Raising $100 billion a year of climate finance by 2020 is challenging, but possible through mechanisms including carbon markets, domestic carbon taxes and a variety of international transportation taxes, a United Nations advisory group said in a report Friday. Earlier this year, the UN's Ban established the panel, which includes U.S. National Economic Council Director Larry Summers, billionaire financier George Soros and Deutsche Bank vice-chairman Caio Koch-Weser. The financing will be used to support mitigation and adaptation efforts in developing countries--in particular, for the poorest and most vulnerable communities.