Kelly Craft told lawmakers at her confirmation hearing she would “be an advocate for all countries to do their part in addressing climate change”.
In the past, she had claimed to believe “both sides” of the climate debate.
Mr Trump has previously called climate change a “hoax” and questioned the scientific consensus on the matter.
Earlier this month, Mr Trump said climate change “goes both ways” and blamed other nations for worsening air and water quality.
In 2017, he pulled the US out of the landmark Paris climate agreement, saying the deal was disadvantageous to US workers.
Mrs Craft who is currently serving as the ambassador to Canada, had offered a similar opinion in 2017, telling CBC she believed “there are scientists on both sides that are accurate”.
But she reversed that viewpoint on Wednesday, telling the Senate Foreign Relations Committee that “human behaviour has contributed to the changing climate”.
“Let there be no doubt: I take this matter seriously.”
She also acknowledged “that fossil fuels have played a part in climate change”.
However, Mrs Craft did support Mr Trump’s decision to withdraw from the Paris accord, saying the US did not have to “be part of an agreement to be leaders”.
She added that the US should not have to assume “an outsized burden on behalf of the rest of the world”.
Mr Trump’s nominee has been under scrutiny over her ties to the coal industry as she is married to Joseph Craft III, the head of Alliance Resource Partners, one of the country’s largest coal companies.
After being grilled by Democrats on how she would handle fossil fuel discussions in the UN, Mrs Craft pledged to recuse herself from such talks if the ethics agreement called for it.
If confirmed, Mrs Craft would replace Nikki Haley, who resigned last October.
Meanwhile, the Trump administration has continued to roll back environmental protections.
The latest such effort on Wednesday loosened restrictions on coal-fired power plants. The measure, signed by Environmental Protection Agency chief Andrew Wheeler, will allow individual states to determine if coal plants should reduce emissions.
The new measure replaces an Obama-era plan that sought to reduce greenhouse gas emissions.
Environmentalists have criticised the new policy, saying it will worsen fossil fuel emissions, while Republican lawmakers from coal industry states praised the move.
New York Attorney General Letitia James has promised a lawsuit.
In a statement, she called the rule “another prime example of the Trump administration’s weak attempt to deny that climate change has caused – and will continue to cause – devastating impacts on both the safety and health of all Americans and the economy”.
Scientists have warned that the world is headed towards a temperature rise of 3C, that would cause significant and dangerous changes to the planet.