It took less than 140 characters for Hillary Clinton to make a convincing case to her environmental critics that she's committed to keeping carbon underground and protecting some of America's most unique, sensitive habitats. 

Clinton’s position against drilling in the Arctic is a clear break from the Obama administration, which gave Shell final permission on Monday to explore for oil in the Chukchi Sea of Alaska through late September.

The environmental group 350 Action, among Clinton’s harshest critics on climate issues, offered rare praise Tuesday for Clinton’s leadership—while noting she still hasn't taken a concrete position on the group’s top target, the Keystone XL pipeline.

"This is a hugely encouraging sign from Hillary Clinton, and it's in no small part thanks to activists in Seattle, Portland, and around the country who've placed their bodies on the line to put Arctic drilling and the broader issue of climate change on the political map,” 350 Action spokesperson Karthik Ganapathy emailed the New Republic. “It's not easy to stand up to Big Oil, nor to break with a sitting President from within your party—so Secretary Clinton deserves real credit for that."

In some ways, a candidate's position on Arctic drilling is more consequential than the Keystone XL pipeline. President Barack Obama's final decision on the proposed pipeline is expected to come soon, whereas the next president will set the agenda for offshore drilling, including in the Arctic. According to federal estimates, the U.S. Arctic contains 30 billion barrels of undiscovered oil: the equivalent of running the Keystone XL pipeline at full capacity for 75 years, even if you count the added carbon emissions from tar sands oil, according to Natural Resources Defense Council Arctic Director Neil Lawrence. If Keystone is ever built, the State Department has put the pipeline’s lifespan at roughly 50 years.

Shell's early drilling activity might already be impacting the environment. Shell has already gained approval from the National Marine Fisheries Service for "incidental harassment" of threatened marine animals. Mostly, the potential for harassment is from the noise its seismic testing and ice-breaking equipment produces. Shell had to prove it “will have a negligible impact” on animals like the bowhead whales, gray whales, and the endangered ringed seals, which are sensitive to the noise. Yet scientists argue there is no way of knowing the full extent of the damage to animals’ hearing. The Obama administration is well aware of threat: in late June, it ruled that Shell would have to abandon plans to drill two wells at once, because of the damage the noise would cause the area’s walrus population and other wildlife.

But the biggest environmental threat, of course, comes from oil spills. Shell and the U.S. government are likely overestimating their ability to clean up a disaster in the remote and unpredictable region. The National Research Council reported in 2014 there's much more we don't know about oil cleanup efforts in icy waters, which would present different challenges from offshore drilling in the usual temperate oceans. The worst oil spill in U.S. history until the 2010 BP disaster was the 1989 Exxon Valdez crash, which leaked more than 11 million gallons of crude oil into Alaska's Prince William Sound. Twenty-five years after the spill, the National Oceanic and Atmospheric Administration concluded that only 15 of 28 damaged species of animals and plants have fully recovered. As NOAA marine biologist Gary Shigenaka explained, "If you thought that Prince William Sound was remote, then responding to a spill in the Arctic would be almost like working on the moon."