A group of socially responsible investment managers and others are urging U.S. senators to ensure that climate and energy legislation doesn't include provisions that would lead to more investments in high-carbon fuels, such as Canadian tar sand oil, liquid coal and oil shale.

In a letter sent to senators today, the group asked that any final climate and energy bill not weaken a current law that prohibits federal agencies from procuring unconventional fuels with higher greenhouse gas impacts than conventional fuels.

Allowing federal agencies to expand use of these fuels implicitly encourages U.S. companies here and abroad to invest in costly energy and infrastructure projects, the letter said.

Also, legislation that makes the United States more dependent on these energy sources could worsen the impact of future volatility.

The group added it supports a low carbon fuel standard. Such a standard would "forestall future price spikes by providing for an efficient transition to cleaner fuels. Rather than dictating the use of particular fuels, it will allow the market to determine the most efficient way to reduce the carbon content of fuel," the letter says.

Those signing the letter included Michael Passoff, associate director, corporate social responsibility program, As You Sow; Bennett Freeman, Senior vice president, sustainability research and policy, Calvert Asset Management Company Inc.; Susan Vickers, RSM VP Community Health Catholic Healthcare West; Steve Scheuth, president, First Affirmative Financial Network; Kristina Curtis, senior vice president, Green Century Capital Management; William C. Thompson Jr., New York City comptroller; Leslie E. Christian, president and CEO, Portfolio 21 Investments; Sister Patricia Marshall, cdirector, social justice office,  Sisters of the Blessed Sacrament; Shelley Alpern, director, social research and advocacy, Trillium Asset Management Corp.; and Patricia A. Daly, executive director, Tri?State Coalition for Responsible Investment.