A More Productive, But Different, Ocean After Mitigation

Warming of the ocean surface under greenhouse gas (GHG) accumulation has been projected to enhance ocean stratification, exacerbate nutrient limitation of phytoplankton, and decrease marine net primary production (NPP) over the next century. Studies of the reversibility of warming further suggest a lagged recovery of global mean sea surface temperatures after GHG mitigation, suggesting that oceanic NPP may also be slow to rebound. In this study, we employ a mitigation scenario in which projected Representative Concentration Pathway (RCP8.5) forcings are applied out to 2100, and then reversed over the course of the following century in a fully coupled carbon-climate earth system model, and find an unexpected rapid increase in global mean NPP, including an "overshoot" to values above contemporary means. The 5.5% NPP overshoot is driven by a similar overshoot (11.8m) in the maximum monthly mixed layer depth arising from a transient imbalance between the cooling surface ocean and waters at intermediate depths (~100-400m) that still carry strong legacy effects of warming in the 21st century. Residual warm subsurface waters at these depths weaken upper ocean density gradients, resulting in deeper mixing and enhanced surface nutrients despite the continued presence of significant legacy warming and freshening in surface waters. Enhanced surface nutrients combine with the positive effects of residual warming on phytoplankton growth and nutrient recycling to drive a global mean NPP overshoot. Regional variations in NPP reversibility exist however, and some regions experience prolonged suppression of NPP. We also find a marine ecosystem regime shift as stark depletion of silica at intermediate depths over the 21st century warming and mitigation period results in increased prevalence of large, non-diatom phytoplankton.