How Growing Cities Warm the World

In 2025, more than half of the people in the world live in cities. Scientists predict that nearly 70% of people will live in cities by the year 2050. According to United Nations projections, cities around the world will need to make space for up to 2.5 billion more people in the next 25 years [1].

Cities are also where most of the world’s energy is used. Energy is needed to power transportation, factories, homes, offices, schools, and shops. Because of this, current cities produce about 70% of all greenhouse gases (GHGs). Examples of GHGs include carbon dioxide (CO₂) and methane (CH₄). These gases trap heat in the Earth’s atmosphere, like a giant blanket, making the planet warmer. This process is often called global warming or climate change.

The Intergovernmental Panel on Climate Change (IPCC) recommends that we should try to limit global warming to 1.5–2°C above global temperatures from before humans started burning lots of coal and oil in factories and cars, about 150 years ago. To do this, many cities will need to reduce the GHGs they emit to net zero [2]. Net zero means that all the GHGs emitted by a city are absorbed by local ecosystems (especially plants and trees). For example, if cars in your city release 1 kg of CO₂ into the air and trees absorb 1 kg of CO₂ from the air in your city, the amount of CO₂ in the air in your city stays the same—in other words, there is net-zero change in CO₂. Even though a small amount of GHGs might still be released, they are absorbed by ecosystems instead of adding to the “blanket” in Earth’s atmosphere.

To achieve net zero, cities must work on emitting fewer GHGs. They also need to increase their ability to absorb the remaining GHGs. This process is called carbon sequestration. Cities face a big challenge: they must give more and more people homes, transportation, clean water, heat, and jobs. At the same time, they also need to figure out how to cut pollution. This can be tricky and costly. We need smarter, greener ways to build and live in cities.

Adapting to a Changing Climate

Cities are also facing another challenge: they also need to adapt to the impacts that climate change has on their local weather conditions. The Earth is getting warmer because of the GHGs that trap heat in the atmosphere. This means that many cities are getting warmer and there are more heat waves in the summer. Cites are extra sensitive to warming because of the urban heat island effect. This is because all the buildings, roads, and concrete trap heat, making the city a few degrees warmer than the surrounding countryside. Cities must take action to help their residents keep cool. This is an especially big problem in places that were historically colder. In these places, houses are designed to keep heat inside, and many do not have air conditioning or even fans.

Climate change is more complex than just warming. It also brings with it more frequent and more severe storms, floods, and droughts (sometimes in the same place). Our cities are designed to deal with expected weather events, but severe storms can be bigger than cities planned for. To keep residents safe and comfortable, cities must adapt. For example, they might need to build bigger pipes to safely take away the water from a big storm.

Learning From Nature

City planners often talk about nature-based solutions, which means using nature, like trees and plants, instead of only concrete and steel. For example, instead of a plain concrete ditch next to the road, cities can build a green ditch with plants that soak up water. Nature-based solutions usually offer many benefits beyond their main purpose. For example, a green roof (with plants on top) offers the same shelter as a regular roof, but the plants can also help to keep the building warm in winter and cool in summer. Since plants absorb rain water instead of letting it run down a gutter to the street, they can help reduce flooding. They can also improve air quality, support biodiversity, and make the building look more attractive for residents and neighbors [3].

According to a lot of research, nature-based solutions can also help to reduce GHGs in many ways [4]. We reviewed several studies to identify the types of nature-based solutions that are the best at reducing GHGs in cities. From these studies, we also learned about the different ways each nature-based solution works to reduce GHGs. We used that research to make a list of the top five nature-based solutions for reducing GHGs in cities (Figure 1).

How much can Nature Actually Help?

We wanted to understand how much nature can help fight climate change. To do this, we used a computer model to add up the benefits. This model counts the direct effects, like trees, green roofs, and forests soaking up carbon. It also counts the indirect effects, like shady streets that encourage people to walk instead of drive, or green buildings that use less energy. By adding up these effects, we can see a fuller picture of the potential climate benefits of nature-based solutions in cities. The computer model also allows us to compare different strategies side by side, showing which types of nature-based solutions are most effective in different urban settings and helping to avoid reliance on only one kind of solution.

