Two companies already capturing CO2 from the air are Climeworks and Carbon Engineering.

Both are World Economic Forum Technology Pioneers - early-stage and growing companies from around the world who are leading the way on new technologies and innovations.

Switzerland-based Climeworks opened its first DAC plant in 2017 and now has 15 machines in operation. It runs the Iceland plant mentioned above - currently the world's biggest - and employs a form of solid direct air capture using filters.

Climeworks is helping companies including jewellery brand Swarovski and online grocer Ocado to reduce, offset and remove CO2 emissions.

Canada-based Carbon Engineering uses a form of liquid direct air captureand is working with partners to develop the world's first large-scale DAC facility. This is in the US Permian Basin, a shale oil and gas producing area between Texas and New Mexico, and is designed to permanently lock away between 500,000 and 1 million tonnes a year of CO2 in rocks deep underground.

Engineering has started on another DAC plant of the same scale planned for Scotland.