President-elect Donald Trump has threatened to pull the U.S. out of the Paris Climate Agreement, called climate change a hoax perpetrated by the Chinese, and more recently, simply “weather.” His Cabinet picks include Rex Tillerson, CEO of oil giant Exxon; Rick Perry, a climate change denier and a supporter of the fossil fuel industry; Scott Pruitt, who wanted to completely do away with the Environmental Protection Agency; and Ryan Zinke, a vocal supporter of gas pipelines.

And on Wednesday, Anthony Scaramucci, a Wall Street investor and a Trump transition team member, tried to equate climate science to the long-debunked medieval theory that said the Earth was flat.

Understandably, a number of people within the country and outside are worried about the climate policies the Trump administration would form, or abandon. Given the structure of governance in the U.S., however, there is hope that states and cities will take their own course instead of simply following the federal government.

And with that in mind, mayors from five international cities, all of which are part of the Carbon Neutral Cities Alliance, wrote an open letter to some of their U.S. counterparts Thursday. Addressed to the mayors of Boston, Boulder, Minneapolis, New York, Portland, San Francisco, Seattle and Washington, D.C., the letter makes an appeal for the cities to continue their fight against climate change.

“We see climate change as a core issue of national interest and a key driver of the march toward a cleaner and more equitable future. Clean energy employment grew 12 times faster than overall job creation in the U.S. in 2015, and this trend is growing. Business leaders understand this. This is why you, the mayors of America’s leading cities, must continue to be at the forefront of climate action in the U.S. going forward. This is increasingly important as your new national leadership abdicates responsibility for protecting Americans and the world from fossil fuel impacts to our people’s health, our economies, and our environment,” the letter said.

While the danger of Trump scuttling the progress on mitigating climate change is very real, it is not an easy thing to do, especially with the increasing adoption of renewable energy.

Stewart Kantor, CEO of Full Spectrum Inc., told International Business Times: “Despite the concern over the new president-elect and his recent appointments, it would be extremely difficult for the federal government to turn back the clock on the adoption of renewable energy sources. Not only have the most populous states set forth their own rules and regulation to reach goals for greater renewable energy usage, but a majority of the consumers, corporations and their shareholders are pushing for the success of clean energy throughout the U.S.”

Full text of the letter, written by the mayors of Oslo, Rio de Janeiro, Stockholm, Sydney and Vancouver, can be read here.