Last month 175 countries signed the Paris climate agreement. In April, cities around the globe added almost 5 million people, and a report released by U.N.-Habitat shows this rise in urbanization continuing.

Cities are not often viewed as environmentally friendly places. Buildings and concrete are much of the landscape, not the typical backdrop in photos from nature lovers. And a quick look at the data presents a stark picture. Cities today cover just 2 percent of the globe’s land yet they are the drivers of 70 percent of global carbon emissions.  

But beneath this tough exterior lies a key building block in the fight against climate change. Per capita emissions in cities are typically lower than those in their home country.

A few years ago, Chicago did an inventory of local carbon emissions (PDF) in partnership with the seven-county metro region. The region includes the city, inner-ring older suburbs and newer communities far from the city center.  

Each county in the Chicago region had lower per capita emissions than the average of the United States overall.

Not all locations, however, performed equally. Per capita emission in Chicago were 20 to 30 percent lower than in the surrounding counties. In the city of Chicago transit is extensive, housing is often multi-family (with shared walls preserving energy and smaller units) and neighborhood layouts are more compact.  

Chicago has been pushing to reduce emissions even farther and leveraging its existing strengths by rebuilding sections of the transit lines, passing ordinances to make it easier to build housing near train stations, accelerating energy efficiency in existing buildings through benchmarking and the voluntary Retrofit Chicago program, and removing coal from the power supply. It is a strong start with more needed.

Per capita emissions in cities are typically lower than those in their home country.

100 years ago about 20 percent of the global population was urban. Today we have hit 54 percent and projections show this rising to two-thirds by 2050. In the process we will add a staggering 2.5 billion new urban residents to the planet.

One million people are joining urban communities each week. In under three weeks we add a new Chicago to the globe and in about three months we add the full metro region.  

The majority of the growth in urban populations will happen outside of the United States. In 1995 there were 14 mega-cities, cities with 10 million inhabitants or more. Today there are 29. Asia alone has 16.

With a swelling global urban population the performance and possibilities of cities are center stage. Many cities must drive current levels of emissions even lower. Chicago performs very well with per capita emissions at just 60 percent of the U.S. average. However, the United States is a high emitting country and Chicago’s level reflects that. It is almost double the national average in China.  

Rapidly growing cities in many lower emitting countries, on the other hand, are seeing urbanization at the same time that national per capital emissions are rising. Building and powering these cities to perform beyond the level of the suburbs and the core of Chicago will shape the emissions trajectory for decades to come.

The opportunity is enormous. Estimates from C40 on the potential impact of low carbon development over the next 15 years show that actions in global cities together can avoid eight times the emissions of the entire United States.  

The Paris Agreement was signed in cities around the world. A tremendous opportunity for delivery is anchored there, too.