U.S. government scientists warned us Monday that the Chesapeake Bay will be washing over our ankles and threatening to dampen our knees within decades because of global warming.

So why is it so hard to do anything serious to cut greenhouse gas emissions to protect the planet?

Two major environmental battles over projects in Maryland are about to reach turning points in disputes that highlight the challenges.

Coincidentally, both are on the Chesapeake’s low-lying shore, which is especially vulnerable to rising sea levels.

In one, Gov. Martin O’Malley (D) must choose this month whether to proceed with plans to build a $200 million wind turbine farm to produce clean electrical energy in Somerset County on the Eastern Shore.

In the other, national environmentalist groups are strongly urging the Obama administration to put the brakes on a $3.8 billion project to start exporting liquefied natural gas from the Cove Point plant in Calvert County. Federal regulators are to release a crucial environmental assessment May 15.

In theory, these two should be easy calls. Of course we should do whatever we can to promote renewable energy and reduce reliance on fossil fuels.

That suggests we should build the wind farm and kill the gas plant.

In practice, however, politicians, businesses and local communities often place a higher value on jobs, profits and tax revenue.

Moreover, the trade-offs are more complicated than usual in the two Maryland cases because national security and America’s foreign trade balance might be affected.

Regarding the wind farm, O’Malley is inclined to give a green light to clean energy. He would do so by vetoing a bill that would effectively kill the project.

But such a veto would anger the state’s most powerful congressman, Rep. Steny Hoyer (D), and top Democrats in the General Assembly.

Hoyer and his allies are unhappy that the wind turbines’ rotating blades would disrupt military radar exercises across the bay at the Patuxent River Naval Air Station. They fear that could eventually mean naval base jobs are sent elsewhere — out of Hoyer’s district.

The concerns are overblown. The wind farm developer has already agreed to avoid the problem by switching off the turbines when radar exercises are underway.

“Science just doesn’t lie — if the turbines don’t spin, there is no impact on the radar,” said retired Air Force Col. David Belote, who was the first executive director of the Pentagon office that handles conflicts between renewable energy facilities and military operations.

Moreover, the Pentagon could block permits for the turbines if it determined in the future that national security were somehow at risk.

O’Malley should veto the bill and burnish his already laudable record for promoting wind power.

The stakes are higher in the controversy over Dominion Resources’ Cove Point project, 65 miles southeast of Washington. It has attracted surprisingly little attention outside of environmentalist circles.

Activists are aghast over Cove Point almost to the same degree that they oppose the better-known Keystone XL pipeline. They warn it would create a risky precedent by helping to pioneer a major new U.S. industry shipping natural gas overseas.

Environmentalists would rather see money invested in renewable energy such as wind and solar power. They also say export demand would encourage increased use of the controversial gas-drilling technique called “fracking.”

“If this facility can get approved in Maryland, a progressive state with a strong environmental record of leadership, then the door will open to fast-track gas exports off our shores without adequate review,” said James McGarry, chief policy analyst of the Chesapeake Climate Action Network.

But the arguments in favor of Cove Point are strong, too. Normally, everybody wants to increase exports, especially of energy. It helps our trade balance and reduces our dependence on foreign oil.

More important, Cove Point’s defenders argue that using natural gas, overall, would help the climate by replacing dirtier fuels such as coal.

Environmentalists contest that, however. They warn credibly that any advantages over coal could be lost because of gas leakage during drilling and transportation to Cove Point, and onward to China and India.

I agree with 16 environmentalist groups that wrote President Obama in March calling for a more rigorous review of the pluses and minuses for the climate before proceeding.

A robust new U.S. natural gas export industry is alluring. But we should fully understand the consequences.

Tough decisions, indeed. Let’s hope we make the right ones before the Chesapeake is up to our waists.

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