Ahead of the Global Climate Action Summit in San Francisco and on the heels of more than 250,000 people around the world joining the weekend's #RiseForClimate demonstrations, New York City Mayor Bill de Blasio and London Mayor Sadiq Khan published an op-ed on Monday calling on all cities to divest their assets from fossil fuel companies and to take bold action to address the climate crisis.

"Okay, this is a seriously big deal... Momentum!"

—Bill McKibben, 350.org

"We believe that ending institutional investment in companies that extract fossil fuels and contribute directly to climate change can help send a very powerful message that renewables and low-carbon options are the future," they wrote in the Guardian. "Both our cities are also investing in a sustainable future."

In addition to outlining the cities' specific efforts on both fronts, the op-ed also announced that "working through the C40 Climate Leadership Group, London and New York will co-chair the Cities Divest/Invest forum, a global network for cities that will not only enable us to share tools, knowledge, and experiences, but also to advocate for action on divestment and green investment."

Many climate campaigners celebrated the mayors' call to action, including 350.org co-founder and longtime divestment advocate Bill McKibben, who tweeted: "Okay, this is a seriously big deal. The mayors of New York and London are challenging every other city on the planet to join them in divesting from fossil fuel. Momentum!"

His group noted the op-ed comes after the #RiseForClimate actions, which featured the West Coast's largest ever climate march in San Francisco, the location of the upcoming summit:

100,000s people set the test for real climate leadership this weekend with 900+ #riseforclimate actions. Two of the world's most iconic cities just responded! #fossilfree https://t.co/GOJ043ax2M SCROLL TO CONTINUE WITH CONTENT Never Miss a Beat. Get our best delivered to your inbox.





— 350 dot org (@350) September 10, 2018

While advocates welcomed the op-ed, they have, at times, criticized officials in both cities for not going far enough to address the worldwide climate crisis—much like California's Democratic Gov. Jerry Brown, who is hosting the San Francisco summit and is under pressure from voters, activists, and other politicians from his state to match his promises with pioneering action.

"As mayors, we are not only committed to taking bold action to tackle climate change and to improve the lives of those we represent, but also to showing others the way."

—New York City Mayor Bill de Blasio and London Mayor Sadiq Khan

California has spent the summer battling massive, historic wildfires that experts say are and will continue to be made worse by the the global crisis—along with temperature increases, dangerous heat waves, intense rain events, and more, according to a recent report by the state's scientists. But California is not alone in facing this—in the words of Brown—"apocolyptic threat."

Pointing to findings that Europe's summer heat wave was made more than twice as likely by climate change, and the "growing evidence" that ties the crisis to other extreme weather," the mayors wrote: "This summer it seemed as if our two cities had changed places. London was hot and dry while New York had days and days of rain."

"It's clear that what we think of now as freak weather in our cities is likely to become the new normal, and that climate change poses a huge threat to the futures of our children, and many generations to come," they concluded. "As mayors, we are not only committed to taking bold action to tackle climate change and to improve the lives of those we represent, but also to showing others the way."