Large scale negative emissions arguably rely on the cost-effective development of direct air capture (DAC) technologies which could draw down Carbon dioxide (CO2) using chemicals that bind with it. The basic technology of DAC has been used in submarines and spacecraft. At its simplest, DAC works by absorbing CO2 from the air onto a chemical receptor, which has a strong attraction for the CO2, in a similar way as a sponge absorbs water. DAC is not expected to be an alternative to conventional mitigation nor to point-source abatement through carbon capture and storage. Rather, it is intended as a supplement to such responses, providing a means to reduce ambient atmospheric concentrations of CO2 or to offset dispersed emissions that are difficult to mitigate directly, such as those from air travel. Large scale DAC remains a "technological imaginary". It is no silver bullet for the climate problem, despite being almost certainly essential to long-term reductions of atmospheric CO2.