Four of the 23 Democratic presidential hopefuls did not participate in the Washington Post's new survey on climate policy, "but Joe Biden remains the biggest enigma of them all," according to an analysis the newspaper published Monday as activists vowed to increase pressure on Biden to back bold plans to combat the global crisis.

"We're going to turn up the heat on Biden and all the candidates throughout the summer."

—Victoria Fernandez, Sunrise Movement

"The former vice president has cast himself as an ardent environmentalist... But his campaign has not yet announced any concrete energy proposals," the Post's Dino Grandoni, who contributed to the survey report, wrote Monday.

Although Biden is the front-runner based on recent polling, he has kept a low profile compared with his competitors.

Juliet Eilperin, another journalist who contributed to the Post's report, tweeted Monday afternoon that Biden, Sen. Kamala Harris (D-Calif.), former Colorado Gov. John Hickenlooper, and Andrew Yang "didn't bother to respond" to the survey, initially published Friday and repeatedly updated with responses from campaigns since then.

The newspaper asked candidates nine questions:

Do you support the Green New Deal resolution?

Should we rejoin the Paris agreement?

Do you support building more nuclear power plants?

Would you support setting a price on carbon, such as with a carbon tax or cap-and-trade?

Would you ban fracking?

Would you ban fossil fuel exports?

Would you end leasing for fossil fuel extraction on federal lands?

Would you eliminate fossil fuel subsidies?

Are you doing something about your campaign's carbon footprint?

The Post's reporters grouped the candidates by similar positions after considering not only their campaign's responses but also "legislative records, action taken in an executive role, and other public comments, such as policy discussion on campaign websites, social media posts, interviews, town halls, and other news reports."

Though four candidates did not respond, Biden is the only one who appears in the "unclear/no response" column for all nine questions.

Joe Biden bats an impressive 1.000 when it comes to not answering WaPo’s questions about climate change. He’s 9 for 9 for “unclear/no response.” https://t.co/iYDPsPewBz — matthew miles goodrich (@mmilesgoodrich) June 3, 2019

Biden is the only candidate in the "unclear/no response" column for the questions about supporting the Green New Deal resolution—introduced by Democrats in Congress earlier this year—and rejoining the Paris agreement, which was negotiated while he served as vice president in the Obama administration.

While every other country on Earth has backed the landmark climate accord since it was finalized in late 2015, President Donald Trump vowed two years ago to withdraw the United States as soon as he can.

Noting the survey results on Twitter Monday, the youth-led Sunrise Movement emphasized Biden's lack of position on the Green New Deal resolution—for which the group vocally advocates—and also pointed out that Biden is "the only major candidate who hasn't taken the #NoFossilFuelMoney pledge."

The supposed Democratic Presidential front-runner is the ONLY candidate who hasn't taken a position on the #GreenNewDeal. AND he's the only major candidate who hasn't taken the #NoFossilFuelMoney pledge. SCROLL TO CONTINUE WITH CONTENT Never Miss a Beat. Get our best delivered to your inbox.





Tell @JoeBiden to do the right thing: https://t.co/oA2SVvcof6 pic.twitter.com/ScQKAQJgsU — Sunrise Movement (@sunrisemvmt) June 3, 2019

"We're going to turn up the heat on Biden and all the candidates throughout the summer, but he's likely to release his plan this week or next," the Sunrise Movement's Victoria Fernandez wrote in an email to the group's supporters Monday, requesting signatures for a petition calling on Biden to back the Green New Deal and sign the No Fossil Fuel Money Pledge.

The group's call for the public to pressure Biden "to do the right thing" comes after Reuters reported last month that Biden's campaign is crafting "a middle ground approach" to the human-caused global climate emergency.

Following the Reuters report, Biden said at an event in New Hampshire, "There's been nothing middle of the road about my record dealing with the environment," Grandoni noted in his analysis Monday. However, the candidate has not yet put out any climate policies.

Other candidates, meanwhile, have drawn attention to Biden's reported plans. At the California Democratic Convention over the weekend—which Biden did not attend—Sen. Bernie Sanders (I-Vt.) declared, "We have got to make it clear that when the future of the planet is at stake, there is no 'middle ground.'" After the senator's remarks, Common Dreams reported, the Twitter hashtag #NoMiddleGround went viral on Sunday.

Sanders's speech came after the senator ranked highest of the Democratic presidential candidates who are in the top six in national polls for a climate policy scorecard released by Greenpeace Thursday. Biden, meanwhile, ranked at the bottom, after Sen. Elizabeth Warren (D-Mass.); former Texas Congressman Beto O'Rourke; South Bend, Indiana Mayor Pete Buttigieg; and Harris.

While the Post's survey elicited criticism of Biden, the group Food & Water Action highlighted a positive takeaway: 10 candidates support a ban on fracking, a process also known as hydraulic fracturing that involves injecting water and chemicals into the ground to access natural gas.

Mitch Jones of Food & Water Action said in a statement, "given that most Americans oppose fracking, for good reason, it's not surprising that more and more presidential candidates are coming to the prudent conclusion that fracking simply must be banned."

"Frontline communities across the country are dealing every day with the human health and environmental risks that fracking inherently brings," Jones added. "Fracking poisons air and water, and makes people sick. Americans know it, and finally Democratic candidates are beginning to get the picture."

Jones called on the 10 candidates who claimed in the Post survey to support a ban "to talk in specific detail about exactly what they would do to halt fracking as soon as they take office," and said that those who haven't yet stated their opposition to the fracking "need to pull their heads out of the sand, and get on board with the will of most Americans."