How many trees does it take to cool a city?

Let’s be honest, we need to implement many different solutions to address climate change. But to do this, we often need more answers. In our current approach, someone has to give a number and that number has to then be budgeted. This is the goal behind research by Wang and colleagues (2024).

Many places around the world have identified tree planting as an important part of mitigating climate change. There is some logic because trees are both a carbon sink, drawing carbon dioxide out of the atmosphere, use the carbon to build the trees structure and therefore store it long term, and a source of cooling through shade and evaporation. They draw water up through the roots and release it through the leaves. The challenge is that things like planning for more trees are done at a city level but there hasn’t been a lot of data on how many trees are needed at that scale.

Wang and colleagues explored how to pin down the number of trees needed for cooling effects. Now. This is, of course, relative. There are many aspects of a city that can impact the warming it is and will experience. But the researchers accounted for this by studying four different cities, two in the US and two in China. They were in three different climates.

At a neighbourhood scale, a 1% increase in tree cover can result in 0.04 to 0.57 degrees Celsius of cooling in an area. The variation relates to things like climate and tree species. But the researchers found that cooling efficiency increased at larger scales in part because it was easier to include large groupings of trees which increase the cooling effects.

What this means is that planners can use the researchers’ analysis to determine how different percentage increases of trees can impact the overall temperature and mitigate temperature increases from climate change. For example, they found that to achieve 1.5° C of cooling in Baltimore, the city would need to increase the tree canopy by 6.39%.

There is a caution though. This math just reveals how many trees are needed. It does not speak to where to place them. For this there needs to be community engagement but there also needs to be equity considerations. If you bike around many cities you will notice a difference in tree cover that is connected to socioeconomic status. More disadvantaged neighbourhoods tend to have less trees. And when you bring trees into these communities it can result in gentrification which will push the previously disadvantaged residents out because they can’t afford to live in the treed version of the neighbourhood.

Obviously trees are only a piece of the solution, but having a way to determine how many trees are needed to achieve specific cooling effects will help support decision making and hopefully result in meaningful action.