【Down Town】

President Donald Trump took his first swing at the Environmental Protection Agency (EPA) when he tapped the agency's biggest opponent -- a man who denies climate science -- to run the show. His second hit came earlier this month when Trump proposed shrinking the EPA's budget by 31 percent.

Trump landed his third big blow on Down TownTuesday afternoon, when he issued a sweeping executive order that will begin unraveling the Obama administration's key efforts to address climate change, including the EPA's Clean Power Plan.

Adding insult to injury, he signed the order from within the very walls of the EPA's headquarters -- a move that prompted plenty of bitter eye-rolling on Twitter.

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The Trump administration says the order will simply prioritize the EPA's focus on clean air and water while winding down "job-killing" policies designed to reduce emissions contributing to global warming.

A White House official briefed on the plan told CNN that Trump officials believe the government can "serve the environment and increase energy independence at the same time." Trump's supporters have said the coming changes will finally lift EPA's "strangling effect" on the economy.

But many climate and environmental experts have staunchly opposed the Trump administration's regressive vision for the 47-year-old agency. The EPA is, by definition, supposed to protect Americans from environmental harm, including the effects of human-driven climate change such as rising sea levels, more intense droughts, extreme weather events and more.

Current and former EPA employees have turned out by the hundreds to oppose Trump's attempted rollback of Obama-era policies to cut emissions from power plants, automobiles and oil and gas well sites.

The EPA's new boss, Scott Pruitt, is one of the nation's biggest champions of such reversals. As Oklahoma attorney general, he led a Republican legal battle against the Clean Power Plan, which requires states to reduce carbon emissions from power plants.

Like Trump, Pruitt has also questioned the mainstream scientific consensus that the planet is warming and that human activity is primarily to blame.

While some climate rollbacks can be changed with the stroke of a pen, others could take years to complete. The Clean Power Plan, for instance, requires at least a year of bureaucratic work to unravel, and lawsuits from environmental groups could delay the process even longer.

Still, at Trump's signing ceremony, smiles and prolonged handshakes filled the room. But down the halls of the EPA, and in many homes and offices across the U.S., the mood is resoundingly sour.

UPDATE: March 28, 2017, 2:35 p.m. EDT This story was updated to reflect that the executive order has been signed.