Biology's growing role

The ocean is a remarkable sink for the increasing amount of carbon dioxide in the atmosphere. Carbon dioxide does not just diffuse into the ocean, it also reacts abiotically with seawater, producing bicarbonate and carbonate and thereby allowing more CO₂ to diffuse into the ocean. All in all, as a result of these reactions the oceans take up roughly a quarter of anthropogenic CO₂ emissions globally, and nearly all of this is converted to bicarbonate and carbonate.

In the Southern Ocean, biological production and decomposition also play an important role in regulating the CO₂ exchange between the ocean and the atmosphere. Although the Southern Ocean became a net CO₂ sink following the industrial revolution, CO₂ fluxes in this region are strongly seasonal. The Southern Ocean is a carbon dioxide sink in summer, when organisms use the dissolved inorganic carbon in the surrounding water — CO₂, bicarbonate, and carbonate — to grow. In winter, when organic material decomposes and releases CO₂ in the process, carbon dioxide is emitted back to the atmosphere.

Credit: © STEVE BLOOM IMAGES / ALAMY

In the 1950s, Roger Revelle and Hans Suess noticed that the efficiency of ocean uptake of atmospheric CO₂ can be quantified by a number that came to be known as the Revelle factor: a ratio relating changes in seawater CO₂ concentrations to changes in seawater concentrations of total dissolved inorganic carbon. But the equilibrium of the reaction between CO₂ concentration and dissolved inorganic carbon changes with pH. As oceans take up more CO₂, they become more acidic and the production of dissolved inorganic carbon becomes less favourable. Over time, the Revelle ratio increases, because a larger increase in the concentration of dissolved CO₂ is required to create a given amount of dissolved inorganic carbon. As a result, the oceans become less efficient at taking up CO₂.

Yet, as the oceans' chemical capacity to soak up CO₂ diminishes, biological activity could play an increasingly important role in regulating CO₂ uptake. Judith Hauck and Christoph Voelker simulated a scenario of large CO₂ emissions and climate change in the twenty-first century with a coupled ocean–ecosystem model (Geophys. Res. Lett. http://doi.org/zzc; 2014). Their approach allowed them to distinguish how CO₂ uptake is likely to change in the future in response to changes in the ocean's chemical capacity to take up CO₂ independent of the effects of changes in temperature, circulation, or resource availability.

It is no surprise that ocean uptake of CO₂ would increase alongside rising anthropogenic emissions of CO₂ over the twenty-first century. But as the ocean's ability to efficiently take up CO₂ diminished, the strength of the seasonal cycle in CO₂ uptake increased throughout the Southern Ocean. Late in the twenty-first century, the biological uptake of dissolved inorganic carbon caused a much larger decline in the amount of CO₂ in the Southern Ocean surface waters than it did early in the century, seawater CO₂ concentrations became more sensitive to changes in dissolved inorganic carbon concentrations: the change in the Revelle ratio means that a given decline in the levels of dissolved inorganic carbon will result in larger uptake of atmospheric CO₂ by the oceans. In total, CO₂ uptake from biological activity increased by roughly 2.5 times over the course of the twenty-first century, even though changes in biological activity were small.

With some effort, we can avoid the large increases in atmospheric CO₂ concentrations described in this high-emissions scenario. But even if emissions grow at a slower pace, CO₂ may eventually reach concentrations that shift the ocean system into a new chemical state, where marine organisms would play an increasingly important role in controlling CO₂ uptake.