rainwater enters the gutters and sewers in the surrounding area. In other words, the roof becomes a sponge that can be wrung out as needed. “In the ‘squeezable’ sponge city, you make the whole city malleable,” says Spaan.

This makes the traditional system of stormwater management more flexible, but also more complicated. So the Resilio project used software from Autodesk to model the impact of blue-green roofs and the risk of flooding in Amsterdam, also adjusting for climate breakdown.

“You can take a look at historical flood patterns and then you can do simulations that will help you understand. If I could take this much capacity out of the drainage network, when the storm comes, I’m going reduce flooding by 10, 15, 20%,” says Amy Bunszel, Autodesk’s executive vice-president of architecture, engineering and construction design solutions. “So our software allows them to do simulations and play with different trade-offs.”

Beyond the sponge-city benefits, blue-green roofs can cool the top floor of a building, essentially “sweating” off the stored water. With the right kinds of indigenous plant, they can help wildlife by catering to native pollinating insects. Going a step further, scientists are experimenting with growing crops on rooftops under solar panels, known as rooftop agrivoltaics. Theoretically, pairing that with blue-green systems could improve the efficiency of the solar panels by cooling them with the evaporating water.

Not every building can go blue-green. The additional infrastructure is not very heavy, but the water it holds is. So while it is relatively cheap and easy to build the system into new construction, accounting for the extra weight, older buildings may need retrofits to accommodate it. In the long term, it can save a building money by reducing the volume of water bought from a municipal system. Like any technology, its cost will fall as it is more widely deployed.

The idea is for places experiencing worsening droughts and flooding to deploy not only sponge city concepts on the ground – like patches of dirt with drought-tolerant plants to absorb stormwater into aquifers – but on top of their buildings as well. “We think the concept is applicable to many urban areas around the world,” says Spaan. “In the south of Europe – Italy and Spain – where there are really drought-stressed areas, there’s new attention for rainwater catchment.”

Cities could even incentivise blue-green roofs by providing tax breaks, rewarding building owners for reducing their contribution of stormwater to overburdened sewer and water systems. US cities such as Los Angeles and Pittsburgh have been rolling out something similar: taxes on the amount of impermeable area on a property, encouraging landowners to develop gardens and other green spaces.

The city of tomorrow, then, isn’t the concrete-smothered metropolis of science fiction, but an increasingly green and spongey landscape that can be squeezed in times of need. “Our philosophy in the end is not that on every roof, everything is possible,” says Spaan, “but that on every roof, something is possible.”

Rapidly rising levels of TFA, a class of “forever chemical” thought to damage fertility and child development, are being found in drinking water, blood and rain, causing alarm among experts.

TFA, or trifluoroacetic acid, is a type of per- and polyfluoroalkyl substance (PFAS), a group of human-made chemicals used widely in consumer products that do not break down for thousands of years. Many of the substances have been linked to negative effects on human health.

Studies from across the world are reporting sharp rises in TFA. A major source is F-gases, which were brought in to replace ozone-depleting CFCs in refrigeration, air conditioning, aerosol sprays and heat pumps. Pesticides, dyes and pharmaceuticals can also be sources.

“Everywhere you look it’s increasing. There’s no study where the concentration of TFA hasn’t increased,” said David Behringer, an environmental consultant who has studied TFA in rain for the German government.

“If you’re drinking water, you’re drinking a lot of TFA, wherever you are in the world … China had a 17-fold increase of TFA in surface waters in a decade, the US had a sixfold increase in 23 years.” TFA in rainwater in Germany has been found to have increased fivefold in two decades.

“I’m worried about this because we’ve never seen in recent history a chemical that’s accumulating in so many media at such a high rate,” said Hans Peter Arp from the Norwegian Geotechnical Institute and the Norwegian University of Science and Technology. “It’s accumulating in our tap water, the food we’re eating, plants, trees, the sea, and all in the past few decades.”

He added: “We all have been experiencing rising TFA concentrations in our blood since the Montreal protocol [banned CFCs]. Future generations will have increasing concentrations in their blood until some kind of global action is taken. Accumulation [in the environment] is essentially irreversible and I’m afraid the impact on humans and the environment won’t be recognised by scientists until it is too late.”

