WASHINGTON (Reuters) - Janet Yellen, U.S. President-elect Joe Biden’s nominee for Treasury Secretary, on Tuesday called climate change an “existential threat” to the U.S. economy and said she would appoint a senior official at Treasury to oversee the issue. “I will look to appoint someone at a very senior level to lead our efforts,” Yellen told members of the Senate Finance Committee. She said doing so would create a hub within Treasury that would focus on financial system-related risk posed by climate change, and tax policy incentives to affect change. “We need to seriously look at assessing the risk to the financial system from climate change,” Yellen said.