Speaking at her Senate confirmation hearing, Kirstjen Nielsen said she is "not prepared to determine causation" on climate change.
Nielsen's comment contradicts mainstream climate science, including a U.S. report last week that concludes the evidence of global warming is stronger than ever and that it's "extremely likely" - meaning with 95 to 100 percent certainty - that global warming is man-made.
As head of homeland security, Nielsen would oversee a sprawling agency that leads the federal response to a range of natural disasters from wildfires to hurricanes.
Democrats criticized Nielsen's answer, which is in line with top Trump administration officials who downplay humans' role in climate change.
Sen. Tom Carper, D-Del., said his state is "sinking" as oceans rise. He called Nielsen's comment troubling. "Ninety-eight percent of our scientists say this is a problem and we as humans are the root cause," Carper said.
Sen. Maggie Hassan, D-N.H., urged Nielsen to "educate yourself" about climate change, adding "What I heard in your answer is politics before science. That concerns me greatly."
Nielsen, a former Homeland security official who now serves as deputy White House chief of staff, told the Senate Homeland Security Committee that she agrees climate change is important and said the Trump administration is revising its climate models to better respond to rising sea levels.
"I can't unequivocally state it's caused by humans," she said. "There are many contributions to it."
A 1,500-page draft report by the U.S. Global Change Research Program said global warming is already sickening, injuring and killing Americans with changes to weather, food, air, water and diseases. And it's expected to get worse, hurting the economy, wildlife and energy supply, the report said.
On other topics, Nielsen said she agreed with former Homeland Security Secretary John Kelly - now White House chief of staff - that a U.S.-Mexico border wall is unlikely to be a physical barrier from "sea to shining sea."
She also condemned white nationalism and refuted Islamophobia and promised to make cybersecurity a top priority.
Nielsen, a former staffer at the DHS, said the scope and pace of cyberattacks on federal networks and critical infrastructure are "continually increasing" and grow more complex and sophisticated each day. She said threats to the homeland "are too many and too varied" for any one agency to confront alone and pledged to work across government and the private sector to find solutions.
Trump nominated Nielsen last month to succeed Kelly. He and others in the administration have spoken about the geographical and logistical obstacles to building a massive wall along the border. Trump repeatedly promised during the campaign that he would build the wall and that Mexico would pay for it, but the administration is seeking billions in taxpayer dollars to finance the project.
Homeland Security has been leading the charge on implementing Trump's aggressive immigration agenda, and Nielsen pledged to continue that work.
"I will work every day to enforce ours laws, secure our borders, coasts and waterways and protect Americans from dangerous criminals, terrorists, cyberattacks and other threats," she said.
After Hurricane Katrina struck the Gulf Coast in 2005, Nielsen led an effort to revise disaster-response and recovery plans, work that drew praise from former Homeland Security Secretary Michael Chertoff.
But Nielsen's involvement in Katrina came under harsh scrutiny a decade ago.
Reports issued two years later by Congress were highly critical of the Bush administration's handling of the storm. The reports faulted the White House Homeland Security Council - where Nielsen directed preparedness and response - for failing to take the lead in staying on top of the unfolding disaster.
Missouri Sen. Claire McCaskill, the committee's top Democrat, said she hopes Nielsen applies lessons learned from Katrina and other disasters to improve oversight of DHS contracting practices.
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