Hillary Clinton's advisers say she plans to take "aggressive" steps on global warming if elected president. Here's John Podesta last week:

Former Secretary of State Hillary Clinton is "quite" involved in climate change policy as a 2016 presidential candidate and will carry on with President Obama's limits on coal-fired power plants if she is elected, her campaign chairman, John Podesta, said yesterday. ... "I have no doubt that she will move forward with an aggressive program to move the country to a cleaner energy system and do what the United States needs to do to meet the target," he said.

Okay, but what would an "aggressive program" entail? One place to look for clues is in this new report by the World Resources Institute. To be clear, the report isn't affiliated with the Clinton campaign. But it does lay out, in detail, what policies the next president could pursue to cut US emissions — even without Congress. If Hillary Clinton or Bernie Sanders or Jeb Bush wanted to go big on climate, this would be the place to start.

First, the backdrop: As part of the UN climate talks, President Obama has pledged that US greenhouse gas emissions will be 26 to 28 percent lower in 2025 than they were in 2005. That's the "target" Podesta is talking about. The Obama administration has already put out a battery of regulations toward that end, like stricter fuel economy standards for cars and the EPA's proposal to curtail CO2 from coal plants.

But as the WRI report notes, all of Obama's climate policies so far aren't yet sufficient to hit that big climate goal. (Right now, US greenhouse gas emissions are only about 8 percent below 2005 levels.) Whether the US gets there would be up to the next president, whether it's Hillary Clinton or whoever else.

10 things the next president could do to cut emissions — without Congress

The bulk of the WRI report looks at various policies the next president could pursue to expand on Obama's existing regulations and hit or even exceed that 2025 target. Most of these things could be done without Congress, mainly by harnessing the EPA's existing authority to regulate greenhouse gases:

1) Follow through on Obama's Clean Power Plan. This summer, the EPA plans to finalize its Clean Power Plan to curtail CO2 emissions from existing coal-fired plants. The WRI report suggests that the Obama administration strengthen this rule before it's finalized, though it concedes this isn't absolutely necessary for the US to hit its 2025 climate target.

The Clean Power Plan would then need to survive all legal challenges, and the next administration would have to make sure the rule gets fully implemented. As I've written before, whoever gets elected president in 2016 will have a ton of leeway over implementation of the power plant rule. Podesta's comments above suggest that Clinton already wants to see these through. So that's step one...

2) Strengthen energy efficiency standards for homes and buildings. Next, the Department of Energy could scale up or tighten various energy efficiency standards for household appliances, residential boilers, commercial ventilation equipment, and so forth. This would help reduce energy demand and CO2 emissions.

3) Expand programs to reduce HFCs. The Obama administration has already begun cracking down on HFCs, a potent greenhouse gas often used in refrigeration and air conditioning. In theory, the next president could direct the EPA to accelerate the phase-out of the worst of these gases and help bolster recycling programs and pursue alternatives.

4) Start regulating industrial CO2 sources. Similarly, the next president could set the EPA loose regulating other sources of CO2 emissions. Under Obama, the EPA has only regulated CO2 from vehicles and power plants. But the agency does have the authority to regulate refineries, cement plants, petrochemical plants, and so on. The WRI report suggests that the EPA under the next president could use this power to improve end-use efficiency and fuel switching in the industrial sector and bring down US emissions further.

5) Crack down on methane leaks from oil and gas infrastructure. Again, the EPA has already begun setting standards for methane leaks from new oil and gas wells. The next president could expand this authority to existing oil and gas wells as well as other natural gas infrastructure.

6) Expand fuel economy standards for cars and light trucks. Federal fuel economy standards for new cars and light trucks are currently set to keep rising each year until they reach 54.5 miles per gallon in 2025. The WRI report points out that the next president could tighten these standards further during the 2017 midterm review, or even extend them further — say, to 63 mpg by 2030.

7) Set new standards for heavy trucks. Similarly, the EPA has already set standards to improve fuel efficiency for medium- and heavy-duty trucks between 2014 and 2018. The Obama administration is currently working on standards for the post-2018 period. This is another area where, conceivably, big reductions in fuel use could be made.

8) Set CO2 rules for aircraft. Countries around the world are planning to come to some sort of agreement on reducing emissions from flying in the next few years. The WRI report notes that either this administration or the next one could work with the EPA to set a rule to improve the fuel efficiency of new aircraft in the range of 2 to 3 percent annually.

9) Reduce methane from landfills and agriculture. The EPA has already proposed regulations on methane emissions from new landfills. It could go further to restrict emissions from existing landfills and coal mines, as well as look into ways to reduce emissions from agriculture (yes, that means tackling cow burps).

10) Tackle miscellaneous greenhouse gases. On a smaller scale, the WRI notes that the EPA and Department of Energy could likely make inroads on other sources of greenhouse gases, like "off-highway vehicles" or "nitric and adipic acid manufacturing." None of these cuts would be massive alone, but together they could add up.

About 70 percent of these expand on policies that Obama has already set in motion. But some of them, particularly CO2 rules for industrial sectors, would involve brand-new regulations on entire parts of the economy.

Add these up, and the US could hit its 2025 goal

It depends on how hard the next president wants to push. Many of these regulations are likely to be extremely controversial, so it's far from a given they'll happen. The WRI report models a number of different scenarios here.

In one scenario, called "core ambition," the Obama administration finalizes its CO2 rule for power plants and whoever gets elected in 2016 fully implements it. The next president also sets new efficiency standards for appliances, bolsters fuel economy rules for trucks, sets new CO2 rules for industry, expands emissions standards for natural gas systems, and cuts down on HFCs. Add it up, and greenhouse gas emissions fall roughly 26 percent below 2005 levels by 2025.

The WRI also models another scenario, called "Targeted Sector Push," in which, on top of the above, the next administration strengthens existing CAFE standards for vehicles during the midterm review in 2017 and sets even stricter CO2 regulations for industrial sources, plus some other steps. The result? Emissions fall roughly 30 percent below 2005 levels by 2025.

Granted, there are other outside variables here that could potentially sway US emissions, too. If, say, solar power prices plummet faster than expected, that might push emissions down even further. Or maybe self-driving cars will revolutionize transportation. The future's never certain. The point of this paper is to show what an aggressive federal climate policy looks like, given what we know now.

Would Clinton actually do any of this?

That's still unclear. All Podesta has said is that Clinton would move forward with an "aggressive program" to hit that 2025 emissions target. The WRI report sketches out, broadly, what regulatory steps could meet that goal. But maybe Clinton's thought of options no one else has.

Mind you, this analysis also assumes that Congress remains gridlocked on climate change for the foreseeable future. The WRI report points out that there's only so much any administration can do on its own to nudge down emissions — particularly for the deeper cuts that will likely prove needed after 2025 for the world to fend off drastic climate change. (Of course, other nations will have to respond in kind.)

"New federal legislation will likely be needed to drive these deeper reductions," the report notes, "for example, a carbon tax, cap-and-trade program, or national clean energy standards." Some of those legislative policies, like a steadily rising carbon tax, would be more cost-effective than having the EPA continue to regulate CO2 from various industrial sectors, chunk by chunk.

In other words, an ambitious climate agenda by the next president could nudge down US emissions quite a bit. An ambitious climate agenda by Congress could go much, much further.

Read more: This earlier piece looks at the flip side — ways in which a more conservative president could dismantle Obama's climate agenda, if he or she so chooses.