Teams have taken other steps to change or reduce their waste: Cleveland, for instance, introduced biodegradable cups made of cornstarch last year and the Mariners have recycled food waste for use as composting since 2007. Other eco-conscious steps include bicycle valet service at some stadiums for fans on game days and using biodiesel—instead of fossil fuels—in field maintenance equipment.

The green zeitgeist has extended beyond the activities inside ballparks to stadium design over the past few years. A prime example is the Washington Nationals's new home in southeast D.C. that opened last year, earning baseball's first leadership in energy and environmental design (LEED) certification. Developed by the U.S. Green Building Council (USGBC), the LEED program's auditors awarded green points based on the Nationals' high-efficiency lighting and air-cooling that saves millions of gallons of water a season, among other conservation metrics.

Nationals Park planners had the environment in mind well before pouring concrete: They chose to build on a reclaimed brownfield site along the Anacostia River south of the Capitol that was once contaminated by chemicals. To prevent stadium-generated pollutants from leaking into the river, workers buried five stories of sand underground to form a state-of-the-art filtering system as well.

On deck for LEED certification include the Minnesota Twins's ballpark opening next year in Minneapolis and the Florida Marlins's new field in Miami (the team will become the Miami Marlins in 2012 when the stadium opens), with the MLB ponying up $1 million toward the Marlins’s facility to ensure greenness, says McHale. The San Francisco Giants, meanwhile, have been upgrading in an attempt to make nine-year old AT&T Field the first environmentally retrofit stadium to get LEED accreditation. (The Giants installed the league's first solar panels in April 2007 a few months before the Indians followed suit.)

Brand-new stadiums opening their gates this season also exhibit some of the latest in green stadium technology and conservation. The lavatories at the New York Mets's newly built Citi Field in Queens, N.Y., for example, feature metered hands-free faucets and waterless urinals. "It's crazy to use drinking water to flush urine," says Hershkowitz.

Across town in the Bronx, the Yankees's new stadium will employ reflectors to amplify stadium light output, using 300 watts per lamp less than standard field-illuminating lights. The Yankees say these advanced lighting systems will save over 100 tons of carbon dioxide emissions per night game—the "ballpark" equivalent, if you will, of planting a tree for every pitch that's expected to be thrown during a home game for the 2009 season.

Even the nation's oldest MLB stadium still in use—Fenway Park, home of the Boston Red Sox, the Yanks's biggest rivals—is getting in on the greening (puns about its legendary "Green Monster" wall in left field aside). Some 28 newly installed solar panels on the fifth floor's slanted roof offset 37 percent of the natural gas burned to heat the stadium's water supply, says Katie Haas, a team spokesperson. The park also sports 11 BigBelly solar-powered trash compacters on the premises.