Earlier this week, New York City Mayor Bill de Blasio and his London counterpart, Sadiq Khan, co-authored an op-ed in the Guardian challenging other cities to join them in divesting their public pensions and other assets from fossil fuels. “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,” the mayors wrote. Less than two per cent of the London Pension Fund Authority’s investments, which total about $7.1 billion, are in extractive fossil fuels; New York is currently the largest city in the world to commit to total divestment, which means removing five billion dollars from the industry within five years.

The divestment movement, which began as little more than a campus protest, has been building extraordinary momentum in recent years. In 2014, institutional investors who had committed to divestment, mostly small cities and colleges in New England and along the West Coast, collectively controlled about fifty billion dollars. According to a report released the same day as de Blasio and Khan’s op-ed, investors representing more than six trillion dollars in assets have now committed to divest, an increase of nearly twelve thousand per cent. Oil companies have started to flag the movement as a material risk in their securities filings.

Divestment advocates—including Thomas Pringle, the Irish Parliament member who introduced a successful bill to divest all of Ireland’s public funds from fossil fuels, Father Paul Moonjely, the executive director of Caritas India, the country’s largest Catholic charity, wealth fund managers, and members of the climate-change nonprofit 350.org—gathered on Monday evening to celebrate the movement’s progress among the old iron presses at the American Bookbinders Museum, in San Francisco. The extravaganza that brought them here, Governor Jerry Brown’s Global Climate Action Summit, a gathering of environmentalists from all over the world to discuss how to prevent climate change from destroying civilization, started Wednesday. But a number of surprising things had already occurred. Brown began the week by issuing an executive order that commits California to reducing its net output of greenhouse gases to zero by 2045. Considering that California is now the fifth-largest economy in the world, Brown’s order is, arguably, the most ambitious climate target in history.

Jeremy Grantham, who co-founded the multibillion-dollar asset-management firm Grantham, Mayo, van Otterloo & Company (abbreviated, perhaps unfortunately, as G.M.O.), was at the party. He famously predicted the dot-com and housing bubbles of 2000 and 2007, respectively. He is sanguine, however, about the long-term economic effects of the coming transition from fossil fuels to renewable energy. A diversified stock portfolio that excludes energy companies will perform precisely as well as one that includes them, according to a recent analysis by Grantham’s firm. “We looked at the real cost of divestment, and there is none,” he told me. “The data is clear.” In the short term, he said, there would be some dips, but the “wonderful thing about the stock market” is that it has priced energy companies consistently well. As for the companies themselves, “if they bend with the wind, admit their problems, they could make good money on the way out,” he said. If not, “rather like dopey Ford and General Motors, they will end up being far behind the new technologies,” and, in the long run, struggle to maintain their profitability.

Kat Taylor, the C.E.O. of a community bank and the owner and founder of TomKat Ranch, which produces sustainable grass-fed beef and offers educational programs on sustainable food systems and farms, introduced herself to Grantham, thanking him for his divestment presentation, and his good humor. “It’s nice to finally meet you—I’m Tom Steyer’s wife,” she said. Steyer is a billionaire and former hedge-fund manager who has given millions to environmental causes, and currently supports one of the biggest Democratic super PACs. “Tom Steyer says hi, too,” Taylor added. “Cheers.”

“Let’s kick ass,” Grantham said, as they clinked glasses.

I asked Grantham who he thought needed to divest next. “Harvard, Yale, Princeton,” he said. “They have the best optics.” He took a long sip of white wine. “The actual economics of divestment are not nearly as compelling as divestment people want you to believe, but the pariah aspects are much more important.” According to Grantham, in the short term, divestment alone is not going to have a major impact on the bottom lines of fossil-fuel companies; petroleum and gas are so deeply integrated with other sectors—shipping, airplanes, steel, cement, petrochemicals—that their decline will likely be slow and irregular. Nevertheless, as climate impacts worsen, and the divestment movement continues to gain steam, investors will have a fiduciary responsibility to address climate risks.

Of course, it will take more than institutional divestment to meaningfully slow rising temperatures. One of the main goals of the summit, which, in addition to Brown, is hosted by Michael Bloomberg, the former New York City mayor, and Xie Zhenhua, China’s top officer for climate-change affairs, is to continue the work of achieving the 2015 Paris Agreement target—limiting warming to no more than two degrees Celsius—despite the Trump Administration’s decision to withdraw from the accord, in June, 2017. Remarkably, there are signs of success. Subnational governments and businesses across the United States have formed coalitions, such as the We Are Still In network, which has nearly three thousand institutional signatories, that are committed to meeting the climate targets set by the Obama Administration in 2015—a twenty-six- to twenty-eight-per-cent emissions reduction, below 2005 levels, by 2025. According to a new report compiled by a group called America’s Pledge (led, like the summit, by Bloomberg and Brown), which was released on Wednesday, the U.S. is almost halfway to those original targets. Based on emission-reduction mandates already in place, the country will likely reach two-thirds of the agreed targets (a seventeen-per-cent reduction) by the 2025 deadline.

At the same time, the America’s Pledge report offers ten “readily available” climate strategies that would lead to a twenty-one-per-cent reduction in over-all emissions. And, its authors suggest, if coalitions can enact even tougher measures and incorporate a broader range of states, cities, and businesses, the country could nearly achieve the targets of the Paris Agreement without any help from the federal government. Coal offers one of the best examples of how Trump and other climate-denying policymakers are swimming upstream; despite Trump’s assurances that his Administration has “ended the war on beautiful, clean coal,” coal-plant retirements are happening at a faster rate than ever before.

Still, the most important outcome of the divestment movement is how it will redirect money into clean-energy and carbon-neutral technology. On Thursday, de Blasio announced a pledge to invest more than four billion dollars of the city’s public-pension funds into renewable-energy companies and other climate-change-related sectors. While this represents only a small percentage of New York’s total pension funds, worth more than a hundred and ninety-five billion dollars, the investment suggests that the next phase of the movement is under way. As Khan and de Blasio noted in this week’s op-ed, “If we want to fund the scale of transformation the world needs, we must foster sustainable investment and use the power of institutional investors.”