AS A state senator in Massachusetts, Scott Brown voted for a regional pact to ratchet down power companies’ carbon dioxide emissions. But as a US senator in Washington, Brown last week voted to strip the Environmental Protection Agency of the authority to regulate any greenhouse gases. This is not the first time Brown has done a Jekyll-Hyde switch between Beacon Hill and Capitol Hill — in 2006 he voted for Mitt Romney’s health care plan, only to oppose the national version of it in 2010. But the turnaround on greenhouse gases is especially disappointing to any Massachusetts voters who thought they saw in Brown a conservative on fiscal issues who was also a conservationist when it comes to protecting the environment.

Instead, Brown has marched in lockstep on this issue with fellow Republicans who are against government regulation across the board. Many congressional Republicans do not even acknowledge that greenhouse gases contribute to climate change and need to be curbed. Clearly, Brown did not share that view when he voted in favor of Massachusetts’ participation in the Regional Greenhouse Gas Initiative, which requires utilities to reduce carbon dioxide emissions or face financial penalties.

The bill Brown backed would also prohibit any improvement in auto fuel-efficiency standards after 2016. Advances in car mileage prompted by the fuel-efficiency standards that began in the ’70s have been a rare success in this country’s halting effort to reduce greenhouse gas emissions.

Fortunately, the bill Brown supported did not pass. If it had, and survived a likely Obama veto, it would have dealt a blow to the Massachusetts and regional economy. The state is home to a promising clean-energy sector, with many companies working on new-generation batteries, sustainable biofuels, and solar technology. Between 2006 and 2009, the Northeast received more than 20 percent of all venture capital investment in clean energy.

Voting to strip the EPA of its authority to regulate greenhouse gases is a favor to Big Oil and Big Coal and the states where they are dominant, not to Massachusetts clean-energy firms struggling to compete with companies in Europe and Asia. When Brown voted for the regional pact on utility emissions in 2008, he said, “Passing this legislation is an important step . . . towards improving our environment.’’ He was right then, and wrong now.

© Copyright 2011 Globe Newspaper Company.