Nanogirl Michelle Dickinson: The heat is on

Construction boom means we need strategies to offset 'urban island' microclimate change. Photo / Nick Reed

With more cranes towering across our city skylines than ever before, New Zealand's construction boom is on show for all to see.

Strong economic growth, high immigration numbers and increasing tourism visitors are just three of the factors pushing us to build more residential and commercial buildings as well as the roads to service them.

This is great news for our economy, more people want to live here, more businesses are growing, and more tourists want to experience our incredible culture and landscape.

Cities are centres of economic and social activity and growing these to cope with our increasing population implies we are building for our future.

However, as our cities grow in size we are losing more of our green spaces and replacing them with dark asphalt roads, heat absorbing concrete buildings and coloured steel roofs.

Current building practices involve constructing buildings out of materials that are good at insulating or holding on to heat, which is great for energy conservation inside the building, but can cause heat retention outside the building.

The new Auckland unitary plan is designed to provide more housing and infrastructure to manage the city's growth; however the type of building materials used and considerations around green environments are also important considerations for our future cities.

When houses, offices and commercial buildings are built close together in a city they can act like a blanket, trapping the warm ground air from the cars, people and machinery between the buildings and preventing it from escaping.

The trapped air held between modern high rise apartments and tall office buildings create what is known as an urban heat island where hotter microclimates occur in high density areas.

According to predictions published this week in the journal Nature Climate Change, when combined with the effects of climate change, this trapped warm air could make 25 per cent of built up cities warmer by 7 degrees Celsius by the end of this century.

Looking at 1692 cities around the world including Auckland, Christchurch and Wellington, the study found urban heat is likely to negatively affect economies over the next few decades.

Increasing city temperatures have been shown to reduce human productivity, increase costs for cooling internal air temperature and increase medical care costs due to falling air quality.

The accumulated costs of both the urban heat effects and climate change are predicted to decrease the gross domestic product (GDP) by 5.6 per cent on average by the end of the century, with the worst affected cities shaving up to 10.9 per cent off their GDP.

So how is a city supposed to grow and accommodate its increasing population while also preparing for the future?

The Nature study performed a cost-benefit analysis for a number of local policies and found the most desirable solutions were also the simplest and involved investing in climate-friendly building practices at a local level.

Simple steps such as installing green roofs, building pavements and roads that reflect more sunlight and absorb less heat, and growing more trees and plants in the city were all easy solutions suggested help maintain the local temperature.

The researchers calculated that by switching 20 per cent of the roofs and half of the pavements to cooler forms over the long term, cities could save up to 12 times the cost of their installation and maintenance fees and bring temperatures down by almost a degree.

As we marvel at the new high density buildings going up around us and we watch our cities grow to accommodate their new residents, this study reminds us that building for our future doesn't just involve housing residents but also ensuring that the city can still breathe on its own.