"The U.S. election will be one of the most consequential moments determining the whole world's ability to limit warming," said Caroline Brouillette, executive director of Climate Action Network Canada.

The razor-thin race to the White House is happening as American communities reel from the devastating impact of two hurricanes that many experts say were worsened by climate change.

The United Nations has said climate change is the largest crisis facing humanity today and Vice-President Kamala Harris and former president Donald Trump have starkly different plans for how, or even if, their administration would respond.

Observers say the vice-president's record presents a leader that takes the issue seriously. She is expected to follow the path laid by Biden's administration, which brought in historic legislation to support and expand the clean energy economy.

On the other side, experts warn, the first Trump administration was a devasting blow to environmental regulations and climate policy. If he wins, they say, then Trump would go even further.

"This time around it would be even worse," said Lena Moffitt, executive director for Evergreen Action, a climate change advocacy group.

Trump's first administration saw sweeping attempts to roll back more than 100 environmental protections. The Republican president also withdrew from the Paris Agreement, an international treaty to cut greenhouse gases.

During Trump's tenure the Environmental Protection Agency shrank and the words "climate change" were removed from its website.

"It was a punch to the gut," said Raul Garcia with Earthjustice Action.

"He undid longstanding protections that had been relied upon since the 70s... to keep our environment as healthy as possible. Regulation after regulation, we saw him in a very ad hoc, convoluted and oftentimes illegal way get rid of these requirements."

It's unlikely Trump has any plans for global warming this time. In the wake of devastating Hurricane Helene, he called climate change "one of the great scams of all time."

The 2024 Republican platform proclaims "DRILL, BABY, DRILL" and says America will become "energy independent, and even dominant again." There's no mention of climate change.

Trump indicated he would go after the Inflation Reduction Act, the landmark environmental economy legislation, and end incentives for the electric vehicle market.

Trump has tried to distance himself from Project 2025, a blueprint for a hard-right turn in American government, but environmental experts suspect it will be the path he follows.

The document calls for "unleashing all of America's energy resources" by eliminating federal restrictions on fossil fuel drilling on public lands and easing environmental permitting restrictions.

It proposes closing the National Oceanic and Atmospheric Administration and steering the National Weather Service to exclusively sell weather data to private forecasters.

"As scary as it is, I think that what we saw under the Trump 1.0 administration was only a test run when it comes to walking back climate action," said Brouillette.

That plan would be a wholesale assault on the climate progress, Moffitt added.

"(It) would take us backwards at a time when we can least afford it," Moffitt said, referencing an expert report from the Intergovernmental Panel on Climate Change which warned the world must take immediate action to slash greenhouse gases in order to limit global warming to 1.5C.

"We are halfway through this decisive decade that scientists say we have to take action, bend the curve of emissions downward if we hope to have a chance of avoiding the worst of the worst of the climate crisis."

When President Joe Biden got to office, he reversed Trump's policies, strengthened environmental regulations and rejoined the Paris Agreement.

The Inflation Reduction Act amounted to the largest infusion of government cash into climate and clean-energy initiatives. Harris provided the tie-breaking Senate vote to pass the legislation.

The vice-president has called climate change an existential threat. During her speech at the Democratic National Convention she listed what was at stake in November.

"The freedom to breathe clean air, and drink clean water and live free from the pollution that fuels the climate crisis," Harris said in Chicago.

She prosecuted oil companies for environmental violations as California's attorney general. As a senator, Harris was an early co-sponsor of the Green New Deal, a non-binding blueprint to transition to clean energy within a decade.

She's previously suggested a climate pollution fee to "make polluters pay for emitting greenhouse gases into our atmosphere."

"Harris has a really strong record on the environment and will take us forward, building the clean energy economy the world is increasingly demanding, but making sure those things are built right here at home," Moffitt said.

"We can expand clean energy while also reducing emissions."

But Harris has also taken a more centrist turn since she rose to the top of the Democrat's ticket. During the presidential debate, she boasted the Biden-Harris administration had overseen "the largest increase in domestic oil production in history."

She has also reversed a 2019 pledge to ban fracking.

Brouillette said there have been "disgraceful attempts by both candidates to out-frack each other" but a Trump presidency would set the world on a dire path.

If the Republican leader prevails in November, Brouilette warned the rest of the world will need to respond.

"It is more important than ever that all other major economies, including Canada, step up and show high ambition leadership on (the) climate."

This report by The Canadian Press was first published Oct. 26, 2024.