Barack Obama has warned world leaders must strike a deal on climate change "while we still can" when they meet for a climate conference in Paris later this year.
Speaking at the start of a three-day tour of Alaska, the US President admitted "the climate is changing faster than our efforts to address it".
He said the US had to acknowledge the part it had played in damaging the environment and "embrace responsibility" to help solve the problem.
"On this issue - of all issues - there is such a thing as being too late," Mr Obama said. "And that moment is almost upon us."
He went on: "Of course we're here today to discuss the challenge that will define the contours of this century more dramatically than any other.
"And that's the urgent and growing threat of a changing climate. Our understanding of climate change advances each day.
"Human activity is disrupting the climate - in many ways, faster than we previously thought. The science is stark. It is sharpening.
"It proves that this once distant threat is now very much in the present."
Mr Obama will become the first US President to visit the Alaskan Arctic as part of a tour designed to highlight the state's melting glaciers and sinking villages.
He will hike to Exit Glacier in Kenai Fjord National Park. Last year it melted at a rate three times the annual average.
But Mr Obama has been criticised for giving the go-ahead to oil exploration in the treacherous waters of the Alaskan Arctic.
Environmentalists say a spill would be catastrophic for the region and that exploration sends the wrong message on fossil fuels.
Campaigner Sweetwater Nannauck told Sky News: "I don't see how we can be a champion for ending this without putting a stop to this drilling, not just here but other sources too.
"You can't do both. It is impossible."
All Alaskans receive an annual dividend from oil income and, with existing supplies dwindling, many have welcomed new drilling licenses.
Joan Naviyuk Kane, a native Alaskan writer, said: "Oil exploration and natural resource development are going to happen whether or not native Alaskans are complicit.
"Either they take a piece of the economic pie or they resist and refuse to have a part in it and also not then get the money that is going to be available from it."
But Fran Ulmer, the chair of the US Arctic Research Commission, denied the issuing of drilling permits represented hypocrisy from Mr Obama.
She told Sky News: "He has taken a leadership role in trying to do whatever he can within his executive authority to reduce and deal with climate change.
"But at the same time we're not going to stop using oil and gas. We hope to reduce its use but that doesn't mean we're going to stop filling up our cars at the tank."
Mr Obama hopes highlighting the impact of climate change in Alaska will garner support for his efforts to secure an international climate treaty at the United Nations Climate Change Conference in November.
The visit will also see him join TV survival expert Bear Grylls for an episode testing his ability to survive in the Alaskan wilderness.