By Sophie Hares
TEPIC, Mexico, Feb 15 (Thomson Reuters Foundation) - Rio de Janeiro is sweltering in blazing temperatures as it gears up for its spectacular carnival later this month, but in its northern favelas, the mercury is spiking even higher as hot, tropical air trapped in the narrow concrete alleys causes stifling conditions and a potential rise in health problems.
Christovam Barcellos, coordinator of the climate and health observatory at Brazil's Fiocruz government health institute, noted that on summer days, when the temperature is around 32 degrees Celsius (89.6°F) in Rio's southern zone, it is 3 to 4 degrees higher in northern parts of the Brazilian city.
"It's really significant - not only for mosquito-transmitted diseases but also in terms of health for vulnerable people with heart diseases (or) hypertension," he said.
In cities across Latin America, haphazard development combined with heat pumped out by cars, factories and buildings is causing "urban heat islands", where city temperatures are higher than their surrounding areas.
Temperature differences are usually greatest at night when stagnant warm air becomes trapped.
While daytime temperatures often differ by a few degrees Celsius, some areas experience substantial spikes. In Rio's hottest suburbs, for example, temperatures can be as much as 20 degrees Celsius higher than around the city, said Andrews Lucena, associate professor at Rio's Federal Rural University.
"This is a problem that's going to become critical," said Jennifer Doherty-Bigara, climate specialist at the Inter-American Development Bank.
"When we think about cities, we shouldn't only think about the economic opportunities. If we don't have climate-friendly cities, it will have an impact on the same productivity we're trying to foster," she said.
While megacities such as Mexico City, Sao Paulo and Rio de Janeiro have long been affected by the heat island effect, other cities such as Santiago, Lima and Buenos Aires are now suffering from it too, climate experts said.
Changing building design and materials, enhancing airflows and simply planting trees could help make cities and their residents more resilient to the urban heat island effect, which can worsen pollution, boost energy demand and even curb economic activity, said experts.
"We think of cities as a series of canyons with the streets lined with buildings on either side so they are very efficient in trapping the heat... You are bombarded by reflections and re-re-re-reflections from every building around," said Rohinton Emmanuel, professor of sustainable design at Glasgow Caledonian University.
"Each of us is equivalent to a 100-watt bulb walking around, and that generates more heat too," he said, explaining that air pollution can also stop heat escaping.
While specific data on Latin America is limited, a 2015 global study showed significant increases in urban heatwaves over a 40-year period, alongside a fall in city winds, according to its co-author Dennis Lettenmaier. Minimum temperatures are rising quicker than maximum temperatures around the world, causing more hot nights, he added.
Urban development is linked to this trend, although the warming of the planet may be a larger contributor, said Lettenmaier, a geography professor at the University of California.
"But if you don't have air conditioning and you're in a major city and you get a big heatwave, does it really make a difference to you why? Whether it's city development or whether it's because of general global warming, it's still a big problem," he added.
While high-rise city centres can generate their own heat islands as glass sky-scrapers reflect the sun, block airflow and pump out warm air from cooling systems, slum areas such as Rio's favelas can also become heat traps with their tightly packed concrete-block houses and lack of trees.
The associated health risks include heat exhaustion, stress, respiratory and cardiovascular problems, while higher pollution levels are a particular threat to children and older people.
Scientists are also studying whether heat islands can expand the spread of mosquitoes carrying dengue, Zika and chikungunya.
"The poorest people are always more affected," said Massimo Palme, associate professor at Chile's Catholic University of the North, who has studied heat islands in Pacific coast cities.
"When a heatwave occurs, if they don't have water supplies (or) hospitals nearby, they are in danger much more than other social classes."
While persuading local governments to invest in climate-related measures is tricky, experts say there are plenty of cheap and effective options available.
Simply painting building and bus roofs white reflects heat, as does planting shady trees which can also cut air pollution. Promoting public transport over private cars, and offering tax incentives for roof gardens are also recommended.
Longer-term, urban planners and architects should focus on expanding green areas, using less glass and positioning buildings to cut heat generation and boost airflow, experts said.
But dealing with existing buildings is a separate challenge, noted Emmanuel of Glasgow Caledonian University.
"It's not possible to build a new city, so it will have to be retrofitted," he said.
(Reporting by Sophie Hares; editing by Megan Rowling. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, climate change, resilience, women's rights, trafficking and property rights. Visit http://news.trust.org/climate)
The Thomson Reuters Foundation is reporting on resilience as part of its work on zilient.org, an online platform building a global network of people interested in resilience, in partnership with the Rockefeller Foundation.