Treasury Secretary Janet Yellen said Saturday that $3 trillion in new capital is needed each year to fight climate change and called the global transition to a low-carbon economy “the single greatest opportunity of the 21st century.”

Speaking in Belém, Brazil, after meeting with G20 finance ministers to discuss economic developments this week, Yellen stressed the need for stronger climate finance policies through 2050 to address the “existential threat” to societies and the economic strain posed by climate change.

“Neglecting to address climate change and the loss of nature and biodiversity is not just bad environmental policy. It is also bad economic policy,” she said.

The speech, delivered as part of an event by the Inter-American Development Bank, reaffirmed US commitment to the IDB’s Amazonia Forever Program on its one-year anniversary, a project intended to promote sustainable development in the Amazon region and defend against deforestation.