03 April 2018

Reversal of Increasing Tropical Ocean Hypoxia Trends with Sustained Climate Warming

Decreases in productivity in tropical surface waters may reduce consumption of O2​ at depth, reversing warming effects on solubility.

Science

Researchers from the University of California Irvine, with collaborators from the National Center for Atmospheric Research, examined the impacts of sustained climate warming on ocean interior oxygen for a “business as usual” scenario of fossil fuel emissions using an Earth system model.

Impact

Our analysis indicates that sustained multi-century climate warming would likely cause declines in the oxygen content of the global ocean. However, in the tropics, the balance between ventilation, which replenishes interior O2, and sinking biological particles, which lead to the consumption of O2, is modified by climate warming. Reduced nutrient flows into the tropics cause a decrease in ocean primary production after 2100. In turn, biological export declines, and this causes less O2 consumption in deeper waters. These processes reverse solubility driven impacts on tropical interior O2.

Summary

Ocean interior O2 responds to changes in solubility, ventilation, and the strength of the biological pump. While climate-driven solubility changes have received considerable attention, the influence of climate on ventilation and net primary production may yield a complex response that is difficult to predict, particularly over a period of centuries. The recovery of O2 in the tropical intermediate waters after 2100 that we describe highlights a possible unanticipated long-term implication of a decoupling of the Southern Ocean and tropical ocean net primary production, and has important implications for biological feedbacks to climate warming. Specifically, denitrification levels in tropical intermediate waters decline below pre-industrial levels, suggesting that the contracting oxygen minimum zones may offset anthropogenic N2O emissions originating from agriculture and other land sources.

Contact
Drs. Weiwei Fu and James T. Randerson
University of California at Irvine (UC Irvine)
Publications
Fu, W, F Primeau, J Moore, K Lindsay, and J Randerson.  2018.  "Reversal of Increasing Tropical Ocean Hypoxia Trends with Sustained Climate Warming."  Global Biogeochemical Cycles 32.  https://doi.org/10.1002/2017gb005788.