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<jats:p> The atmosphere is only one component in an enormous complex of nested physical and chemical processes, some of which remain poorly understood. <jats:italic>Science</jats:italic> offers this user's guide to the carbon cycle. </jats:p>

<jats:p> <jats:italic>Science</jats:italic> has created a map showing some of the major carbon capture and storage projects around the world, either completed, in operation, or scheduled for the near future. </jats:p>

<jats:p>Two new projects, one in Inner Mongolia and the other in Tianjin, mark the coal-hungry country's first major steps toward trapping carbon emissions.</jats:p>

<jats:p>The capture of carbon dioxide at the point of emission from coal- or gas-burning power plants is an attractive route to reducing carbon dioxide emissions into the atmosphere. To commercialize carbon capture, as well as transport of liquified carbon dioxide and its storage in exploited oil fields or saline formations, many technological, commercial, and political hurdles remain to be overcome. Urgent action is required if carbon capture and storage is to play a large role in limiting climate change.</jats:p>

<jats:p> Amine scrubbing has been used to separate carbon dioxide (CO <jats:sub>2</jats:sub> ) from natural gas and hydrogen since 1930. It is a robust technology and is ready to be tested and used on a larger scale for CO <jats:sub>2</jats:sub> capture from coal-fired power plants. The minimum work requirement to separate CO <jats:sub>2</jats:sub> from coal-fired flue gas and compress CO <jats:sub>2</jats:sub> to 150 bar is 0.11 megawatt-hours per metric ton of CO <jats:sub>2</jats:sub> . Process and solvent improvements should reduce the energy consumption to 0.2 megawatt-hour per ton of CO <jats:sub>2</jats:sub> . Other advanced technologies will not provide energy-efficient or timely solutions to CO <jats:sub>2</jats:sub> emission from conventional coal-fired power plants. </jats:p>

<jats:p> Air capture is an industrial process for capturing CO <jats:sub>2</jats:sub> from ambient air; it is one of an emerging set of technologies for CO <jats:sub>2</jats:sub> removal that includes geological storage of biotic carbon and the acceleration of geochemical weathering. Although air capture will cost more than capture from power plants when both are operated under the same economic conditions, air capture allows one to apply industrial economies of scale to small and mobile emission sources and enables a partial decoupling of carbon capture from the energy infrastructure, advantages that may compensate for the intrinsic difficulty of capturing carbon from the air. </jats:p>

<jats:p> The possibility that substantial quantities of CO <jats:sub>2</jats:sub> can be injected into subsurface porous rock formations has been investigated sufficiently to show that pore space available to contain the CO <jats:sub>2</jats:sub> is abundant. Multiple rock types and physical mechanisms can be used to trap the CO <jats:sub>2</jats:sub> indefinitely. With careful site selection and operations, leakage to the near-surface region can be avoided. The next step is to test these injection processes at the scale of a large power plant. </jats:p>

<jats:p> The battle to reduce greenhouse gas emissions and prevent the most dangerous consequences of climate change will be waged across multiple fronts, including efforts to increase energy efficiency; efforts to deploy nonfossil fuel sources, including renewable and nuclear energy; and investment in adaptation to reduce the impacts of the climate change that will occur regardless of the actions we take. But with more than 80% of the world’s energy coming from fossil fuel, winning the battle also requires capturing CO <jats:sub>2</jats:sub> from large stationary sources and storing that CO <jats:sub>2</jats:sub> in geologic repositories. Offshore geological repositories have received relatively little attention as potential CO <jats:sub>2</jats:sub> storage sites, despite their having a number of important advantages over onshore sites, and should be considered more closely. </jats:p>

<jats:p>Satellite tracking technology has allowed scientists to map part of the migration route of the European eel.</jats:p>