Why the world needs a better way to measure cities' emissions

Earlier this week, the world welcomed a new protocol for how to measure the greenhouse gas emissions from an entire city. Thanks to the leadership of C40, ICLEI, and the World Bank, among others, this new protocol will for the first time provide city governments with clear and consistent guidelines for measuring emissions. It couldn't have come at a better time -- a new measurement protocol is critical for empowering city governments to take the next step on climate change action.

Why is it important for cities to measure their greenhouse gas emissions? Significant evidence -- from academic research to C40's work to CDP's own data -- shows that city governments are often the ideal entity to take action on climate change. Many already are.

An important first step for cities is to account for the greenhouse gas emissions within their cities. Just like financial accounting allows companies to understand where their money comes from and where it goes, greenhouse gas accounting enables cities to understand the contribution of their buildings, energy, transport and people to global climate change.

Up until now, however, cities have not had good guidance on how to measure these emissions. Last year, CDP and C40 together surveyed the largest cities in the world. We asked them if they were measuring their emissions, and if so, what methodologies they were using.

The results were positive -- 27 out of 42 C40 member cities measured and reported city-wide emissions. However, in all but a small number of cases, city governments were measuring their emissions in different ways. Some were including emissions from their airport, for instance, while others were excluding their airport.

Some categorized their emissions by scope (the traditional categorization used by corporations), while other cities divided their emissions by economic sector. Overall, the most common methodology reported to CDP last year was "Other".

These differences in how cities account for their greenhouse gas emissions are important for several reasons. First, the differences mean that it is challenging to compare city emissions to one another. Want to know if New York or London has more emissions from buildings? You will be stuck for hours trying to patch together raw data in a consistent and comparable way.

Second, the differences make learning and knowledge sharing among cities more difficult. Measuring city-wide emissions is a complex and difficult exercise for any city government. The current lack of clear guidelines simply makes the process harder, sets a higher barrier to entry, and has encouraged city governments to create their own accounting methods. As a result, it is difficult for cities to share best practices and understand the most effective ways of reducing emissions.