As a top-level international climate summit starts later this month in Paris, Gore - who helped negotiate the 1997 climate treaty that didn't control the problem - is sure this time will be different.
"I'm optimistic," Gore said in a sit-down interview this week with The Associated Press. "We're going to win this. We need to win it faster because a lot of damage is being done day by day. We continue to put 110 million tons of global warming pollution into the atmosphere every 24 hours as if it's an open sewer."
In 35 minutes, Gore - portrayed by critics as a preacher of doom and gloom - uses versions of the words "optimistic" or "hopeful" or "positive" at least 16 times.
Even when he ticks off the alarming impacts of global warming, he finishes with a note of confidence.
"The number of extremely hot days has multiplied dramatically," Gore said. "The large downpours, floods, mudslides, the deeper and longer droughts, rising sea levels from the melting ice, forest fires, there's a long list of events that people can see and feel viscerally now. Every night on the television news is like a nature hike through the Book of Revelation."
But he added: "Increasingly people are connecting those dots. And even if they don't use the phrase climate crisis or global warming, more and more people are feeling that this is going to have to be addressed."
On Friday, Gore will take his mixed message of alarm and hope to Paris, a bit ahead of world leaders. He will host a 24-hour-telethon of sorts from the Eiffel Tower to raise awareness about global warming, featuring Elton John, French President Francois Hollande, actor Jared Leto, Energy Secretary Ernest Moniz, former United Nations chief Kofi Annan, actor Ryan Reynolds and California Gov. Jerry Brown.
But he said it's no longer just about convincing people to act - it now makes sense economically, too. He says solar and wind energy is dirt cheap - even free in Texas at certain hours. Businesses and developing countries are taking climate change seriously, he said.
"There really is a wave in corporate America moving rapidly toward a low carbon economy," Gore said.
Unlike the Kyoto treaty in 1997, which mandated emission cuts for rich nations but not poor, what's likely out of Paris won't require ratification by the U.S. Senate and is based on countries setting their own goals. And that, Gore insisted, is "more productive."
But is Gore, himself?
Dana Fisher, director of the Program for Society and the Environment at the University of Maryland, said Gore's "role is limited at this point. There was a moment in time when he was pushing a wave of attention." But now, she says, she didn't even know that Gore was organizing his Paris telethon.
"I never thought of him as central person in the climate movement," Fisher said.
Gore insists that he is. He said he's trained "many thousands" of activists and still consults with leaders in the U.S. and other governments at all levels. He plans to be at the Paris climate talks "until the last dog dies."
Some experts suggest Gore's stint as the public face of climate change activism - especially with his 2006 documentary "An Inconvenient Truth" - may have turned off some people because the messenger was so associated with Democratic politics.
"Climate change science is demonized because of Al Gore," said Erik Conway, a NASA historian who co-wrote the book "Merchants of Doubt." Conway doesn't fault Gore, but said, "If John McCain had become the titular leader of the climate change movement instead of Al Gore, we might have a different world."
But Naomi Oreskes, a Harvard historian who is a co-author of the book with Conway, disagrees. "He's become demonized because he is effective," she said.
For his part, Gore said, "Whoever becomes highly visible as a spokesman for change gets the slings and arrows and all of the anger directed at the messenger to try to get at the message."
In addition to his Climate Reality Project, his main advocacy group (which is co-producing the telethon), Gore is chairman of Generation Investment Management, a boutique investment managing firm. He is on Apple's board of directors and is a senior partner at a Kleiner Parkins Caufield & Byers, a major Silicon Valley venture capital fund.
Fifteen years later, the 67-year-old Gore claims he doesn't dwell on 2000 election, when he won the popular vote but lost the Electoral College: "I started moving forward the day after the Supreme Court decision and I'm excited about the future."
So was losing the presidency was all for the best?
"No, I wouldn't say that," Gore said, laughing. "I don't think there's any position with as much potential to create positive change as much as president of the U.S., but that was not to be. I feel very fortunate other ways to make a positive difference."
Then he returned to the subject of his telethon. Hozier will be among the performers; Gore whips out his iPhone to play the singer's 2014 hit, "Take Me To Church."
He closed his eyes and listened. The song's final verse: "In the madness and soil of that sad earthly scene, only then I am human, only then I am clean."