The only way to limit global warming to less than two degrees is to combine renewable energy and energy efficiency with a large expansion in the use of carbon capture and storage, writes Christian Friis Bach. Christian Friis Bach is executive secretary and under-secretary-general of UN Economic Commission for Europe (UNECE).I will admit that just a few years back I was very sceptical. Today I am convinced that we must do it. We must capture the carbon dioxide emitted from burning fossil fuels.Oil prices tumble. Coal resources are vast. Large new gas reserves have been found. Fossil fuels will be with us for many decades and will continue to underpin social and economic development around the world. We need to invest heavily in energy efficiency and in renewable energy sources, but the only way we can hope to limit global warming to less than two degrees is to combine it with a significant expansion of the use of carbon capture and storage (CCS).