Last month, the German chemical regulator informed the European Chemicals Agency that it wanted TFA classified as reprotoxic, meaning it can harm human reproductive function, fertility and foetal development.

Denmark and Germany have set limits for TFA in drinking water but the UK has not. England’s water companies have been asked to assess their drinking water sources for 47 types of PFAS but TFA is not on the list.

Britain’s Health and Safety Executive has identified TFA as “a substance of concern, since there are indications that it might cause developmental toxicity” and the Environment Agency says it is planning a targeted programme to test for TFA in surface and groundwater.

A Department for Environment, Food and Rural Affairs spokesperson said it would continue to “assess levels of PFAS occurring in the environment, their sources, potential risks and to inform policy and regulatory approaches.

“Regulations require that drinking water must not contain any substance at a level which would constitute a potential danger to human health. Should TFA be detected in drinking water we would expect companies to react in the same way as for other PFAS compounds.”

But TFA is incredibly difficult to remove from water. “There’s no way to get TFA out,” said Behringer. “Reverse osmosis is massively expensive and not scalable, so the logical course is to stop the input.”

The European Fluorocarbons Technical Committee, representing the F-gas and chemicals industry, says TFA occurs naturally in large quantities in the environment. It says industrial use of TFA is limited and environmental releases are very low. It did not respond to a Guardian request for comment.

But these assertions have been disputed. The US Environmental Protection Agency says TFAs are a breakdown product of F-gases. Moreover, studies of Arctic ice cores show TFA levels have been rising sharply since F-gases replaced CFCs in the 1990s.

“Every time the industry says it’s natural, they quote certain scientific papers,” says Prof Shira Jourdan, an environmental analytical chemist at the University of Alberta. She said she had studied these decades-old papers and found they only suggested it was possible that TFA was naturally occurring because of a lack of knowledge of its origins at the time of the studies.

“None of the evidence says it’s natural,” said Jourdan. “When industry says it’s natural it’s a danger, because then no one takes accountability for the pollution.”

Ariana Spentzos, of the NGO Green Science Policy, said: “We’re following the familiar PFAS playbook by allowing reckless environmental contamination and only figuring out after the fact the trail of harm left behind. We are just beginning to understand the health hazards associated with TFA.”

Environmental groups are calling on the UK government to take more action to tackle PFAS substances. “PFAS presents a global chemical pollution crisis which requires urgent action,” said Hannah Evans, from the campaign group Fidra. “We’re calling on the UK government to prevent PFAS emissions at source, which includes revising both F-gas and pesticide regulations to phase out PFAS.”

The German Environment Agency recommends using natural refrigerants instead. Its president, Dirk Messner, said: “TFA is found everywhere – in water, soil, food and the human body. It does not break down and can hardly be removed from drinking water. However, TFA-forming chemicals are numerous and on the rise. Persistent substances from multiple sources like TFA fall through the regulatory cracks. To reduce the release of TFA into the environment, we need consistent, precautionary regulation, cross-sectoral minimisation and a substitution with TFA-free alternatives wherever possible.“

Blue-green algae, or cyanobacteria, come in many forms and have generally got a bad press, mainly because five of the 2,000 identified species can produce some of the deadliest toxins known to science.

At the same time, they are among the oldest organisms in the world, dating back 2.1bn years, and we owe them a debt of gratitude.

Cyanobacteria were the first organisms to use photosynthesis, turning carbon dioxide into oxygen. They are responsible for creating the oxygen-rich atmosphere that enabled life on Earth to flourish and humans to evolve.

In their untold trillions, in almost every environment where there is water, even on damp rocks in deserts, they continue this valuable service, keeping the atmosphere safe for mammals to breathe.

But in nutrient-rich water, created by farm waste or sewage released into rivers and lakes, blue-green algae multiply fast, especially in warm sunshine. This is dangerous in still waters where they form dense rafts of scum that deprive the waters below of oxygen, killing fish.

