The most attention-grabbing event during the week of the U.N. Climate Summit was the People’s Climate March, of course. But the second most attention-grabbing event was perhaps the Divest-Invest coalition’s announcement that at least 656 individuals and 181 institutions and local governments had signed onto their pledge, from actor Mark Ruffalo to the British Medical Association to the World Council of Churches. These investors collectively control more than $50 billion, and they have promised to make no new investments in the largest 200 oil, gas, and coal companies, sell their existing fossil fuel assets within five years, and invest in clean energy.

Among the groups to join was the Rockefeller Brothers Fund, which drew the attention of major media like The New York Times and The Washington Post, since the family’s fortune was made from oil. (The Rockefellers cleverly spun that apparent irony by saying that John D. Rockefeller was merely at the forefront of energy development for his time, and if he were alive today he would be investing in renewable energy.)

But does divestment actually accomplish anything? When you divest your shares of a fossil fuel company, you sell them to someone else. “In the early stages, you’re not starving companies of capital because if you divest, somebody else invests,” observes Nathaniel Bullard, director of content for Bloomberg New Energy Finance, an energy market news and analysis service. It would take an extraordinarily large amount of divestment to actually hurt a company’s stock price. And if it did, less scrupulous investors might see a deal on the undervalued stock and just prop it back up.