US Treasury Secretary Janet Yellen said private financing, and not just government spending, will be needed to tackle the “existential threat” of climate change, warning that the overall cost of achieving net-zero carbon emissions by 2050 could run to $2.5 trillion over 10 years for the US alone. Al Jazeera reports that Yellen pledged that the US will help developing countries that are especially vulnerable to threats from climate change, but didn’t offer any specific new pledge of additional US government funding to help developing nations adapt to a warming planet or build clean-energy projects.