President Obama is passionate, and vocal, about combating climate change. As his tenure draws to a close, science correspondent Miles O’Brien reviews the administration's environmental policy -- from the 2009 “cap-and-trade” climate bill, to the 2015 Paris accord, to executive orders on greenhouse gas emissions -- in assessing the president's legacy.

STEVE INSKEEP: And finally tonight, we continue our series about The Obama Years.

In his farewell address last night, the president spoke of his actions against climate change, including a global accord to reduce emissions.

He did more as his time in office went on, despite opponents who criticized the costs or doubted the science.

Miles O’Brien reports, as part of our weekly look at the Leading Edge of science and technology.

PRESIDENT BARACK OBAMA: And that’s why I invited Luther, my anger translator, to join me here tonight.

(CHEERING AND APPLAUSE)

MILES O’BRIEN: In the long, heated debate over global warming, this was a night to remember.

PRESIDENT BARACK OBAMA: But we do need to stay focused on some big challenges, like climate change.

KEEGAN-MICHAEL KEY, Actor: Hey, listen, you all, if you haven’t noticed, California is bone-dry.

MILES O’BRIEN: At the White House Correspondents Dinner in 2015, Barack Obama became the first president to openly scorn climate change deniers, with the help of comedian Keegan-Michael Key playing the president’s anger translator, Luther.

PRESIDENT BARACK OBAMA: Instead of doing anything about it, we have got elected officials throwing snowballs in the Senate.

KEEGAN-MICHAEL KEY: OK. OK. OK. I think I got it, bro.

PRESIDENT BARACK OBAMA: It is crazy. What about our kids? What kind of stupid, shortsighted, irresponsible bull…

KEEGAN-MICHAEL KEY: Whoa, whoa, whoa, whoa, whoa! Hey!

(LAUGHTER)

MILES O’BRIEN: By all accounts, the president’s frustration and anger were real.

CAROL BROWNER, Former Senior Advisor to President Obama: I think that this president believes that climate change is real. He believes there is a moral and ethical imperative to act. And he has taken the laws on the books and implemented them to their fullest.

MILES O’BRIEN: Carol Browner ran the Environmental Protection Agency in the ’90s. She joined the Obama White House as a senior adviser. She helped guide the administration as it aggressively employed existing laws to reduce greenhouse gas emissions.

They raised the bar on mileage for automakers, made appliances more efficient and tried to control carbon dioxide emissions at power plants as if it were any other kind of pollutant.

CAROL BROWNER: He said, “This is a serious problem, and I am committed to taking every step.”

MILES O’BRIEN: But, for Browner, the rule changes were a consolation prize. She came to the White House to shepherd a sweeping climate change bill through Congress.

CAROL BROWNER: We had looked economy-wide to look at all sources of carbon pollution. At the time, it really seemed like the thoughtful and wise thing to do, because, obviously, this is an economy-wide problem.

MILES O’BRIEN: She envisioned an economy-wide cap-and-trade system that would have set a nationwide limit, or cap, on greenhouse gas emissions. Companies would be granted allowances to produce these climate-altering gases. Those that produced less than their allowance could sell or trade permits to emit more to companies that could not reach the goal.

But the American Clean Energy and Security Act of 2009, sponsored by Democrats Henry Waxman and Ed Markey, ran into a political buzz saw.

Economist Douglas Holtz-Eakin is the president of the American Action Forum and a veteran of the John McCain presidential campaign.

DOUGLAS HOLTZ-EAKIN, American Action Forum: It was a very intrusive, heavily regulatory bill, where literally, at the key moment, John Boehner literally went to the podium, started flipping through and reading pages randomly, and he could find something bad on everyone.

REP. JOHN BOEHNER, Then-House Minority Leader: Twenty percent of the electricity that goes into every federal agency has to come from renewable sources. Do we have any idea whether this is possible? I can’t find the answer here.

MILES O’BRIEN: The money that would change hands in cap-and-trade was supposed to stay in the private sector, not go to the U.S. Treasury. But in its first budget message to Congress, the administration implied it would auction the emission allowances to companies, making it look like a tax.

