In 2009, when representatives from around the world gathered in Copenhagen, Denmark to discuss global action on climate change, wealthy countries pledged $100 billion a year to help developing nations adapt to the impacts of rising temperatures and curb carbon emissions.
The number was arbitrary, tossed into the fray by then-Secretary of State Hillary Clinton as tensions rose over rich countries' responsibility to pay for the problem they had largely caused. But it stuck, and 2020 was set as a goal for delivering the funds.
