The secret weapon for extreme heat on the North Shore? It's probably growing on your street.

As the North Shore warms, the first line of defence against urban heat are our trees.

In the context of climate change, the process of photosynthesis in trees and vegetation is crucial in removing carbon dioxide from the atmosphere. However, in an urban or suburban setting, trees play another significant role in reducing heat.

On average, we currently experience four days of extreme heat a year. Extreme heat days are defined as days where the average Australian temperature is in the top one percent of records.

According to data from the Bureau of Meteorology, this figure has risen steadily since the 1910s.

The federal government’s National Climate Risk Assessment, released on Tuesday, stated that if the climate warms by 3C, we could be experiencing up to 18 days of extreme heat a year.

How does it work?

Tree canopies are important enough for councils to keep data on how much coverage there is within their boundary.

The most significant effect of tree canopy is the shade they provide to our suburbs, according to Sebastian Pfautsch, Professor of Urban Management and Planning at Western Sydney University.

A tree shields you from the sun's rays but also works in much more subtle ways. 

When the sun's rays hit the earth’s surface, they do not simply dissipate, but are absorbed into the surface. The materials we build much of our urban spaces with — such as concrete and asphalt — have a high thermal mass, meaning they can absorb a lot of heat. When they radiate that heat back out, the entire atmosphere gets hotter. 

If the sun's rays hit the tree canopy first, however, that energy goes towards photosynthesis, and less heat is absorbed into the atmosphere. A small amount of additional cooling is also produced through the process of transpiration. This is when plants release water back into the atmosphere as vapour.

Some trees are bigger and better than others

Pfautsch said that “generally, any tree is better than no tree”, but added that specific species  provide a greater reduction in urban heat, based on the size of their crown and density of canopy. 

A eucalyptus tree, for example, due to its pendulous, downward pointing leaves, and “open cauliflower like canopies”, provides less shade, and therefore less cooling, than a fig tree, with its dense, large canopy.

Image: City of Sydney Council, Ku-ring-gai Council, DO'Neil via Wikimedia