NEW YORK - Quick: How green is your 401(k)? If you can't tell, you're not alone.

Mutual funds rarely provide details about the greenhouse gas emissions that are tied to their investments - an oversight that could become costly as cap-and-trade laws gain a foothold in the United States.

On Wednesday, environmental researcher Trucost published what it says is a first-ever ranking of mutual funds according to their carbon footprints. Trucost's analysis of 91 funds is meant to help investors gauge how emissions laws could affect a fund's holdings.

"Carbon emissions are a real financial issue that will soon have a real price in the U.S.," said James Salo, a Trucost researcher who wrote the report. "Investors can use this to protect their assets."

The report, which examined 75 major equity funds and 16 sustainability/socially responsible investment funds with a combined value of $1.55 trillion, will feed a growing appetite for emissions data from Wall Street.

Aside from environmental concerns, investors say they need help identifying who could be hit the most as companies start to pay for the amount of carbon they send into the atmosphere.

The U.S. hasn't established a national cap-and-trade system, though regional initiatives are set to begin in the next few years. A recent plan by Democratic lawmakers also proposes cutting greenhouse gases by a fifth over the next decade.

"Investors are keenly interested in seeing companies be more transparent" about how they'll be affected, said Mindy Lubber, president of Boston-based Ceres, a network of investors and environmental groups.

Ceres started the Global Reporting Initiative, which asks multinational companies to volunteer information about environmental sustainability. It now wants the Securities and Exchange Commission to require companies to disclose their climate risk in regulatory filings.

"When it's mandated by the government, everybody's got to do it," Lubber said.

An alliance of faith-based groups known as the Interfaith Center on Corporate Responsibility also has partnered with Trucost in compiling "climate risk profiles" of major companies.

ICCR members, who control about $100 billion in investments, use the climate profiles to judge if they've put money in the right places, said Patricia Daly, executive director for the tri-state coalition.

"Asking companies to be socially and environmentally responsible, to plan their future in a sustainable way, is a critical piece of how they're going to perform financially," Daly said.

For its ranking system, Trucost used an extensive database that included environmental information on 4,500 companies around the world.