As the fossil fuel divestment movement enters its fourth year, nearly a third of the 28 schools that signed on have completely purged oil, natural gas and coal investments from their endowment portfolios.

Many of the nine schools that have fulfilled their divestment pledges joined the fast-growing campaign as early as 2011; this group mostly involves small New England and California colleges that are overseeing relatively modest endowments.

Unity College, a small private school in Maine, is one of them. In 2012, the college pledged to divest from fossil fuel companies––then 3 percent of its total exposure­––over the next five years. But it finished the task in 2014, three years ahead of schedule. And so far, the school's portfolio "is performing past our expectations," said Bob Mentzinger, a Unity College spokesman.

Organizers of the movement, including 350.org and scores of affiliated groups around the world, are pushing for more converts.

This week, they won the support of the United Nations group in charge of ongoing talks aimed at creating a global climate treaty. And with the addition of eight universities so far in 2015, the fossil free movement expects this year to top last year's divestment surge.

"This campaign...gets more momentum every day," said Karthik Ganapathy, a spokesman for 350.org, the group credited with launching the movement and helping support similar campaigns overseas.

Beyond academia, fossil fuel divestment has been embraced by 41 cities, 72 religious institutions, 30 foundations and more than 650 individuals—representing investment portfolios worth an estimated $50 billion. They include a broad spectrum of interests, from the Rockefeller Brothers Fund and the Church of England to the Council of Canadians and the Great Old Broads for Wilderness.

Organizers hope to add two heavyweights to the list this year: Harvard University and Oxford University.

Repeatedly, students at Harvard have pushed for the school's divestment of fossil fuels from its $36 billion endowment––and the university has refused. In an October 2013 letter rejecting divestment, Harvard president Drew Faust wrote: "the endowment is a resource, not an instrument to implement social or political change." Since then, the university has increased its publicly traded holdings in the energy sector.

Meanwhile, Harvard students have expanded their campaign. In recent months, they've held protests on campus, secured support from faculty and alumni, and sued the school over its refusal to divest. The court hearing was held in late February; a decision is expected in the coming weeks. Cambridge, Mass., Harvard's host city, has already agreed to divest from fossil fuel companies.

Oxford officials recently delayed a decision on divestment until May.

Winning divestment pledges from those storied universities would be a "cultural tipping point," according to 350.org's Ganapathy. The movement, he said, would "explode."

Schools and others who pledge to divest set their own terms, but most of them plan to sell off stocks and bonds tied to oil, natural gas and coal companies. In addition, they typically plan to make the transition over several years—or as long as a decade in the case of very large portfolios.

The primary argument for divestment, in a nutshell, is: "If it is wrong to wreck the climate, then it is wrong to profit from that wreckage," according to 350.org's website.

At the moment, fossil fuel divestment looks like a shrewd financial move, too. The value of coal stocks has dropped precipitously amid stiffening regulation and competition from power plants using natural gas and renewable energy sources.

In the oil market, crude prices worldwide have dropped to between $40 and $50 a barrel—down more than 50 percent since mid-2014. Amid the turmoil, oil company share prices have fallen to levels that weigh down portfolio profits. Some believe those trends will continue, arguing that tackling climate change will cut fossil fuel demand and render some company reserves worthless.