Divestment has become a rallying call by environmental campaigners in the fight against climate change. It's when environmentally aware investors put pressure on their fund managers, employers and governments to move money away from polluting industries. An estimated $11 trillion have been divested from fossil fuel stocks since the 2015 Paris climate summit, but has that divestment made a difference? With emissions continuing to soar, wouldn't it be better just to tax energy companies more? Others argue that investors should put money into innvovative technologies that help solve climate change. Justin Rowlatt discusses these issues and more with guests Mark Lewis, Global Head of Sustainability, BNP Paribas Asset Management, Ahmed Mokgopo, Campaigner, 350.org and Gayle Peterson, Associate Fellow, Said Business School, Oxford.