How much carbon is in your investment portfolio?

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.