London: Making cities and farm fields more reflective – including by painting buildings white or leaving more land unploughed after harvests – could reduce extreme heat by up to 3 degrees Celsius in areas where the techniques are used, scientists say.

While such measures would not lower global temperatures – which are rising as a result of climate change – such simple changes could provide substantial local or regional relief on the hottest days, according to research published this week in the journal Nature Geoscience.

"This is clipping the upper tail of extreme temperatures," said Andy Pitman, director of the ARC Centre of Excellence for Climate Extremes at the University of New South Wales in Australia.

The measures could mean fewer heat deaths in cities during scorching weather and fewer threats from everything from heat-warped rail lines to electrical outages from excess demand for electricity, said Pitman, one of the authors of the study.

"If you can bring the extreme temperature down a degree or two you reduce the risks. And that might mean you don't trigger the 'brownout' – or a human health catastrophe," he said.

Painting homes white – something common across parts of the Mediterranean, for example – is hardly a new idea to combat heat.

"Italians and Greeks worked this out a thousand years ago," Pitman noted.

But as climate change brings more heat threats around the world, many other parts of the world may need to "rediscover what some countries or populations have done for a long time", he said.

New advances, such as more reflective road paving materials and roof tiles, could also help, he said.

On farms, leaving fields unploughed after harvest to increase the amount of sunshine they reflect, could also help lower local temperatures, said Sonia Seneviratne, the study's lead author and an expert on land and climate dynamics at ETH Zurich, a Swiss university.

Changing the types of crops planted – or developing ones designed to reflect more light from their leaves, potentially using genetic modification – could also help, she said.

"With the harvest happening in August in Europe, the weeks that follow often tend to have heatwaves," she said. "So in Europe, this could be useful."

With the world struggling to hold global temperature increases to below 2 degrees Celsius above pre-industrial times, in line with the Paris Agreement on climate change, such measures could provide a fairly benign way to cut risks from rising heat, she said.

That's particularly true in comparison with more controversial and global "geoengineering" technologies being discussed, such as spraying sulfate aerosols into the planet's stratosphere to block some of the sun reaching the planet, the researchers said.

Scientists warn such measures could have hard-to-predict regional or global impacts, such as unintentionally shifting monsoons in Asia.

Changing the reflectivity of cities and fields, in comparison, would have a local – rather than global – impact on temperature peaks, so would likely be much less controversial and less likely to provoke regional or global disputes, the researchers said.

"Compared to some other things proposed, modifying albedo (reflectivity) could be done with less risky techniques," Seneviratne said.

Even so, the report noted that modifying the reflectivity of cities and farm fields appears to work best in North America and Europe, where it has no clear negative side effects.

In parts of Asia, however, where contrasts between land and sea temperatures help drive monsoon rains, changing local temperature extremes might have some impact on monsoons, the report warned.

Pitman said the findings do not mean that "someone should take a paintbrush and wander over all the Earth's cities painting them white".

Instead, as cities renovate and build new infrastructure, they should "consciously make the city brighter".

At the moment that is not happening widely, he said – including in his own steamy Sydney neighbourhood where a new electrical substation was just built in dark grey materials.

Taking measures to curb temperature extremes also will not work in the long run unless efforts to curb emissions are stepped up, the researchers said.

"Land-based climate engineering is not a silver bullet," Seneviratne said. "It's just one part of a possible climate solution."

Thomson Reuters Foundation