Every year, we human beings emit large quantities of carbon dioxide (CO2). Roughly half of it remains in the atmosphere, where it drives global warming; the other half is absorbed by the ocean and by plants on the land. In this way, both systems provide invaluable services for us human beings. Without their absorbing and storing carbon dioxide, the current concentration of the greenhouse gas CO2 wouldn’t be 412 ppm (parts per million, as of December 2020), but roughly 600 ppm. As a result, the global warming would be between 3 and 4 degrees Celsius – and not the 1.1 degrees Celsius measured to date.
Since 1850, the ocean alone has absorbed the same amount of carbon dioxide as was released in the same timeframe by burning mineral oil (75 ppm). But researchers expect to see this essential climate service provided by the ocean begin to decline due to global warming. The causes are complex: first of all, gases dissolve much more poorly in warm water than in cold; in other words, from a purely physical standpoint, a warming ocean simply can’t absorb as much as a cold one. In addition, the constant absorption of carbon dioxide affects the chemical balance of seawater. This phenomenon, referred to as ocean acidification, further reduces the ability to absorb CO2. A smaller ocean carbon sink can also be the outcome when climate changes affect major ocean currents, or when, due to changes in the marine environment (warming, acidification, nutrient and light availability), fewer microalgae (phytoplankton) grow, reducing the ocean’s biological productivity.