The oil and gas boom is reducing U.S. greenhouse gas emissions, Energy Secretary Ernest Moniz said Monday.

"The increased production of oil and natural gas in the United States has, obviously, been a major story in terms of our economy, and also our environment," Moniz said at a field hearing in Seattle convened by Sen. Maria Cantwell of Washington, the top Democrat on the Senate Energy and Natural Resources Committee.

"The natural gas boom, in particular, has led to the displacement of high-carbon coal with low-carbon natural gas producing fewer [carbon dioxide] emissions," Moniz said.

Moniz's comments follow those by the head of the Energy Information Administration, the Energy Department's analysis arm, earlier this month, indicating that carbon emissions are lower than they have been since 1992 because of increased reliance on natural gas from hydraulic fracturing, known as fracking.

Environmental groups have been heaping pressure on the Obama administration, Democrats and Republicans to support a ban on fracking, saying that the natural gas and oil produced from the process are raising greenhouse gas emissions and exacerbating climate change. Instead, they want 100 percent renewable energy.

Monday's hearing was meant to examine the increased pressure that oil and gas production has placed on the nation's infrastructure, especially for Cantwell's state that has become a key chokepoint for shipping U.S. fossil fuels and other commodities to foreign markets.

Moniz said the increased demand for moving oil and natural gas around the country has placed "big strains" on the nation's existing transportation infrastructure, where rail has been increasingly tapped to make up for the lag in pipeline capacity.

Nevertheless, slowing production has reduced pressure of rail shipments nationwide, except for the Northwest and Cantwell's state where traffic remains steady.

Cantwell, who supports taking steps to fight climate change and reduce emissions, did not appear opposed to keeping the oil trains from continuing to roll into her state. In fact, she said the oil and gas boom has achieved "energy independence" in making the nation less dependent on foreign oil.

"This is a pretty big shift for the United States of America," Cantwell said. "We wanted energy independence. But now we got it."

The issue in front of her is "our own safety and security issues in the Northwest," she said.

The priority is making sure it is transported safely, especially in her state where many of the trains move through highly populated areas to reach Washington's ports.

Her state's concern rises from the possibility of oil train derailments, which have become almost common place in recent years. The derailments have caused fires, explosions and even deaths.