BETTER LATE than never, Gov. Robert L. Ehrlich Jr. (R) has decided to become something other than an obstacle in Maryland's intensifying debate on tougher air pollution rules. Last month the governor, who has opposed tough legislative efforts to reduce toxic emissions from coal-fired power stations, proposed a more modest regulation that would compel six of Maryland's nine plants to cut their production of three major pollutants -- nitrogen oxide, sulfur dioxide and mercury. Mr. Ehrlich's initiative had the somewhat rushed feel of political expediency; he announced it without producing a text of the regulation itself, and he did so just two days after a rival in next year's gubernatorial race, Montgomery County Executive Douglas M. Duncan (D), endorsed the tougher legislative approach that the governor has opposed. Still, Mr. Ehrlich's about-face, whatever its motivation, represents a step in the right direction, albeit a limited one.

The chief limitation in his "Clean Power Rule" is that it does nothing at all about a fourth major pollutant, carbon dioxide gases, which scientists have identified as a major cause of global warming and rising sea levels. In this, at least, Mr. Ehrlich has been consistent; his administration has refused to enlist Maryland in a coalition of Eastern Seaboard states drawing up emission limits for carbon dioxide gas. Opposition to tough measures to combat carbon emissions is understandable since, in the case of coal-fired plants, it would mean cutting power generation or shutting down plants altogether. But by failing to include even a modest goal -- say, a reduction in carbon emissions of 5 or 10 percent over 15 years -- Mr. Ehrlich misses an opportunity to encourage investment in renewable energy sources and more energy-efficient consumer products and manufacturing equipment.

The governor's proposal, although its details remain unknown, appears to be the right move on slashing ozone and fine particle pollutants. Nitrogen oxide, which contributes to smog and causes asthma, bronchitis and other breathing disorders, would be cut by 69 percent; sulfur dioxide, an ingredient in soot and a contributing cause of heart attacks and strokes, would be cut by 85 percent. Filtering out most of those substances could cost hundreds of millions of dollars, and one of the two power companies that own the affected plants suggested that at least some of the cost would be passed along in the electricity bills paid by customers over time. If so, it's a price worth paying. And in any event, Mr. Ehrlich's proposal is hardly revolutionary; it would simply nudge the state toward meeting standards for 2010 already set by the Environmental Protection Agency.

As a byproduct of those reductions, the governor's regulation would also slash mercury emissions by 70 percent by 2018, in line with the Bush administration's goals. But those goals are a shadow of much stronger ones set by the Clinton administration, and the technology for filtering mercury from plant emissions is relatively cheap. Mr. Ehrlich acknowledged that his plan represents a "balance" of what was doable without crippling two of Maryland's biggest power providers. That may be the case. Still, lawmakers should regard the governor's blueprint as a work in progress, ripe for improvement.