Hot future prompts new ideas for cool cities

Higher temperatures must mean more energy just to cool cities – which means even more heat. But ingenuity is already proposing answers.

LONDON, 15 August, 2019 − The world could need a quarter more energy by 2050, to cool cities and survive the global heating expected by then. And that assumes that nations will have taken steps to control greenhouse gas emissions and that the rise in temperature will be moderate.

If, on the other hand, the world goes on burning fossil fuels under the notorious “business as usual” scenario, then according to new research the people of the planet could demand up to 58% more energy, just to drive the extra air conditioning and refrigeration in ever more frequent and ever more intense extremes of heat.

The latest study, by researchers based in Boston, Massachusetts and Venice in Italy, helps to settle one of the more intricate questions that accompany climate projections and energy demand: yes, there will be more people and bigger cities which demand more power anyway, and yes, warm zones will get hotter and demand more expense on keeping cool. But chilly and temperate nations will enjoy milder winters and spend less on staying warm. Which wins?

The new paper, in the journal Nature Communications, either settles the matter or provides fellow scientists with a methodology and a set of results to examine more closely.

Risky faster heating

A warmer world will also be vastly more energy-expensive. And if nations invest in coal, oil or natural gas to provide the extra electricity to provide the air-conditioning, drive the electric fans and refrigerate food and medical supplies, then global heating would accelerate to ever more dangerous levels.

“At this point, we don’t know. To cool my house, I could buy a bigger air-conditioner. Or if higher demand makes electricity more expensive, I could choose to open my window or run a fan,” said Ian Sue Wing, an earth and environment scientist at Boston University, who led the study.

“We could use coal or we could use renewable sources, and those two choices mean very different things for our future. With coal, it will mean more greenhouse gas emissions. That’s what keeps me up at night.”

By 2050, there could be between 8.4bn and 10bn people on the planet. Gross domestic product per person (an economist’s measure of income and spending) could have all but doubled or even in some places more than trebled. Tropical and mid-latitude zones could, if warming is only moderate, experience as many as an extra 50 uncomfortably hot days each year. If the warming is vigorous, the number could soar to 75.

“We could use coal or we could use renewable sources. With coal, it will mean more greenhouse gas emissions. That’s what keeps me up at night”

Researchers have warned, consistently and repeatedly, that even a modest rise in average planetary temperatures will take the form of longer and more intense heat waves. By 2100 three out of four people on the planet could be exposed to heat extremes, and those most at hazard will be living in the tropical and subtropical megacities.

Extremes of heat can kill – one group has already identified 27 ways in which to die of rising temperatures – and scientists began warning years ago that ever more needed investment in air-conditioning equipment would only make energy demand, and perhaps greenhouse gas emissions, worse, while also contributing to ever greater outdoor temperatures.

So researchers have been looking at other approaches. The puzzle has already tested the levels of ingenuity and fresh thinking in the world’s energy laboratories. Researchers have cheerfully proposed reflector roofs that could send 97% of the sunlight back into space.

They have explored nature’s answer to the unforgiving sun: more trees in cities could take temperatures down by as much as 5°C and even make cities wealthier and healthier. And already this month, scientists and engineers have suggested two new ways to address the challenge of the overheating cities.

One US team at the University of Buffalo, working with the King Abdullah University in Saudi Arabia, has devised an inexpensive polymer-aluminium film that keeps itself cool, packed in a specially designed solar shelter. The film absorbs heat from the air and converts it to thermal radiation that can be beamed back into space.

Deep cuts possible

The researchers report, in the journal Nature Sustainability, that in the laboratory temperatures could be lowered by up to 11°C. On a clear, sunny day in New York state, they achieved outdoor all-day temperature reductions of 2°C to 9°C.

This exercise in entirely passive cooling – no electricity, just rooftop boxes – is in its infancy. But there are other approaches to the “heat island effect” that already makes modern cities uncomfortable.

Researchers at the University of Rutgers in the US simply looked at the ground beneath their feet. Pavement and road surfaces made of concrete or asphalt cover 30% of most cities and in high summer these surfaces can reach 60°C.

So, the Rutgers engineers report in the Journal of Cleaner Production, roads could be made of permeable concrete, through which water could drain. It might give off more heat on sunny days, but after rainfall the water could run through, and evaporate through the pores, to reduce pavement heat by up to 30%.

And in addition, their concrete treated with fly ash and steel slag would make a huge difference to stormwater management and reduce the risk of urban flash floods. − Climate News Network