Wrack your brain. What issue or bundle of issues do President Donald Trump and the Democratic presidential candidates see the most differently? Go on. It’s not easy.

But consider the twin topics of energy and the environment.

Since taking office, Trump has nominated fossil fuel lobbyists and coal-friendly politicians to lead the Environmental Protection Agency and the Interior and Energy departments. He has denied climate science, including NASA research from his own administration, and rebuked U.N. climate agreements, including the Paris agreement of 2015 and the Kigali amendment, which has bipartisan and industry support.

Steadily, he has made it easier for industries to raise their emissions. In 2018, Trump dismissed the conclusions of researchers at 13 federal agencies in the latest National Climate Assessment, which said climate change would punch 100-billion-dollar-plus holes in the domestic economy.

“I don’t believe it,” he said before blaming other nations. “Right now we’re at the cleanest we’ve ever been, and that’s very important to me. But if we’re clean but every other place on Earth is dirty, that’s not so good. So I want clean air. I want clean water. Very important.”

[Climate-focused Democrats hope for November reward]

Democrats vying for the nomination to challenge Trump have responded by trying to out-muscle one another to become the most hawkish climate candidate, with some unveiling multi-trillion-dollar plans to tackle emissions and decarbonize the economy.

The effort to stand out in a Democratic primary fight, especially when considering that climate steadily ranks as a top issue for young voters, particularly those who lean left. But the yawning gap between Trump and those seeking his job obscures the differences between the campaigns.

Every Democratic candidate supports eliminating fossil fuel subsidies, boosting fuel-economy standards and — in what is often mentioned as a “day one” initiative — rejoining the Paris climate agreement. But a closer look shows the rivals’ views are more nuanced. Here are five ways Democratic presidential candidates differ on energy and environmental affairs.

1. Carbon tax

Support is almost universal in the field for placing a price on greenhouse gas emissions, whether through a cap-and-trade market or a carbon tax.

Former Vice President Joe Biden, former South Bend Mayor Pete Buttigieg, Sen. Amy Klobuchar, billionaire activist Tom Steyer, former New York City Mayor Mike Bloomerg and technology entrepreneur Andrew Yang each support taxing emissions.

Sen. Elizabeth Warren has said she would back a carbon tax, though one does not come up in her campaign climate plan.

While Sen. Bernie Sanders has supported carbon taxes before, he has said such a tax must be part of a sweeping climate plan because the window in which to prevent catastrophic damages is closing.

Klobuchar, Buttigieg and Yang have been more detailed than their opponents over how the money should be spent.

2. Tax revenue

While most Democrats want to price emissions somehow, they disagree about where the revenue should go.

Buttigieg would send it back to the public through dividend checks to offset rises in gasoline and electricity costs. Yang would split the money, sending half to citizens as part of his universal-basic-income policy and half to projects that decarbonize the economy. Klobuchar would use some of the revenue on public works, she said in September.

3. Nuclear power

The future of America’s nuclear industry divides the Democratic field into three clear paths.

Sanders and Warren oppose building new reactors or plants and want to phase out the plants already online.

On the opposite flank is Yang, who wants to expand nuclear power domestically by, in part, using thorium rather than uranium. “Nuclear isn’t a perfect solution, but it’s a solid solution for now, and a technology we should invest in as we move to a future powered primarily by renewable energy,” his campaign website reads.

Biden, too, has made bullish sounds about nuclear, saying in his climate plan that he would support research to lower the costs of building nuclear facilities and disposing of fuel.

The rest of the field has either unclear stances on the topic or has staked out a centrist position that the nuclear power, which generates 20 percent of the nation’s electricity, should be maintained until it can be replaced by other zero-emissions sources.

Bloomberg, Buttigieg, Klobuchar and Steyer land in this no-new-plants-at-this-time camp. “I would look at those plants and make sure they’re safe and figure out what upgrades we have to make to the plants, but I wouldn’t expand nuclear unless we can find safe storage,” Klobuchar told a CNN town hall in September.

4. National emergency

When Trump declared national emergency to access funding for a U.S.-Mexico border wall, conservatives warned a Democratic president could do the same over climate change.

So far, only Sanders and Steyer have pledged to do so, though Sen. Michael Bennet, who did not qualify for the latest debate, said at a Georgetown University event, “I would declare a public health emergency.”

Sanders, Warren, Klobuchar and six other Senate Democrats introduced a resolution in July declaring climate change triggered an “emergency” that “demands a national, social, industry and economic mobilization” across the country as a “massive” scale.

In a plank of his climate plan, Steyer makes his intentions clear: “On day one of my presidency, I will declare the climate crisis a national emergency.”

5. Carbon capture

Some climate scientists and technical experts, such as former Energy Secretary Ernest Moniz, say equipment to trap and hold carbon dioxide emissions — broadly known as “carbon capture” technology — is vital to tackle climate change.

Yet the issue is the subject of contention among left-leaning and liberal groups and candidates. Some view it as a financial lifeline to the coal, oil and gas industries that should be rapidly shrunk. Others say carbon capture is a pragmatic solution to trap dangerous emissions, adding that it’s already working at a small number of oil facilities.

Second to nuclear power, carbon capture is the most polarizing environmental issue for 2020 Democrats.

Only four — Biden, Buttigieg, Klobuchar and Yang — have explicitly endorsed the idea. It didn’t even come up at CNN’s 7-hour climate forum in the fall.

At a national debate in July, Biden, asked if fossil fuels, coal and fracking included, would have a place in his administration, gave a murky reply.

“No, we would work it out,” Biden said. “We would make sure it’s eliminated and no more subsidies for either of those, any fossil fuel.”

After the Trump campaign swiftly pounced — “Bye bye coal, Democrats & @JoeBiden just said they are done with you. How do you feel about that Pennsylvania?” Brad Parscale, the manager, said on Twitter — the Biden campaign clarified its position.

The former vice president “supports eliminating subsidies for coal and gas and deploying carbon capture sequestration technology to create economic benefits for multiple industries and significantly reduce carbon dioxide emissions,” the campaign said.

Away from the political center, the Sanders campaign has called carbon capture a “false solution.”