US President Barack Obama on Tuesday viewed Alaska's Exit Glacier, in a bid to drive home the impact climate change is already having on America.

Obama visited the Kenai Fjords National Park in southern Alaska, where he stood against the backdrop of the vast, but receding, Exit Glacier.

He pointed to signposts marking the glacier's retreat since 1815.

"This is as good of a signpost of what we're dealing with when it comes to climate change as just about anything," Obama said.

"This place has lost about a mile and a half over the last couple hundred years. The reduction in glaciers has accelerated each and every year" thanks to a changing climate that has brought less snow and longer, hotter summers, he said.

That in turn, the president noted, has had an impact on flora and fauna in the spectacular park, as well as melting glacier ice, that has raised sea levels.

"We want to make sure that our grandkids can see this," Obama said.

Obama is in Alaska to build support for domestic carbon reduction rules and a global pact to cap global temperature increases by two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

In December, representatives from around the world will gather in Paris to try to thrash out the deal.

On Monday, Obama warned that climate change is no longer a problem of the future, but rather a challenge for now and one that will define the next century.

Describing the "urgent and growing" threat that he said was not being addressed quickly enough, he sketched the problems already facing people living in one of America's last wilderness frontiers.

The challenge "will define the contours of this century more dramatically than any other," Obama told a conference in Anchorage.

"Human activity is disrupting the climate, in many ways faster than we thought," he said, with one eye on Republicans who reject humans' role in heating the planet.

"The deniers are increasingly alone, on their own shrinking island."