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. It analyzed nine greenhouse gases, and rated companies based on the number of metric tons of carbon dioxide, or equivalent emissions, relative to sales.

For each fund, Trucost adjusted carbon dioxide figures according to the amount the fund invested in a particular company. Thus, a fund that invested in a tiny percentage of an oil company would only be tagged for an equally tiny amount of carbon.

The Trucost report, which used data from 2007 and 2008, showed wide variations in the carbon footprints for leading mutual funds. The fund with the biggest footprint was 38-times more carbon intensive than the fund with the smallest footprint, the report said.

The ranking also showed that funds with the smallest carbon footprints naturally steered away from power companies and the oil industry, investing instead in financial services, banks and health care.

The Financial Select Sector SPDR Fund topped the list with the smallest footprint. Its investments, which were worth $7.8 billion at the end of 2008, emitted an equivalent of 40 tons of carbon dioxide per $1 million in revenue. It was followed by the Vanguard Heath Care Fund and PowerShares QQQ Trust.

The fund that was the most carbon intensive was the iShares FTSE/Xinhua China 25 Index Fund. Its investments, worth $5.9 billion at the end of 2008, emitted 1,549 tons of greenhouse gases per $1 million of revenue, according to Trucost.

That was roughly double the footprint of the next largest emitter, the Fidelity Capital Appreciation Fund, which had 758 tons of carbon dioxide, or equivalent gases, per $1 million in revenue.

The report also noted that Sustainability/SRI funds, which typically screen investments for ethical and environmental practices, posted a variety of carbon footprints. They filled out the middle of Trucost's ranking system with neither the best nor the worst performances when it comes to keeping low carbon emissions.

Salo said he expects some of them to re-evaluate their investments now in the future.

"This data hasn't been available before," he said. "They've been flying blind."

Trucost Chief Executive Simon Thomas said the report will be the first in a series of carbon footprint rankings. The purpose, he said, is to give investors and fund managers enough information to fine tune their investments in emerging carbon economy.

"Our hope is that investors start referring to this habitually in their investment decision making," Thomas said. "We think they should."

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On the Net:

http://www.trucost.com/carbonfootprint.html

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