Stock investors have long enjoyed a smorgasbord of options for investing in their beliefs. There are mutual funds and exchange-traded funds for the teetotaler, the peacenik and the conservationist. But when it comes to the $96 trillion bond market, the opportunities for so-called socially responsible investing can be pretty slim. Of course, that's not to say clever bond issuers haven't done their best to get in on this feel-good market nonetheless.

Witness the creation of green bonds -- and, along with them, a fresh debate on both their purpose and their value. This spring, for the first time, Bank of America Merrill Lynch made World Bank issued green bonds available to its wealth-management clients, who can now buy the 10-year bonds in denominations as low as $1,000. A handful of fund managers are also investing in the bonds, which finance endeavors such as an eco-farming initiative in China or an irrigation project in Tunisia. Some U.S. municipalities are also looking into funding environmental efforts with their own version of green bonds.

World Bank green bonds are designed for conservative investors. The AAA-rated bonds typically yield about the same as comparable U.S. Treasury bonds. Benji Bailey, comanager of the $302 million Praxis Intermediate Income fund, likes the World Bank's green bonds for diversification. "We're not just investing because of the positive story," Bailey says. The one-year total return of a green bond he owns, which matures in 2013, is 2.4 percent, or $240 in income a year on a $10,000 investment, about $20 less than what a comparable Treasury would return. Bailey also owns some utility-issued bonds that fund green power generation; those are lower-rated and yield more than the World Bank bonds.

But critics say green bonds aren't all rosy. Steven Kyle, a professor of applied economics at Cornell University who once worked as a bond trader for the World Bank, says, "It's just a marketing ploy": In a large, complex organization, who's to say which bond money finances which projects? What's more, the green-bonds market is tiny, at an estimated $6 billion to $12 billion -- barely a drop in the ocean-size global bond market. And finally, the bonds don't trade as often as highly liquid Treasury bonds. That limits the options for selling individual green bonds before they mature -- should the need arise.

A World Bank spokesperson, meanwhile, says that although the organization can't pinpoint which bond proceeds support which particular project, the money goes into an account that funds only environmental initiatives. And investors who hold their bonds to maturity need not worry about selling their bonds at a discount. Neither argument totally sways critics, but those such as Kyle say they do see an advantage to the bonds: "If it makes people happy, then why not?"