Engineering reflectivity can be a way to protect human safety during dangerous heat waves. In baking rural regions, put off plowing those freshly harvested wheat fields. And in sweltering cities, paint houses white, install shiny roofs and plant pale-leafed tree species.

In a word, increase reflectivity. Bounce more sunlight back into space to save lives, tempers and energy costs. This would, in effect, constitute local geoengineering—without the unimaginable costs of spraying aerosols into the stratosphere or placing solar reflectors in orbit; and without unwelcome side effects.

The idea is based on the simple observation that a harvested wheat field is brighter than plowed croplands, and that reflective roofs are less likely to absorb sunlight than dark tiles or slates.

“These measures could help to lower extreme temperatures in agricultural regions and densely populated areas by up to two to three degrees Celsius [35 F to 37 F],” said Sonia Seneviratne, professor of land-climate dynamics at ETH Zurich, the Swiss federal institute of technology.

Her co-author, Andy Pitman, of the University of New South Wales, who directs the Australian research council’s Centre of Excellence for Climate Extremes, said: “Extreme temperatures are where human and natural systems are most vulnerable. Changing the radiative properties of land helps address this issue with fewer side effects.

“This research suggests that by taking the regional approach, at least in temperate zones, policy and investment decisions can be pragmatically and affordably focused on areas of greatest need.”

The Swiss and Australian scientists report in the journal Nature Geoscience that they worked with computer models to simulate changes in the albedo of the land and the cities (“albedo” is the climate scientist’s word for the reflectivity of ocean, ice, desert or forest).

What the researchers found was that the higher the temperatures, the stronger the effect of enhanced albedo, or reflectivity.

A Growing Problem

The big heat is on the way. Researchers have repeatedly confirmed that heat extremes pose increasing threats, especially to megacities—urban centers with more than 10 million people.

They also have confirmed that such extremes of heat can be lethal,  especially when matched with rising humidity.

Cities in any case are more at risk, because of the notorious urban heat island effect. Greater investment in air conditioning plants is not the answer, because that could only increase energy demand driven by fossil fuel combustion and thus raise urban temperatures even higher and increase long-term global warming even more.

So there has been more emphasis on natural responses, such as greater investment in the “urban forest” of park, garden and avenue trees to keep urban populations a little cooler.

Reducing Extremes

The latest study found that large-scale alteration of rural and urban albedo had no significant effect on average temperatures, and made hardly any difference to rainfall in Europe and North America. But as the thermometer soared, so did the effect: Such alterations did significantly reduce the extremes of heat.

In Asia, this form of what might be called grass-roots geoengineering may not suit: Monsoon rainfall fell in the simulation, and the monsoons are crucial to the economies of China and India.

“Regional radiation management can be effective, but even here we have to consider any potential effects on food production, biodiversity, CO2 absorption, recreation areas and much more,” Seneviratne said.

“Even this climate technique is no silver bullet; it’s just one potential tool among several others in the battle against climate change.”

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