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.

Third and most important, a commonly accepted framework for emissions accounting could be the key to unlocking additional finance for city governments. Despite significant effort by the World Bank and others, no funding has yet to flow to city governments for city-wide emissions reduction projects under the UN's Clean Development Mechanism (CDM), in part due to issues of methodology.

Private investors, who are often looking for large scale projects that can be conducted at the city level, also require certainty about emissions reductions. We need an improved measurement protocol to unfreeze capital, allowing it to flow to a level of government that we know is effective at taking action on climate change.

But a new protocol is just one part of how we are going to achieve commonalities among reporting cities. A good methodology must be married to transparent, public, annual reporting. Here, the private sector provides a good model.

Ten years ago, when CDP first began providing a reporting platform for companies, very few companies measured their emissions, and many used different methodologies. Today, of over 3,000 companies that report annually to CDP, the great majority use the same measurement protocol, the Greenhouse Gas Protocol. This standardization has come about partly due to the quality of the Greenhouse Gas Protocol.

But another key factor is that, thanks to CDP's annual disclosure process, companies know which protocol their peers are using. These companies can now make an informed decision to choose (or not) a common protocol. In this way, disclosure is a key driver of standardization. Simply put, it gives reporting entities a clear view of what everyone else is doing.

A new measurement protocol will not be a panacea for urban GHG measurement and reporting. It will likely take years before we have broadly common measurement from cities. But the release of this protocol is a gigantic leap in the right direction.

_{City skyline photo via Shutterstock.}