Published 26 January 2023

Urban heat islands – what they are and why they matter

By Brayden J Lewellen

Have you ever walked through the city with the sun beating down, feeling the heat radiate off of the dark asphalt under your feet? The uncomfortable lack of shade making you wish you had brought a water bottle?

If so, then you’ve experienced something called the Urban Heat Island effect.

This effect, or UHI for short, is the technical term for why cities are significantly hotter than the surrounding countryside. The concept is relatively straightforward – all of the darkly paved roads and footpaths absorb sunlight better than grass and trees, and warm up. These warm surfaces then radiate this stored heat, increasing the temperature of the air. The mirrored glass of skyscrapers also concentrates heat in the narrow streets between towering buildings. Many urban activities, like driving, construction, and air conditioning, produce waste-heat which is pumped into the atmosphere. Finally, these concrete jungles, with their efficient drainage systems, funnel rainwater into underground pipes rather than allowing it to evaporate and cool the surrounding air. Together, these processes¹ ² lead to hotter urban environments.

However, we all enjoy a warm sunny day. Why does a bit of extra heat matter?

It is a well-known fact that our planet is experiencing global warming. And as the earth’s temperatures continue to rise, the added heat in cities from the UHI effect is going to make a big difference to our urban dwellers. One of the few studies on UHI in Aotearoa New Zealand showed that in Ōtautahi Christchurch, the UHI effect can add 5°C to residential areas and up to a whopping 11°C in the city centre.³

Climate change is going to make this worse. Not only is the average global temperature rising, but Aotearoa New Zealand can expect more extreme temperatures and heat waves in the coming decades.⁴ In Tāmaki Makaurau Auckland, we can expect 70 more hot days (>25°C) per year by 2100.⁵ This kind of heat can severely impact the very young, elderly, and chronically ill, and during heat waves, cooling systems will be used more heavily and more frequently, which in turn adds waste heat to the already warmer urban environment. 

So what can we do?

Thankfully, there are three excellent options to help decrease the UHI of a city:

• Reflect the light back into space. One easy method is to paint buildings and infrastructure white.⁶ All future re-roofing projects in the city should consider using pale colours. 

• Enable evaporation to cool us down. Water features can decrease nearby surrounding temperatures by 1 to 3°C.⁷ But it has only a localised effect and is problematic when water is scarce, a common occurrence in summer throughout urban centres in Aotearoa New Zealand.

• Allow nature to cool us through both shade and evaporation. Although only trees provide significant shade, all plants cool through evaporating water from their leaves. Green space includes public parks or private gardens, but it also includes the more mundane road-side vegetation and stormwater swales, and the more exotic green roofs and living walls.⁸

Although all three of these methods are effective, adding green space has the most co-benefits⁹ for humans and the environment:

• Improve health and wellbeing – nature is relaxing and reduces stress and encourages walking.

• Enhance biodiversity – green space increases habitat for our native birds and plants.

• Improve air quality – trees filter out harmful gases and particulate matter.

• Carbon sequestration – as trees grow, they remove carbon from the air and solidify it in wood.

• Increase property values – tree-lined streets attract tenants and buyers.

• Reduce flood risk – green space absorbs rainwater, reducing pressure on stormwater systems. 

• Reduce energy costs – trees provide shade and reduce the need for air conditioning.

• Sustain and enhance mauri – bringing nature to the city builds connections to our Taiao.

Due to the numerous co-benefits that green space and especially trees can provide, Tāmaki Makaurau Auckland has created a 30-year forest strategy¹⁰ to improve the canopy cover across the city. The goal is to increase overall canopy cover from 18% to 30% by 2047. Currently, 61% of the total urban canopy cover in Tāmaki Makaurau Auckland is on privately owned land, and yet only ~15% of private land has canopy cover.¹¹ Thus, there is space for individual property owners to make a difference in the UHI as they own most of the urban canopy and yet only 15% of their land is covered. The council as the minority landowner needs to work with private property owners to make a substantial impact in overall forest coverage. 

Unfortunately, Te Whanganui a Tara Wellington has never completed a study on its UHI,¹² and has no long term plans to decrease its urban heat.¹³ This makes our capital much more vulnerable to global warming in the future with increased heat waves and the resulting poorer health outcomes. Clearly, this needs to change. Aotearoa New Zealand’s National Adaptation Plan¹⁴ to climate change highlights the need to reduce the UHI of our cities to meet our international climate change obligations and to protect the well-being of our people. Yes, it is a multi-decades long project to reforest our cities, but the time to start is now. Our cities need concrete plans to reduce our concrete jungle.

Footnotes

1 - Learn About Heat Islands, United States Environmental Protection Agency

2, 4, 5 & 12 - What we know and do not know about New Zealand’s urban microclimate: A critical review, ScienceDirect

3 - Modeling the Winter Urban Heat Island Over Christchurch, New Zealand, AMS Journals

6 - Climate explained: how white roofs help to reflect the sun’s heat, The Conversation

7 - Reducing heat with water, Urban Green Blue Grids

8, 9, 10 & 11 - Auckland’s Urban Ngahere (Forest) Strategy, Auckland Council

13 - Our Natural Capital: Wellington’s biodiversity strategy and action plan 2015, Wellington City Council

14 - Urutau, ka taurikura: Kia tū pakari a Aotearoa i ngā huringa āhuarangi. Adapt and thrive: Building a climate-resilient New Zealand, Ministry for the Environment