Elon Musk, Microsoft and oil giants like BP, Occidental and Exxon Mobil are investing in carbon capture technologies. Carbon Engineering and Climeworks are two start-ups that have built machines to suck CO2 straight out of the atmosphere in a process known as direct air capture. But the technology faces a number of challenges, one of which is that there’s currently no market for the captured CO2. As a result, some companies are selling their captured CO2 to oil companies, which use it to produce even more oil.