Democratic Senator Harry Reid of Nevada, one of the most vocal and active climate leaders on Capitol Hill, announced Friday morning he would not seek re-election in 2016.

The Senate minority leader's retirement is a major loss for the climate movement, several political and environmental experts told InsideClimate News.

Reid, 75, has championed dozens of environmental initiatives during his five terms in office, including designating more than 3 million acres of federally protected wilderness, promoting renewable energy, and thwarting three new coal-fired power plant projects in his home state. In recent years, he's become an outspoken advocate for climate action, calling global warming, "one of the greatest challenges of our time."

"Reid has provided tremendous leadership in making Nevada a leader on solar, geothermal and wind," said RL Miller, chair of the California Democratic party's environment caucus and founder of Climate Hawks Vote, a super PAC that works to elect climate-conscious candidates. "Equally important has been his leadership in the Senate in promoting a climate-friendly agenda. Simply put, we're going to miss him."

He has also been a strong critic of the fossil fuel industry's attempts to sow doubt about climate science and delay action on global warming. Last May, Reid called oil and gas billionaires Charles and David Koch "one of the main causes" of climate change.

He has sponsored several pieces of legislation calling for more renewable energy and climate adaptation. As minority leader, he also blocked several votes aimed at speeding up a decision on the controversial Keystone XL pipeline, which would transport carbon-heavy tar sands fuel from Alberta, Canada to refineries in Texas.

Earlier this month, Reid penned an editorial in USA Today with Democratic Senator Sheldon Whitehouse of Rhode Island, calling out the Republican Party's anti-climate agenda. The letter was in response to Senate majority leader Mitch McConnell, R-Ken., and his campaign against the Clean Power Plan, which aims to curb greenhouse gas emissions from coal-fired power plants.

"As the evidence becomes clearer and clearer, and the American people move farther away from the deniers, the Republican Party must have its own day of reckoning, when it will finally again be able to address—or even mention—climate change," the senators wrote. "That day can't come too soon."

Mike Brune, executive director of the Sierra Club, said in a statement that Reid has shown "tenacious leadership to protect our air, water, and climate" during his three decades in office. The senator "has left an immeasurable impression on our nation's climate and clean energy future," Brune said.

League of Conservation Voters president Gene Karpinski called Reid "a true environmental champion" who has "led the charge beating back repeated attempts to weaken the laws that protect our air, water and public lands."

Reid's absence will be keenly felt in the climate movement, particularly if Republicans win the White House and keep the majority in Congress in 2016, because they will undoubtedly try to dismantle most, if not all, of President Barack Obama's global warming initiatives. Without strong climate leaders in the Senate—which loses another when California Democratic Senator Barbara Boxer retires in 2016—these efforts could face little resistance.

The early contenders to fill Reid's shoes as Senate Democratic leader don't have strong climate credentials, said Miller. These include Senators Chuck Schumer of New York, Dick Durbin of Illinois, and Patty Murray of Washington.

Nevada is also swing state, so it's not a given that its voters will replace Reid with a Democrat. A Republican is likely to toe the party line on climate and environmental issues.

There are some lawmakers who could help fill the void left by Reid and Boxer, experts say. Among them are Senators Bernie Sanders of Vermont, Brian Schatz of Hawaii, Maria Cantwell of Washington, Jeff Merkley of Oregon, Ed Markey of Massachusetts and Whitehouse of Rhode Island. All are Democrats except Sanders, who is an independent.

"No one can replace Harry Reid," said Dave Willetts, a spokesman for the League of Conservation Voters. "He was and is a huge figure in the Senate. But we see a class of senators willing to step up and fill his shoes."

In the meantime, Reid made it clear in his retirement announcement Friday that he has no plans of slowing down during his final two years in office.

"I am going to be here for another 22 months, and you know what I'm going to be doing? The same thing I've done since I first came to the Senate."

An earlier version of this story incorrectly grouped Senator Bernie Sanders with other Democrats who could help fill the void of leadership on climate. Sanders is an independent who caucuses with the Democratic party.