It was the world’s most famous movie built around a slide show: Al Gore, in a darkened auditorium, clicking through images that warned of a heating planet. Within the film’s first minutes, Mr. Gore described a “moral imperative to make big changes,” a call he echoed several times onstage.
Twenty years after the release of “An Inconvenient Truth,” his Oscar-winning documentary, it is remembered mostly for its cultural resonance — for bringing awareness of global warming into the mainstream. Mr. Gore and his intense focus on climate change energized both supporters on the left and critics on the right. In a self-effacing bit on “Saturday Night Live,” Mr. Gore warned viewers about glaciers on the attack. A year later, he shared the Nobel Peace Prize.
As the years passed, Mr. Gore has kept going with his slide show, giving presentations in hundreds of cities worldwide, most recently in Nashville earlier this month.
And with time, the slide show has changed in ways that reflect how the conversation about climate change has shifted over the course of a generation.
Onstage in Nashville, Mr. Gore made a central argument that would have been inconceivable two decades ago. Rather than directly invoking morality, he led with economics.
The cost of renewable energy had plunged. He talked about “market forces” and about the “spectacular, unprecedented” technology revolution — including low-cost solar panels and wind turbines — that now make aiding the planet an affordable choice.
“We’re in a different world now,” Mr. Gore said in Nashville. “The options are terrific.”
The moral aspect of climate advocacy has had a long legacy, burnished not only by Mr. Gore but also by Pope Francis, who portrayed a link between environmental degradation and societal rot. In the late 2010s, a wave of youth protesters argued that political leaders and corporations had a duty to safeguard the planet for future generations.
But as that movement waned, some felt the moralizing had at times brought a political backlash. After the documentary’s release, Mr. Gore was criticized in some right-wing circles for hypocrisy given that he traveled widely and lived a lifestyle reliant on fossil fuels for energy. Later, to attend climate events, the activist Greta Thunberg twice crossed the Atlantic by sailboat in a conspicuous effort to avoid polluting air travel, a move that some critics called a publicity stunt out of reach for noncelebrities.
Environmentalists, meantime, made a new case: that wind and solar energy were becoming cheaper than fossil fuels. Bill McKibben, a founder of the campaign group 350.org, said climate advocates no longer had to “fight against the force of economic gravity.”
“This is the place where we have the most leverage,” he said. “Instead of having the economic wind forever blowing at us, now it’s in our sails. We do finally have a tool to work with.”
Johan Rockström, the scientist who leads the Potsdam Institute for Climate Impact Research, went so far as to say that the “winning argument” in pushing for cuts to greenhouse-gas emissions “is not the ethical and moral one, it is the economic one,” in part because of the political blowback the moral argument can attract.
In an interview, Mr. Gore spoke at length about his evolving slide show, which he estimated he had given thousands of times. Did he move away from the moral message?
“It remains the central argument,” Mr. Gore said, pointing out that he was still making this case in speeches and at meetings. But he also noted that “experience says there are a lot of people who will be more easily be persuaded by the impact on their pocketbooks.”
He described his target audience as people “whose minds are changeable,” and that’s where he thinks an economic argument, as well as one about health implications, has grown more effective. He also reasoned that the moral case, as strong as he felt it was, had perhaps reached its persuasive limits.
President Trump has forcefully rejected the idea that renewable energy has an economic advantage, last year calling green energy a “scam” and describing climate change as the “greatest con job ever perpetrated on the world.” During his first term, Mr. Trump criticized “prophets of doom and their predictions of the apocalypse,” with Ms. Thunberg, the climate campaigner, in the audience.
The consequences of climate change, of course, still raise moral questions. Wealthy countries are responsible for the bulk of emissions, while poorer nations disproportionately face the damage. Deadly heat waves singe countries where most can’t afford air conditioning, while hurricanes barrel into small islands and slice their G.D.P.s. Last year, the top United Nations court ruled that nations had a duty to prevent environmental harm and that any breach constituted a “wrongful act.”
“The idea that there is one way to talk about climate, it never was true and isn’t true now,” said Rachel Cleetus, a senior policy director at the Union of Concerned Scientists.
Mr. Gore still begins his slide show as he did in the “An Inconvenient Truth” days, with an image of Earth taken during the Apollo 8 mission in 1968. After that, the differences in content are immediately apparent. Back then, though he cited examples of how climate change was driving extreme events, he said the phenomenon was just “beginning” to show itself. Now, he devotes a full hour to current events such as fast-intensifying hurricanes, wildfires and shrinking glaciers that are reducing water supply.
In the 2006 documentary, he noted that the 10 hottest years on record had come over the previous 14 years, since the 1990s.
Compare that with his current slide show, in which he notes that the 10 hottest years have all happened since 2015.
“It’s still getting worse much faster than the solutions are being deployed,” Mr. Gore said in the interview.
Mr. Gore said he had been presenting some version of the slide show since the 1980s and had performed it more than 1,000 times even before the documentary’s release, using a Kodak Carousel projector in the early days. He said that “one of the most important presentations” was on the deck of a houseboat in Center Hill Lake in his home state, Tennessee. He was with longtime friends, with beer flowing. They weren’t the most obvious audience for a climate talk. But, he said, they were interested.
“It emboldened me to start taking it on the road in a much more forceful way,” Mr. Gore said.
A scene in the documentary showed him at a desk, laptop open, jotting notes on paper. “I guess the thing I’ve spent more time on than anything else is trying to identify all those things in people’s minds that serve as obstacles to them understanding this,” Mr. Gore said at the time.
Asked why he continued with the slide show 20 years later, he described it as a “challenge of a particular kind.” But this time he was talking about himself, not the planet. “It’s one of those, when you pick it up, you can’t put it down,” he said.
