The Carbon Cycle on Two Timescales

Whether a transfer counts as an addition depends on the system being counted. For the atmosphere alone, respiration adds carbon and photosynthesis removes it. For air, living things and the ocean counted together, those transfers move carbon between stores within the system.

Now include a coal deposit outside that active system. Burning coal moves geological carbon into the active stores. An animal respiring carbon from recently grown food moves carbon already inside them. Both release CO2, but only the coal transfer draws on the geological store in this comparison.

This accounting does not assign a special lifetime to fossil-derived CO2. Once mixed into the same air, CO2 behaves alike regardless of its source.

A rapid exchange can also leave a surplus in place. Carbon leaves the atmosphere while other carbon enters. To assess an emissions pulse, follow the change in the total amount stored, rather than asking only how quickly individual molecules cross a boundary.

A useful carbon budget therefore names its boundary, the stores being counted and the transfers across that boundary.

Consider a rise in atmospheric CO2. The resulting warming can change the rate at which silicate minerals react with water containing dissolved CO2. Where water and fresh rock are available, warmer conditions can increase this chemical weathering.

Rivers carry dissolved products to the ocean. Carbonate formation and burial complete a pathway that removes CO2 over geological time. Breaking a rock into pieces exposes more surface, but physical breakup alone does not complete that chemical removal pathway.

An increase in long-term removal opposes the initial CO2 rise. This is a negative feedback. Its rate also depends on water supply and access to fresh minerals, so temperature alone cannot determine the response.

Geological stores have return routes as well. Volcanic and metamorphic degassing can release CO2. Over long periods, the balance between geological sources and sinks helps shape atmospheric carbon and climate.

A feedback can oppose a change without reversing it quickly. Land and ocean uptake can reduce an atmospheric addition earlier, while geological removal of the remaining tail takes much longer. Calling weathering a thermostat does not mean that it can cancel a year's emissions within a season.

Follow added atmospheric CO2 into seawater. Its uptake reduces some of the atmospheric addition, but the carbon remains in Earth's system. Transfer into surface water also does not mean immediate burial in rock. Circulation can carry carbon into deeper water, and some ocean carbon can later return to air.

Extra dissolved CO2 changes seawater chemistry. It lowers pH and reduces carbonate-ion availability. Seawater can become more acidic while remaining alkaline overall.

Corals and many shell-building organisms combine calcium and carbonate to build calcium carbonate structures. Reduced carbonate availability can make that construction harder. This does not mean that every shell immediately dissolves; responses depend on the organism and surrounding conditions.

These chemical effects are different from ocean warming, although both can affect marine life. A claim about ocean carbon uptake should say what is stored, for how long and with what chemical consequences.

The linked TED-Ed talk concerns oil-industry communication about climate change. It provides social context for fossil-fuel use. It does not replace evidence about carbon transfers, ocean chemistry or removal times.