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This has been a big week for climate-change activists. Some 400,000 people marched in New York on Sunday ahead of a United Nations summit on curbing carbon emissions and global warming. And the news coverage brought attention to a new protest tool: Divestment.

In recent years, college students—taking a page from campus anti-apartheid activists in the 1980s—have been pushing their schools to stop investing in fossil fuel-producing companies. The movement has spread to other kinds of nonprofits and investors. This week, the Rockefeller Brothers Fund, a philanthropy built on the Standard Oil family fortune, announced that it was already divested from coal and tar sands and was moving out of other fuels. Earlier, Stanford University said it wouldn't put money in coal, and several cities have joined the divestment charge.

So let's say you too want to join this burgeoning movement and get fossil fuels out of your own investment portfolio. How would you do it? It's not going to be easy. If you have your 401(k) or IRA invested in a diversified U.S. stock fund, there's a good chance Exxon Mobil and Chevron are among your biggest individual stocks holdings. Those two oil giants alone represent 4% of the S&P 500 index, a standard market benchmark. Most professional fund managers think they simply have to hold energy stocks.

There's a niche of mutual funds and ETFs that invest with the environment in mind. You have to look at them carefully to make sure they fit both your social and investment goals. Some are way too specialized and risky to use as a core piece of your portfolio. The PowerShares WilderHill Clean Energy ETF, for example, invests specifically in firms that use or develop alternative energy sources or technology. It lost an annualized 7.8% over the past five years, compared to an annualized gain of over 16% for the broad market. In short, says Morningstar analyst David Kathman, such specialty industry funds are "not something that should take up too much of portfolio."

Other funds consider climate change alongside several other environmental and social issues in their selection of stocks, and manage not to veer too far from what you'd get in, say, a plain-vanilla S&P 500 index fund. If you have a low-cost, well-diversified index investing approach—and you should— some of these mutual funds and ETFs can indeed be a sound choice.

Just know that they won't necessarily get you totally out of fossil fuels. The Vanguard FTSE Social Index mutual fund, which charges a low 0.28% management fee and follows an index of 376 U.S. stocks, doesn't hold Exxon Mobil or Chevron, and is relatively light on energy stocks in general. Nonetheless, it recently included Occidental Petroleum and Anadarko Petroleum , both of which are on the Carbon Underground 200, a list of coal, oil and gas producers cited by many divestment activists.

Occidental and other fossil fuel companies can also be found in another solid-looking fund, TIAA-CREF Social Choice Equity. It's not an index fund, but cuts a similar profile to one: It has low expenses of 0.48%, and doesn't try to beat the market, but instead to get as close as possible to replicating the benchmark while removing companies that don't pass its social and environmental screens. It's actually done a much better job than the Vanguard fund at keeping pace with the gains of the broad stock market—over the past decade it has earned an annualized 8.4% to the S&P's 8.3%. (Vanguard earned 7%. ) But also similar to the general market, the fund has about 10% of assets in energy companies, though again minus Exxon Mobil and some other big energy companies.

Both TIAA-CREF and FTSE, the company that runs the custom index Vanguard tracks, say that even though they own some fossil-fuel extractors, they hold significantly less than you'd get in similar funds.

The point isn't necessarily that these funds aren't green enough, but that they are trying accomplish something different from the aims of the new divestment ethos. TIAA-CREF, for example, will give an energy company credit for improving its environmental profile. Divestment, on the other hand, is tightly focused on spreading the idea -- popularized by the writer and organizer Bill McKibben -- that if the carbon reserves held by the world's coal, oil, and gas companies were actually burned, climate change would go catastrophic. "The focus of the divestment movement is on [fossil-fuel] reserve ownership, because most of that has to stay in the ground," says Stuart Braman of Fossil Free Indexes, which maintains the Carbon Underground 200 list.

If you really want pure fossil-free right now, there's a handful of smaller funds that avoid fossil fuels. But they tend to have higher annual fees and most are actively managed, so you'll have to make a judgment about managers' stock picking skills alongside your social criteria. It's hard to find managers who beat the market in general—all the more so when you are choosing from a small set of green funds.

More options may be coming. Braman's company recently launched an index that is basically the S&P 500 minus the Carbon Underground list. The new index is now being used by an asset manager for institutions and high-net worth individuals. Meanwhile, FTSE has also built several indexes that exclude fossil fuels as an investment tool for the Natural Resources Defense Council, and the asset management giant BlackRock is offering strategies based on the indexes to institutions.

Neither of these new indexes are available to ordinary investors right now. But given how how many new index strategies are being tuned into exchange-traded fund these days, it may not be long before a low-cost, fossil-free ETF hits the market.

For now, though, the fact that investing fossil-free is a bit difficult isn't entirely a bad thing for divestors. After all, their end goal isn't to create a convenient new option for the investment equivalent of Whole Foods shoppers. Selling your stock in Exxon Mobil, by itself, doesn't take any money away from that company. If all that happens is that some people quietly shift their IRAs to fossil-free funds, it will merely swap assets from one set of investors to another.

The point of divestment isn't just to do it, but to fight over it. As Matt Yglesias argued at Vox.com this week, divestment will be most effective if it changes the conversation. It aims to draw attention to oil, gas, and coal companies and to stigmatize them, chipping away their social and political power. ("Revoking their social license," McKibben has called it.) New fund options could help the divestment movement mainly by making their demands on schools and institutions more plausible.