relating to substances that are heated to higher than their critical temperature and pressure (= the highest temperature and pressure at which a gas can be changed into a liquid), resulting in changes to the way they behave

Although much denser than a gas, the supercritical CO2 is less dense than water so it will start to migrate upwards. 虽然比气体密度大得多，但超临界二氧化碳的密度比水小，因此它会开始向上移动。