Australian researchers found during an extreme heatwave that backyard gardens lowered land surface temperatures by five to six degrees Celsius more than similar non-vegetated areas.

Hot summers can cause city temperatures to soar one to three degrees higher than surrounding areas, creating "urban heat islands" due to excess concrete, people, air conditioning, machinery and the resulting local climate.

Not only can this be uncomfortable, and even unlivable, it can also place a significant burden on public health.

Green infrastructure and other nature-based solutions are proposed as cost-effective, sustainable approaches to mitigating the impacts of climate change.

"Urban trees in particular are an effective tool for reducing land surface and air temperatures for entire suburbs, and even cities," says researcher Alessandro Ossola from Macquarie University, Australia.

"But as yet we don’t know much about their localised effects, particularly in the places where cooling is most important – our residential neighbourhoods – and when needed the most – during extreme heatwave events."

To investigate this, Ossola and colleagues from the university’s collaborative Smart Green Cities hub analysed thermal mapping data collected from an aircraft in summer 2017 at the peak of a three-day long 40-degree heat wave in Adelaide.

The results, published in an online report, were surprisingly positive.

"We found that the humble home garden is more than pulling its weight when it comes down to urban cooling," says Ossola. "Although they only cover about 20% of urban land, domestic yards account for more than 40% of tree cover and 30% of herbaceous cover, in the form of grass."

The tree canopy cover is notably more than typical parks or other urban green areas which tend to have more grass, and the cooler temperatures were particularly pronounced in the hottest suburbs that don’t enjoy cooling sea breezes.

The researchers estimate that increasing green foliage in residential gardens could reduce local heat by several degrees Celsius.

On the other hand, they calculated that removing existing vegetation through urbanisation, infill and densification could increase local land surface temperatures by three to four degrees, particularly during the day and for the most vulnerable communities.

The considerable, localised cooling benefits of backyard gardens have important implications for policy, which needs to account for extreme temperatures in urban planning, Ossola says.

"Our results clearly indicate that encouraging, protecting and expanding urban greening on private land is a simple, effective means of mitigating the negative effects of climate change on cities and people.

"However this is a strategy that needs to begin now: urban forests don’t grow quickly, and we need to be encouraging low-water use herbaceous cover as a stopgap until a large array of shade trees can take over the job of green cooling."