The Obama administration's final approval of Royal Dutch Shell's drilling for oil in Alaska's Chukchi Sea provoked an angry reaction on Monday from environmentalists who had come to consider President Obama a champion in the fight against climate change.

The decision comes two weeks after the release of the United States' most aggressive attempt to limit greenhouse gas emissions, known as the Clean Power Plan, and just days after Obama announced he will visit Alaska later this month to highlight the impacts of climate change, which he recently referred to as "one of the greatest challenges we face this century."

"I'm flummoxed," said Jamie Henn, co-founder and director of strategy and communications of the green group 350.org. "Arctic drilling is so blatantly out of line with the President's stated goals that the only possible explanations seem to be that he truly doesn't understand the issue or that the White House is somehow convinced that the project won't go forward."

Obama has shown "historic leadership on climate change," said Alex Taurel, deputy legislative director for the League of Conservation Voters. "But his decision to allow drilling in the Arctic is at odds with everything else he's done."

"We are deeply disappointed," Taurel added.

Obama has been calling for America to reduce its reliance on fossil fuels since before entering the White House in 2008. He told a crowd of journalists gathered at the Newspaper Association of America in 2006 that "unless we free ourselves from a dependence on these fossil fuels and chart a new course on energy in this country, we are condemning future generations to global catastrophe."

In recent years, Obama-driven initiatives have helped increase the nation's solar-generated electricity 10-fold and tripled wind capacity. The administration has also pushed for increased energy efficiency measures and higher fuel economy standards. The Clean Power Plan, which will limit how much carbon dioxide pollution power plants can spew into the air, is expected to reduce greenhouse gas emissions 32 percent by 2030. Obama has also pledged to play a key role in international climate treaty talks in Paris this December.

"While President Obama has made some progress during his term on reducing emissions through measures like the recent Clean Power Plan, his environmental legacy will be determined by the steps that he takes to keep fossil fuels in the ground," Greenpeace USA executive director Annie Leonard said in a statement. "The Obama administration should know better than to bend over backwards to approve such a reckless plan."

This won't be Shell's first attempt to drill in the Chukchi Sea, a remote, harsh region nestled between Alaska and Russia. Shell has spent an estimated $7 billion in recent years to search for oil in the Arctic, which is estimated to contain approximately 20 percent of the world's undiscovered fossil fuel reserves. The company first drilled there in 2012 with disastrous results. In the span of a few months, it's drilling fleet failed to meet air pollution standards, miles-long ice floes halted drilling several times, a rig broke and later ran aground, and the company was found to have discharged polluted water into the ocean. Shell did not return to the Arctic in 2013 or 2014.

In January, Obama banned all future oil and gas lease sales in 9.8 million acres of the Arctic Ocean, including parts of the Beaufort and Chukchi seas. The ban, however, did not impact existing leases, such as Shell's.

Earlier this year, the company was granted permission by the Department of the Interior to drill down to—but not into—the oil zone. The permit, however, required that a capping system be on hand in the Arctic to plug a seafloor spill before the company could commence drilling the final stretch to hit oil.

Environmental groups have led an aggressive months-long campaign to stop Shell's Arctic drilling. The efforts culminated two weeks ago when 13 Greenpeace activists hung suspended from a Portland, Ore. bridge to stop the icebreaker MSV Fennica from leaving port with the capping system after undergoing repairs to a gash in its hull. The activists were forced down by the Coast Guard after 40 hours of protest and the Fennica proceeded to join Shell's fleet.

Given Shell's spotty environmental record in the Arctic in 2012, public opposition to the drilling and Obama's recent campaign to reduce America's fossil fuel consumption, environmentalists had remained hopeful that the administration would deny the company's final drilling permit even with the capping system in place.

"It is disappointing that he would undercut his legacy like this," said Athan Manuel, director of Sierra Club's lands protection program. "Science has said with near certainty that any drilling in the Arctic not only furthers climate disruption, but it carries a 75-percent chance of a major spill. There is no valid reason to approve any sort of drilling in the Arctic."

The decision puts Obama at odds, to varying degrees, with the three Democratic frontrunners for the 2016 presidential race. Vermont Senator Bernie Sanders and former Maryland Governor Martin O'Malley have said they oppose drilling for oil in the Arctic. Former Secretary of State Hillary Clinton told a New Hampshire television station in late July that she has "doubts about whether we should continue drilling in the Arctic."

Green leaders are now left wondering what the Arctic drilling decision could mean for the other major energy decision lingering on Obama's docket: the Keystone XL pipeline, which would carry approximately 800,000 barrels of carbon-intense tar sands oil per day from Alberta, Canada to refineries in the Gulf of Mexico.

Most remain hopeful the president will reject the project, citing his veto of legislation earlier this year that would have fast-tracked the pipeline's approval process.

Obama's decision to green light Shell's Arctic drilling "makes his decision on Keystone XL even more critical," said Henn of 350.org. "We need to see evidence that the President understands that in order to prevent catastrophic climate change we need to keep oil in the ground."