ROBERT STAVINS, Harvard University: Some staffers on the Hill or someone saw that, and within six months, cap-and-trade became labeled as cap-and-tax.

MILES O’BRIEN: Robert Stavins is the director of the Environmental Economics Program at Harvard.

ROBERT STAVINS: And that was the theme that conservative Republicans and coal state Democrats used to fight against Waxman-Markey in the House of Representatives and to stop it in the Senate.

MAN: I sued the EPA, and I will take dead aim at the cap-and-trade bill, because it’s bad for West Virginia.

MILES O’BRIEN: The cap-and-trade climate bill passed in the House in 2009.

MAN: The bill is passed.

MILES O’BRIEN: But it never even reached the floor of the Senate for a vote.

DOUGLAS HOLTZ-EAKIN: Had they scaled it back and answered the question, what can we get the votes for, that would have been very different than Waxman-Markey.

MILES O’BRIEN: After Democrats lost control of the House in the 2010 midterm elections, the administration’s lawmaking prospects were fading.

So, Mr. Obama started issuing executive orders aimed at reducing greenhouse gases.

John Holdren is the president’s science adviser.

JOHN HOLDREN, Senior Advisor to President Obama: I think it was the only sensible decision to make, to ask, what can we do using executive authority to carry us until we get a Congress that’s more willing to consider these kinds of actions?

MILES O’BRIEN: The biggest of these rule-making steps was the Clean Power Plan.

PRESIDENT BARACK OBAMA: The single most important step America has ever taken in the fight against global climate change.

(APPLAUSE)

MILES O’BRIEN: It uses the Clean Air Act, first enacted in 1963, to control other pollutants to encourage states to reduce carbon dioxide emissions generated by the electric utilities.

CHELSEA HENDERSON, RepublicEN: I think it was his only chess move, at that point.

MILES O’BRIEN: Chelsea Henderson is with an organization called republicEn, conservatives who are seeking government action on climate change.

CHELSEA HENDERSON: In the end, I would have liked to have seen him overreach earlier in the legislative process, rather than on the regulatory side.

MILES O’BRIEN: Twenty-seven states took the Obama administration to court to try and stop the plan. Early last year, the Supreme Court ordered the EPA not to enforce it.

JOHN HOLDREN: I think the president did the best he had with the tools he had, which were the tools of executive authority. I don’t think he exceeded the legal extent of those tools.

But those are questions that will continue to be tested in the courts obviously, and some of them may be tested in the Congress.

MILES O’BRIEN: And, of course, executive orders that can be enacted with the stroke of a presidential pen can be undone in similar fashion.

But Mr. Obama’s defenders say changing existing rules was a more nimble tool than passing new laws.

CAROL BROWNER: If we had waited to start the existing law, cars wouldn’t have gotten more efficient for at least another year or two. And, more importantly, the president wouldn’t have had the kind of efficiency standards, the proposal on power plants that then allowed us to go to Paris and really establish our leadership.

MILES O’BRIEN: Paris, the site of a pivotal meeting of 195 nations that led to an agreement to curb greenhouse gas emissions, limiting the increase in the global average temperature to two degrees Celsius or below, it is the first comprehensive climate agreement in history.

JOHN HOLDREN: It happened only because the United States and China stood up together. President Obama and President Xi in Beijing in November 2014 stood up, said: We’re the two biggest economies, we’re the two biggest emitters. This is a huge problem. We are jointly going to lead.

MILES O’BRIEN: U.S. participation in the Paris agreement cannot be quickly revoked. A withdrawal has to be announced three years in advance, and then it takes another year for it to become official. But the agreement is voluntary, with no teeth, besides global peer pressure. And there is a loophole.

ROBERT STAVINS: If the Trump administration decided instead not just to remove itself from the Paris climate agreement, but from the overall U.N. Framework Convention on Climate Change that goes back to 1992, ratified by the Senate, signed by Republican President George H.W. Bush, that takes only one-year delay.

MILES O’BRIEN: Barack Obama leaves a climate legacy that is bold, yet fragile. History will likely remember him as the first climate president, but, in today’s political climate, that moniker could very quickly become a footnote.

Miles O’Brien, the “PBS NewsHour,” Washington.