In some circumstances they also create toxins that can poison animals and humans that drink it. Only laboratory tests can establish whether such algae blooms are toxic but anyone seeing one is advised to avoid it and report its presence.

Campaigners are blaming developed countries for capitulating at the last minute to pressure from fossil fuel and industry lobbyists, and slowing progress towards the first global treaty to cut plastic waste.

Delegates concluded talks in Ottawa, Canada, late on Monday, with no agreement on a proposal for global reductions in the $712bn (£610bn) plastic production industry by 2040 to address twin issues of plastic waste and huge carbon emissions.

They agreed to hold more discussions before the last summit on the treaty in Busan, South Korea, in November.

But two years on from a historic agreement in Nairobi to forge a global treaty to cut plastic waste, delegates said countries were just wasting time. A proposal from Peru and Rwanda to address for the first time the scale of plastic production in order cut waste was supported by 29 countries including Australia, Denmark, Nigeria, Portugal, the Netherlands and Nigeria, who signed a declaration, “the Bridge to Busan”, calling on all delegates to ensure plastic production was addressed.

The UK and US did not support the proposal to cut plastic production.

Juliet Kabera, the director general of the Rwanda environment management authority, said: “Rwanda’s vision for the treaty is to achieve sustainable production of plastics. We need a global target based on science to measure our collective actions.”

But as talks headed into the night on Monday, there was no agreement on putting plastic production at the centre of the treaty.

David Azoulay, the director of environmental health at the Center for International Environmental Law (CIEL), said while a handful of countries had taken a stand to keep ambitious proposals alive, most countries accepted a compromise at the last minute that played into the hands of petrostates and industry influences.

“From the beginning of negotiations, we have known that we need to cut plastic production to adopt a treaty that lives up to the promise envisioned … two years ago,” he said. “In Ottawa, we saw many countries rightly assert that it is important for the treaty to address production of primary plastic polymers.

“But when the time came to go beyond issuing empty declarations and fight for work to support the development of an effective intersessional programme, we saw the same developed member states who claim to be leading the world towards a world free from plastic pollution, abandon all pretence as soon as the biggest polluters look sideways at them.”

The US was singled out for criticism for blocking talks on cutting plastic production.

“The United States needs to stop pretending to be a leader and own the failure it has created here,” said Carroll Muffett, the president of CIEL. “When the world’s biggest exporter of oil and gas, and one of the biggest architects of the plastic expansion, says that it will ignore plastic production at the expense of the health, rights and lives of its own people, the world listens.”

He said that despite signalling at the G7 summit this month that it would commit to reduce plastic production, in Ottawa the US failed to follow through on its promises.

The failure to pursue ambitious cuts to plastic production came after a record number of fossil fuel and petrochemical lobbyists attended the summit in Canada.

Graham Forbes, Greenpeace’s head of delegation to the global plastics treaty negotiations, said: “The world is burning and member states are wasting time and opportunity. We saw some progress, aided by the continued efforts of states such as Rwanda, Peru, and the signatories of the Bridge to Busan declaration in pushing to reduce plastic production.

“However, compromises were made on the outcome which disregarded plastic production cuts, further distancing us from reaching a treaty that science requires and justice demands.”

Rich Gower, a senior economist at the NGO Tearfund, said: “An ambitious and effective treaty is still possible, but negotiations are on a knife-edge: time is short and strong opposition remains from the petrochemicals industry and states connected with it, even as their products pile up on street corners and in watercourses around the world.”

Representatives of the petrochemical industry said they were committed to a global treaty to cut plastic waste. But they pushed back on reductions in plastic production, an industry worth $712bn in 2023.

Chris Jahn, the council secretary of the International Council of Chemical Associations (ICCA), speaking on behalf of the industry group Global Partners for Plastics Circularity, said: “Our industry is fully committed to a legally binding agreement all countries can join that ends plastic pollution without eliminating the massive societal benefits plastics provide for a healthier and more sustainable world. We will continue to support governments’ efforts by bringing forth science-based and constructive solutions that leverage the innovations and technical expertise of our industry.”