(CNN) Candidates for the Democratic presidential nomination and most scientists say the climate crisis is the existential threat of our time, while President Donald Trump once claimed it's a hoax cooked up by the Chinese.

Trump also said at last month's G7 meeting that "I'm an environmentalist," citing his experience filing environmental impact statements as a businessperson, though he skipped an actual session about climate change that his fellow world leaders attended.

That pretty much sums up the difference between how a Democrat would treat climate change compared with Trump: as an emergency as opposed to as a joke. That, in itself, would be an enormous switch.

This is a worldwide issue, and solving it will take cooperation from countries like China and India. In addition, the most important actions the US could take would require congressional approval, no easy task when the Senate floor is controlled by a Republican who represents coal country. But presidents actually have quite a bit of power to change US climate policy on their own, without waiting for Congress.

A first set of steps would be to roll back numerous Trump actions that either sidelined US leadership on the climate or rescinded new standards that had been implemented during the Obama administration.

CNN gathered input from advocacy groups, former government officials and academics. Here are some things any president could do on Day One in office:

1. Rejoin the Paris climate agreement

climate agreement was Promising to withdraw the US from the landmark 2015climate agreement was Trump's benchmark action rebutting the responsibility to act on climate change . He said, without much evidence, that it was a demeaning failure for American workers, who would be expected to do more than those in other countries. The result is the US is one of just a few countries not taking part in the effort, in which nations committed to varying amounts of carbon reduction.

Every candidate for the Democratic presidential nomination has said they would bring the US back in. The US was supposed to reduce carbon emissions by 26% from 2005 levels by 2025 under the agreement. It probably won't get there with the willful sabotage of the Trump administration.

But with certain states, cities and companies trying to fill the void, the US could get most of the way there, according to groups like America's Pledge, the coalition pushed by former New York City Mayor Michael Bloomberg and former California Gov. Jerry Brown.

2. Declare a national emergency on climate

Sen. Bernie Sanders of Vermont was the first presidential candidate to argue that Trump, instead of declaring a national emergency at the border, should do it on the climate instead. Others, like Tom Steyer, have followed suit. This would certainly focus national attention and could free up some funds without congressional approval.

The Pentagon has embraced renewable energy and efficiency in the name of defense strategy. But simply striking a tone of concern about climate change could do a lot to change the way the government operates. It would be a massive departure from the current situation, where the words "climate change" are scrubbed from reports and releases and the entire US government has pivoted to ignore that climate change exists

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3. Set a carbon-free goal

The President could easily declare a goal of 100% decarbonization by a given year -- say, 2050 -- and direct his or her Cabinet secretaries to do everything in their power to meet it.

4. Reverse Trump's rollback of fuel economy standards

At the request of automakers, the Trump administration worked to roll back fuel economy standards for cars and trucks. A different President could push things again in the other direction.

California has a waiver from a clause of the Clean Air Act that gives it special authority to set its own fuel economy standards. It negotiated increased fuel economy standards with four companies ( Ford, Honda, Volkswagen and BMW ) to meet a 51 mpg by 2026 standard. Since it's the largest US auto market, there's a good chance other companies will follow suit and this will become the new de facto standard. But it's not as strong as the 54.5 mpg standard set by the Obama administration, which a new administration could restore.

5. Set a zero-emission deadline for the US

Ten states have set full zero-emission goals for automobiles by 2030. Colorado became the latest in August . The federal government could follow suit or go even further.

6. Embrace energy-efficient light bulbs

Calls for new infrastructure spending or a Green New Deal all share the goal of making US buildings more energy efficient. This sounds like a great idea, but even the smallest standard can spark a huge backlash, as Obama found regarding incandescent light bulbs in 2011 . The country has since moved farther away from them.

The Trump administration is relaxing standards for light bulbs . Trump also tried to cut funding for energy efficiency research at the Department of Energy. And his Department of Energy wants to let manufacturers skirt energy efficiency tests

Trump actually signed an executive order intended to make federal buildings more efficient. But that is a small portion of the buildings in the country. Pushing efficiency standards on a larger scale would help cut down on emissions.

7. Reinstate the Clean Power Plan

The Clean Power Plan was an Obama-era regulatory scheme that required states to meet specific carbon emission-reduction standards based on their individual energy consumption. The plan includes an incentive program for states to get a head start on meeting standards on early deployment of renewable energy and low-income energy efficiency.

The Trump administration, under former Environmental Protection Agency Director Scott Pruitt, found that the plan exceeded a president's authority and reversed it. A coalition of states are suing the government to stop some of these regulatory rollbacks.

8. Reverse Trump's plan to speed up oil and gas pipelines

Trump is extremely proud that the United States now produces more oil and natural gas than any other country on Earth, although it reached that threshold during the Obama administration. He's rolled back safety regulations for oil and gas producers, used executive action to make it much easier to approve pipelines and given the federal government more power over states that might try to block them under the Clean Water Act.

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Workers in the Bureau of Land Management worked through the partial government shutdown early this year to approve more and more applications for oil and natural gas drilling on public lands. A new administration could move in the opposite direction.

9. Reimpose a moratorium on coal sales from public lands

Trump reversed an Obama-era moratorium on new leases of public land for coal development. His administration argued this year that new coal coming from public lands would not affect US greenhouse gas emissions. A large percentage -- 40%, according to the federal government -- of the coal mined in the US comes from public lands.

10. Stop trying to push coal altogether

That hasn't gone according to plan, as market forces and cheaper natural gas and renewable plants have pushed the coal industry to the brink. Trump's EPA tried to help the coal industry when it rolled back new emission rules for coal plants , but that faces a challenge by 22 states, who point to provisions of the Clean Air Act.

11. Undo Trump's efforts to increase offshore oil drilling

While he has mocked the idea of offshore wind turbines, Trump has pushed offshore oil rigs. His administration upset decades of precedent when it pushed for new drilling on every coast of the US, although it exempted some states, such as Florida, where he owns beachfront property. The administration appears for now to have put the plan on hold after it suffered some trouble in court, when a federal judge in Alaska froze plans for new drilling in the Arctic Ocean

12. Accelerate production of renewable energy on public lands

While the Trump administration has pushed oil and gas exploration on public lands, there are proposals in Congress to use public land, particularly in the Southwest, for renewable energy production in the form of wind or solar farms.

The Obama administration had a rule that would have moved in that direction without congressional approval.

13. Use the SEC to scrutinize banks and investors for climate risks

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Most attention for climate change goes to the energy sector, but banks and investors have an important role too. The Securities and Exchange Commission asks companies what risks climate change and new climate change regulations pose to their investments.

As the world transitions away from oil and natural gas, companies in that industry and reliant on it will face challenges and may need to adapt. The president could direct the SEC to require more serious answers. Several presidential candidates have endorsed this move

14. Tap into growing public sentiment and work with corporations

States and cities are addressing climate change, be it with zero-emission-vehicle goals or the regional cooperative of states in the Northeast that is trying to cap and reduce carbon emissions on its own. Corporate America is also moving on its own. It's setting up coalitions of companies and lobbying Congress for new action on climate change. As it stands right now, the federal government is not even a part of that conversation.

15. Finally, get Congress on board

Pretty much everyone who agrees that climate change is a problem agrees that Congress will ultimately have to address it in a big way. That means placing some kind of price on carbon. The House voted to do this in 2010, but the so-called cap-and-trade proposal didn't get a vote in the Senate and then Democrats lost control of the House for eight years.