Coastwide
Number of urban climate mitigation case studies, grouped according to city size. The 12 most frequently studied cities are labelled. Population data from UN World Urbanisation Prospects (2018 revision), using agglomeration data where available. Credit: William Lamb

What Do we Know about Climate Change Mitigation in Cities?

The emission reduction goals of the Paris Agreement demand rapid action at all scales and levels of governance – from individual to international – and cities are no exception. Many urban governments recognise this and are embarking on projects to improve public transport services, mandate efficient buildings, or produce energy from local renewable sources.

So what do we actually know about climate mitigation in cities? In our new paper, published in Nature Climate Change, we take stock of all the city case studies currently available in the peer-reviewed literature.

We find that knowledge on the topic is exploding, with few attempts to grapple with the vast flow of studies. We see that the vast majority of research is on larger cities, in wealthier countries, on specific topics. And we suggest that computer-assisted methods are key to tracking work on cities and learning about solutions at a large scale.

A tidal wave of studies

Cities used to be a marginal topic in climate change mitigation. This is no longer the case. Since the fifth assessment report (“AR5”) from the Intergovernmental Panel on Climate Change (IPCC) first included a dedicated chapter on cities (pdf), the literature has jumped from a few dozen articles per year, to almost 3,000 in the past five years alone.

As the chart below shows, the number of urban climate mitigation case studies published in time to be included in each IPCC assessment report – from the second (“AR2”) to the sixth (“AR6”) – has risen dramatically over time.

To say we are unprepared for this development is an understatement. Urban case studies are usually treated as colourful curios in major climate reports. They are placed as dedicated boxed sections that make interesting, if anecdotal reading – such as “How did London’s congestion charge reduce emissions?” and “How does Manizales manage disaster risks?”.

But treating case studies in this anecdotal way suggests we are not really learning about why cities succeed or fail in mitigating climate change. There have been no systematic attempts to map out the existing work, let alone synthesise it. We are generating hundreds of new case studies per month without even knowing what exists.

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