It is "critical" for Washington and Beijing to keep working together on climate finance, US Treasury Secretary Janet Yellen said Saturday, urging deeper cooperation in addressing the "existential threat" of global warming. Yellen is on a four-day trip to Beijing, as the United States seeks to cool tensions and stress areas of collaboration between the world's two largest economies. "As the world's two largest emitters of greenhouse gases and the largest investors in renewable energy, we have both a joint responsibility –- and ability -– to lead the way," Yellen told a roundtable of experts in China, underlining a key…