We did our calculations for 54 cities in Europe. We found that nature-based solutions could reduce GHGs by an average of 17% in these cities. The nature-based solutions did even better in some cities and, in one, nature-based solutions could take away a quarter of their GHGs! While that might not sound like a lot, even a 10% reduction in GHGs can be a very big deal when we are aiming for net zero. Nature-based solutions will not be enough for cities to achieve net zero on their own, but they can work together with other actions. Such actions include using greener ways to produce electricity (called renewable energy) and improving trains and buses to cut down on car use. These things, combined with nature-based solutions, can help cities achieve their climate action goals more quickly. Figure 2 shows how much nature-based solutions can potentially reduce GHGs in 14 European cities.

Does It Matter Where Nature-Based Solutions are Placed in the Urban Landscape?

We were also interested in figuring out where nature-based solutions should go to make the biggest difference. Instead of putting the same solution everywhere, we used a smart map that matches each type of green idea to the place where it works best. For example, street trees are most helpful along busy roads. Green roofs work well in crowded city centers. And greenbelts (big areas of protected nature) are best at the edge of the city, to stop it from spreading too far. By mapping these ideas before implementing them in reality, city planners can save time and resources while still making well-informed choices. In this way, our computer model and map both demonstrate that nature-based solutions have multiple benefits and can also guide city planners on how and where to use them for the greatest impact (Figure 3).

Working With Nature for a Better Future in Cities

Cities are home to most of the world’s people now, and even more people will live in cities in the future. At the same time, cities are also a major source of GHGs, which means they have a significant impact on global warming. This makes cities both part of the challenge and part of the solution. By rethinking how cities are designed, planned, built, and managed, city planners have the opportunity to create cities that are healthier, greener, and more sustainable for future generations.

Nature-based solutions can help cities to move in this direction. Urban parks and gardens, green roofs, and street trees can reduce GHGs, protect residents from heatwaves and floods, promote sustainable behavior, and improve the quality of daily life. They also offer benefits that go beyond climate action, such as supporting biodiversity, cleaning the air, supporting food resilience, and making the city a nicer place to live. Our analysis of 54 cities in Europe showed that nature-based solutions can be very effective in reducing GHGs, even though they cannot reduce them to zero on their own. Nature-based solutions can make a significant contribution to net-zero goals when combined with other measures like renewable energy, sustainable transport, and better building design.

The path to net-zero cities will not be simple. It requires political will, creativity, investment, and collaboration. But nature-based solutions show us that by working with nature, not against it, we can develop cities that can adapt to climate change, achieve net zero, and become more attractive places to live.

Glossary

Greenhouse Gases (GHGs): ↑ Gases that gather in Earth’s atmosphere and create a kind of “blanket” that traps heat from the sun close to the Earth’s surface.

Climate Change: ↑ Changes to weather patterns over a long period of time, including temperature, rains, droughts, and storms.

Net Zero: ↑ All of the GHGs released into the air in a city or country are absorbed again by plants and trees inside the city or country.

Carbon Sequestration: ↑ The process by which plants and trees take in carbon dioxide from the air and store it in their leaves, trunks, and the soil in which they grow.

Nature-based Solutions: ↑ Ways that people can use nature, like trees and parks, to deal with environmental problems, such as flooding and overheating in cities.

Computer Model: ↑ A computer programme that builds a digital copy of something real (like a city) that we can use to learn more about the real world.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was supported by a grant from the Swedish funding agency (Formas Grant 2021-00293; HP, JP, and ZK). We would like to thank all the young reviewers and their mentors for their valuable feedback which helped to improve this article.

AI Tool Statement

The author(s) declared that Generative AI was not used in the creation of this manuscript.

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