A computer simulation shows a net increase in primary production by phytoplankton if climate change were mitigated by 2200 but also indicates big changes in the makeup of those species.

It is well established at this point that anthropogenic factors are warming the planet at an unprecedented rate. Reducing and eventually eliminating our role in climate change is a lofty and necessary goal, but the planet’s climate—especially the oceans—carries an enormous amount of inertia. Here, using computer models, John et al. investigate how the oceans and the microscopic organisms that inhabit them would respond to a mitigation of anthropogenic warming over the course of about 100 years.

The properties of Earth’s oceans are the product of a complex balance of surface temperature, currents, biogeochemical processes, and numerous other factors. Warming and cooling of the climate influence each variable in unique ways, with some reacting quite quickly and others experiencing long lag times before any measurable response. These differences in inertia could lead to scenarios where the oceans’ properties continue to change for centuries or even millennia, even if anthropogenic warming could be mitigated entirely. To examine such a scenario and attempt to parse out these “legacy effects,” scientists simulated a 95-year period of warming (2006–2100) followed by a symmetrical period of cooling (2101–2195) and looked at how the oceans differed at the start and end point of the experiment period.

Overall, the team’s results show that net primary production by phytoplankton in the ocean is likely to rise in such a scenario. This increase in productivity is caused by increased mixing between layers of ocean water: As the planet cools, ocean waters near the surface respond faster than those deeper down. This results in a cooler top layer and relatively warmer waters between 400 and 800 meters depth. This, in turn, brings the density of the ocean’s layers closer together and allows for easier mixing. As the deep waters circulate more freely toward the surface, they carry new supplies of nitrate along with them, replenishing the vital nutrient that allows the phytoplankton to thrive. The magnitude of this effect varied considerably by geographic location, with the most extreme production increases occurring in the subtropics and Arctic.

The makeup of these phytoplankton populations, however, was quite different from contemporary norms. The models predicted a persistent depletion of silicates in the ocean waters, which allowed larger phytoplankton, especially the nondiatoms—which do not require silicates—to dominate.

The simulation results show that anthropogenic forcing will continue to influence Earth’s climate for a long time, even if it can be mitigated within the next hundred years. Understanding and predicting the future state of the planet should both prepare us for the future and serve as a reminder of the long-lasting impact our action today may have on the planet. Even if we find a way to completely stop anthropogenic warming today, the legacy effects of the change we’ve already caused would continue to echo many years into the future. (Geophysical Research Letters, doi:10.1002/2015GL066160, 2015)

—David Shultz, Freelance Writer

Citation: Shultz, D. (2016), How oceans could change if we reverse anthropogenic warming, Eos, 97, doi:10.1029/2016EO047305. Published on 4 March 2016.