Climate scientists and economists have long argued that the world’s cities have some of the greatest leverage when it comes to deciding our planet’s climate future. The United Nations estimates that cities produce nearly three quarters of global greenhouse gas emissions already, and rapid urban migration means this number has the potential to keep growing.

But it also has the potential to shrink, saving trillions of dollars in the process, according to a new report from the New Climate Economy, a project of the Global Commission on the Economy and Climate. The report claims that cities can save a collective $17 trillion or more by mid-century by investing in more efficient buildings, transportation and waste management. And that’s not all: Over the same amount of time, the report argues, these investments could also cut annual greenhouse gas emissions by 8 gigatons (that’s 8 billion tons). Currently, the world emits about 32 billion gigatons of carbon dioxide every year, the most commonly emitted greenhouse gas, according to the International Energy Agency.

Finding ways to cut carbon emissions in cost-effective ways is one of the challenges of the century. Specifically, world leaders hope to slash emissions enough to keep global temperatures from rising more than 2 degree Celsius above their pre-industrial levels. Reducing the burning of fossil fuels and investing in alternative energy sources are some obvious ways of doing so — but cities have some unique opportunities to lower their carbon outputs, according to the New Climate Economy report.

The report breaks down these opportunities into three sectors: buildings, transport and waste. Included in each sector are a handful of specific low-carbon strategies, such as installing solar panels and improving the energy efficiency of buildings, expanding public transportation options and improving recycling rates.  

The report estimates that implementing these low-carbon actions will require gross costs of about $977 billion per year from 2015 to 2050, but they’ll also reduce annual energy expenditures by trillions of dollars. The investments are expected to pay for themselves within 16 years — and the net savings they generate between 2016 and 2050 are estimated at $16.6 trillion, across 99 percent of the world’s urban areas (which include nearly 4 billion people now, and will contain more than 6 billion by 2050).

And that’s a low estimate, according to the report’s authors. “$17 trillion in savings is actually a very conservative estimate because it only looks at direct energy savings generated from investment, which are a small proportion of the wider social, economic and environmental benefits of these investments,” said Nick Godfrey, head of policy and urban development at the New Climate Economy and one of the authors, in a statement.

The report also doesn’t factor in savings from the investment costs that these low-carbon actions will eliminate, such as the costs of building new roads to accommodate more traffic, which would be necessary in a world with fewer public transit options. And the analysis only projects savings up until 2050 — these energy savings would be expected to continue giving back far beyond that point.

On the other hand, there are some limitations, too. For example, the analysis also fails to factor in a phenomenon known as the rebound effect, which is when people take advantage of increased energy efficiency to use more energy. For example, people who usually use low amounts of heat in the winter in order to save money might take advantage of improvements in the energy efficiency of their buildings to use more heat at a lower cost. These means, in some cases, real-life energy savings could be lower than the projections.

Even so, the report recommends that all cities commit to developing and implementing low-carbon strategies by 2020. But, in fact, many cities have already begun the process by engaging in networks or partnerships, such as C40 Cities Climate Leadership Group, or the U.N.’s Compact of Mayors. The report highlights cities that have already made major moves toward a low-carbon economy. For example, Johannesburg recently implemented a bus rapid transit system that returned nearly $900 million in its first phase.

Such initiatives could be one important focus of the U.N.’s climate summit, which will be held in Paris at the end of this year. The goal of the Paris meeting will be to complete negotiations on an international climate agreement to move toward a low-carbon society.

Nations around the world have been submitting individualized carbon emissions reduction goals since last year’s climate meeting, but so far most experts acknowledge that they aren’t enough yet to keep global warming within the 2-degrees Celsius limit. So the huge potential of cities when it comes to reducing emissions and saving energy is more relevant now than ever.