On Thursday evening, the two remaining Democratic candidates for president, Hillary Clinton and Bernie Sanders, paired off at the Brooklyn Navy Yard for their most sophisticated debate yet over setting the right energy and climate-change policy for the country.

The location made sense: Seven decades ago, the shipbuilders there earned a national reputation for their hardworking speed and can-do attitude. Seven decades from now, the shipyard could very well be under water—a victim of a crisis that humanity didn’t act fast enough to stop.

Their exchange got right to the weaknesses of the two candidates’ climate positions. To her critics, Clinton is too entrenched with fossil-fuel interests to do what needs to be done for the climate. Notably, she opposes a tax on carbon emissions, a measure so mainstream that it was sought by President Obama early in his first term. Sanders, meanwhile, can act more like an old-school green than a modern climate realist, opposing more pragmatic paths to quickly reducing carbon emissions (like nuclear power) due to their local environmental risks. And here as elsewhere, his calls for revolution can devolve into mere incrementalism when he’s pressed.

Clinton’s industry ties were the focus of the first half of their exchange. Sanders, echoing a criticism that his campaign has repeatedly leveled in April, hammered her for accepting campaign donations from oil-and-gas industry employees and lobbyists.

“Some people say, well, given the hundreds of millions of dollars she raises it’s a small amount,” he said. “That’s true. But, that does not mean to say that the lobbyists thought she was a pretty good bet on this issue.”

It’s a classic Sanders attack, focusing on the appearance of soft corruption. And Clinton has accepted about $3 million in donations from lobbyists associated only with the fossil-fuel industry (though that’s a smaller number than the Sanders camp has claimed). According to an NPR fact check, employees in the oil and gas industry have donated $300,000 to Clinton’s campaign. By comparison, they’ve donated more than $1 million to Ted Cruz, about $50,000 to Sanders, and less than $10,000 to Donald Trump.

But later in the exchange, Sanders was able to tie the appearance of corruption to a specific policy, asking: “Are you in favor of a tax on carbon so that we can transit away from fossil fuel to energy efficiency and sustainable energy at the level and speed we need to do?”

Clinton, who does not support a carbon tax, didn’t answer directly. He pressed her again, and she still didn’t answer—though the moderators cut them both off. Her opposition is somewhat confusing. Carbon taxes are considered by many economists to be the ideal small-c conservative response to climate change: They price in global warming’s externalities, while allowing the market to choose how to divert resources elsewhere. (For her part, Clinton says she supported dropping subsidies for the oil industry while she was secretary of state.)  

But when they got to what energy policy should include beyond a carbon tax, things got more muddled.

As secretary of state, Clinton pushed to expand hydraulic-fracture mining for natural gas (“fracking”) around the world; now, she wants to limit its use, but not ban it outright. Sanders, meanwhile, opposes fracking in all its forms. NY1’s Errol Louis asked Clinton why she changed her view.

“No, well, I don’t think I’ve changed my view,” she answered, saying she has always seen natural gas as a “bridge” between dirty-burning coal and clean, renewable energy. She added that expanding the use of fracking worldwide helped the United States strategically, as it decreased Russia’s ability to sell oil and gas to countries in Eastern Europe.

Though Clinton was at pains to make it look like her views haven’t changed, she could have just said that the expert understanding of fracking has changed. When drilling was new, many environmentalists supported it. There’s a good reason for that: Fracking has reduced U.S. carbon emissions by six percent this decade, devastated the coal industry, and changed the global economics of energy. And though fracking can have horrifying local environmental consequences, they do not seem to be nearly as widespread as initially feared. (In 2008, solar and wind power also just weren’t efficient enough as technologies to generate the electricity they can now.)

Now, many more climate hawks oppose fracking outright. Recent studies have indicated that the although it may have helped the U.S. reduce its carbon emissions, those gains were largely offset by increases in methane leaks from the fracking process. While methane doesn’t last as long in the atmosphere as carbon dioxide, it is much more effective at trapping heat in the atmosphere for short periods of time. (Because of this, the Obama administration is also starting to regulate methane leaks from fracking operations.)

Then the moderators turned to Sanders, who opposes not only fracking but also all nuclear-power generation. Nuclear generates 20 percent of U.S. electricity without emitting any greenhouse gases, so this policy strikes some climate-concerned folks as a little strange. (It makes much more sense as a holdover view from Sanders’s 60s-era activism.) Louis, the moderator, asked Sanders:

You’ve said that climate change is the greatest threat to our nation’s security. You’ve called for a nationwide ban on fracking. You’ve also called for phasing out all nuclear power in the U.S. But wouldn't those proposals drive the country back to coal and oil, and actually undermine your fight against global warming?

This is the most important question to ask about Sanders’s energy policy. He didn’t answer it directly, instead turning to the broad language of emergency: “We have a global crisis. Pope Francis reminded us that we are on a suicide course,” he said. Then he started talking about the need for infrastructure spending and the plight of coal workers in a transitioning economy. (A plight, it should be added, that is getting worse and that should be addressed by public policy.)

But Louis pressed him, asking how, “with less than 6 percent of all U.S. energy coming from solar, wind and geothermal, and 20 percent of U.S. power coming from nuclear,” he would keep carbon emissions from rising again.

And he relented, admitting, “Well, you don’t phase it out tomorrow. And you certainly don't phase nuclear out tomorrow.”

It wasn’t a wrong answer, but it sounded indistinguishable from the “incrementalism” he had lambasted in the same exchange.

Throughout the back-and-forth on climate, Sanders compared global warming to warfare. The United States should respond to melting icecaps like it would respond to a foreign invasion, he said:

If we approach this, Errol, as if we were literally at a war—you know, in 1941, under Franklin Delano Roosevelt, we moved within three years, within three more years to rebuild our economy to defeat Nazism and Japanese imperialism. That is exactly the kind of approach we need right now.

The idea that climate change will require a mass economic mobilization on the level required by World War II is an increasingly popular idea among environmentalists. It’s been written about in these pages, and Al Gore has called for it as a way to end the global savings glut. It is also the opposite of the world’s current approach, which is effectively to divert investment from fossil fuels to clean energy. Seeing this idea enter mainstream Democratic rhetoric so forcefully bodes well for its chances.

Not that a mass mobilization—or a carbon tax—looks any likelier to happen in the short term. This may be the the only nuanced debate that American voters will hear this year over how to respond quickly to climate change. That’s because, while Democrats volley over how to respond to the problem, many leading Republicans straight-up don’t recognize the reality of global warming. Both Donald Trump and Ted Cruz deny that climate change is either real or human-caused, rejecting the consensus of NASA researchers, of 97 percent of all climate scientists, and of every major American scientific academy, including the American Medical Association. (Cruz’s claims here are especially bizarre.)

In most other developed countries in the West, the debate over climate change tends to happen between climate “hawks” and “doves”: those who think the economy must be restructured away from fossil fuels immediately, and those who think a slower transition is more feasible while remaining safe. Strangely, that’s not quite the debate that happens here yet, even within the Democratic party.  

But it hardly matters. In the general election, one of these two candidates will call for a mix of policy solutions to address one of the gravest geopolitical threats this century. Their Republican opponent will likely say that the threat doesn’t exist.