Scott Pruitt, Donald Trump’s pick to lead the US Environmental Protection Agency, has claimed there is still “some debate” over the role of human activity in climate change and has defended his relationship with the fossil fuel industry during a combative Senate confirmation hearing.

Pruitt, the Oklahoma attorney general, has sued the agency he is now set to lead 14 times over the EPA’s smog, mercury and other pollution regulations. Several of these cases are still ongoing and Pruitt said he would recuse himself in dealing with these cases if instructed to do so by the EPA’s ethics board.

In testy exchanges with senators including Bernie Sanders and Ed Markey, Pruitt said there was “some debate” over how much influence human activity has upon the climate but rejected the president-elect’s claim that climate change is a “hoax”. Pruitt also said the EPA had a “very important role” in regulating carbon dioxide.

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“Science tells us that the climate is changing and that human activity in some manner impacts that change,” he said. “The ability to measure with precision the degree and extent of that impact and what to do about it are subject to continuing debate and dialogue ... so it should be.”

Last year was the warmest on record, scientists announced on Wednesday, with Nasa and Noaa both stressing the primary driver of the warming trend is the burning of fossil fuels and other human activity. Of the 17 hottest years on record, 16 have occurred this century.

Pruitt also seemed uncertain over how much lead can be safely ingested by children, in the wake of the toxic water crisis in Flint, Michigan. “I don’t know,” Pruitt said. “I’ve not looked at the scientific research on that. That’s not something I’ve reviewed nor know about.” The EPA itself states that any amount of lead consumption can be harmful.

Democrats on the Senate committee on environment and public works questioned Pruitt over his repeated challenges to the agency he now seeks to head, as well as his ties to the fossil fuel industry. The oil giant Exxon and coal firm Murray Energy have both given the maximum allowable amount of money to Pruitt, with the Oklahoma attorney general siding with donors 13 times in court cases against the EPA.

One Oklahoma firm, Devon Energy, even drafted a letter for Pruitt that he sent on to the EPA in 2011 under his letterhead with minimal alterations. The letter criticized federal regulations on greenhouse gas emissions from oil and gas producers. A boom in gas fracking activity in Oklahoma has contributed to a surge in earthquakes in the state.

Questioned over this letter by Democrat Jeff Merkley, Pruitt said: “I was representing the interests of the state. It was protecting the interests of the state, it wasn’t sent on behalf of any one company. It was particular to an industry – there’s an oil and gas industry that is vibrant and vital to the state.”

In his opening statement, Pruitt said: “We must reject as a nation the false paradigm that if you’re pro-energy you’re anti-environment and that if you’re pro-environment you’re anti-energy. In this nation we can grow our economy, harvest the resources God has blessed us with as well as being good stewards of the land, air of water by which we’ve been favored.”

Pruitt said he wanted a better partnership with the states, which he said had been subject to “duress and punishment” from the EPA. He said the states had the “resources and expertise” to safeguard America’s environment but accepted that pollution does cross state lines.

Republicans on the committee backed Pruitt, with Senator James Inhofe, who once brought a snowball to the senate floor in an attempt to disprove global warming, citing Pruitt’s fighting of “federal overreach” as praiseworthy. Fellow Republican attorney generals from other states have also supported Pruitt’s nomination.

However, Christine Todd Whitman, who was EPA administrator under George W Bush, warned there may be “war” within the agency unless Pruitt adopted a more conciliatory posture.

“I wish he hadn’t been nominated,” Whitman, a Republican, told Guardian US. “Mother Nature doesn’t care about states’ rights. You need some area of federal oversight to protect human health and the environment. You can’t just turn it back to the states; most of them don’t have the budget to do the scientific research.

“I think this new administration will try to back down some of the regulations and slow down enforcement of the regulations they don’t like. They will starve the agency for money.

“I would hope that Scott Pruitt will understand how important and complicated the EPA is once he gets in there but EPA people are assuming it will be a war. That won’t be pretty for anyone. Unless he makes real strides to outreach and respect the mission, it will be war.”

Thirteen former state EPA chiefs have urged the Senate to reject Pruitt, citing his “deeply troubling” position on climate change and his repeated courtroom challenges to EPA clean air and water standards.

“Rather than EPA acting as our partner in state-led efforts to ensure clean air and water for our residents, we fear that an EPA under Mr Pruitt would undermine the rules that help to make sure that our state regulations are successful,” the group wrote in a letter to the environment and public works committee.

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More than 170 environment groups, including the Sierra Club and the Clean Air Task Force, have written a separate missive to senators decrying Pruitt’s views that “run counter to the EPA’s critical mission to protect our health and the environment”. The letter also calls for the Senate to reject the Oklahoma attorney general.

Pruitt is the “worst nominee ever tapped to lead the US Environmental Protection Agency”, according to Rhea Suh, president of the Natural Resources Defense Council.

“He doesn’t have a single environmental achievement to his name, doesn’t believe in the agency’s mission, and has made a career out of suing the EPA to try to block it from doing its job as the guardian of our environment and health,” she added.