Current climate targets require negative carbon dioxide (CO2) emissions. Direct air capture is a promising negative emission technology, but energy and material demands lead to trade-offs with indirect emissions and other environmental impacts. Here, we show by life-cycle assessment that the commercial direct air capture plants in Hinwil and Hellisheiði operated by Climeworks can already achieve negative emissions today, with carbon capture efficiencies of 85.4% and 93.1%. The climate benefits of direct air capture, however, depend strongly on the energy source. When using low-carbon energy, as in Hellisheiði, adsorbent choice and plant construction become more important, inducing up to 45 and 15 gCO2e per kilogram CO2 captured, respectively. Large-scale deployment of direct air capture for 1% of the global annual CO2 emissions would not be limited by material and energy availability. However, the current small-scale production of amines for the adsorbent would need to be scaled up by more than an order of magnitude. Other environmental impacts would increase by less than 0.057% when using wind power and by up to 0.30% for the global electricity mix forecasted for 2050. Energy source and efficiency are essential for direct air capture to enable both negative emissions and low-carbon fuels. Direct air capture (DAC) of CO2 has garnered interest as a negative emissions technology to help achieve climate targets, but indirect emissions and other environmental impacts must be better understood. Here, Deutz and Bardow perform a life-cycle assessment of DAC plants operated by Climeworks, based on industrial data.