Electric vehicles are a useful tool for fighting sweaty city summers, according to a study published on Thursday.

Switching from vehicles powered by fossil fuels to plug-in equivalents would ease a phenomenon called urban heat island, it said.

The term describes what happens when city temperatures are driven higher by heat from traffic and air conditioners and by warmth, stored during the day in roads and buildings, which is released at night.

In some places, the buildup combines with a summer heatwave to inflict sweltering discomfort and heat stress.

Writing in the journal Scientific Reports, specialists in China and the United States said urban heat island creates a vicious circle.

The hotter it is, the more people crank up their air conditioning, which in turn disgorges more heat into the street, and so on.

The team calculated what would happen in Beijing if petrol- and diesel-powered cars and light trucks were replaced by their electric equivalent.

Their simulation was based on the weather in the Chinese capital in the summer of 2012, when the city was three degrees Celsius (5.4 degrees Fahrenheit) warmer than the surrounding countryside.

Switching vehicles would have caused this to fall by 0.94 C (1.7 F), according to their model.

"Heatwaves kill, and in terms of climate change, even one degree can make a difference," said Jianguo Liu of Michigan State University, who took part in the study.

The theoretical switch would also save the city 14.4 million Kilowatt-hours in electricity each day -- the daily equivalent of 10,686 tonnes of carbon dioxide (CO2) emissions.

The calculation took into account heat from the extra fossil fuel that would have had to be burned at local power plants to charge the electric cars.

Electric vehicles emit only about a fifth of the heat of conventional cars.

"The replacement can mitigate heat island intensity, which can reduce the amount of electricity consumed daily by air conditions, benefitting the local and global climate," the study said.

The researchers sounded a note of caution, pointing out that urban heat island has a basket of factors, some of which are sketchy.

Particulate pollution, city layout, building design, energy efficiency and parks are also believed to play a part in worsening or mitigating it.

The computer simulation focused only temperature change and energy savings.

It was not designed to factor in the cost of introducing electric vehicles and their charging infrastructure, nor did it estimate the health benefits from less pollution.