At a Glance
A serious threat from global warming and anthropogenic activities is the loss of oxygen in the world’s oceans (Levin 2018). The Resplandy group leverages cutting edge high-resolution ocean and Earth System Models to constrain future ocean de-oxygenation and anticipate the occurrence of “dead zones” threatening coastal ecosystems and populations in the Indian Ocean.
Warming makes oxygen (O2) less soluble and limits its exchanges with the atmosphere. A major concern is that this loss of O2 will expand tropical oxygen minimum zones (OMZ). These are areas where O2 levels are already very low and unsuitable for most organisms. The expansion of tropical OMZs threatens the survival of marine organisms that rely on dissolved oxygen for respiration (Diaz and Rosenberg 2008). It also affects the biogeochemical cycling of carbon and nitrogen, potentially amplifying global warming.
Earth-system models (ESM) currently project dramatically different changes in OMZ volume (ranging from −2% to +16% by 2100) (Bopp et al. 2017). In fact, ESMs agree on many aspects that control future OMZs such as the reduced oxygen solubility tied to surface warming and reduced oxygen supply. Uncertainties, however, arise from the differences in the magnitude and timing of these changes (Resplandy 2018). Using a high-resolution ocean (10 km) ESM developed at NOAA-GFDL, the Resplandy group showed that OMZs in ESMs are too sensitive to ocean circulation changes. This compromises the ability to predict the evolution of OMZs and explains the disparate trends in their future projections (Busecke et al. 2019).
The tropical Indian Ocean is at greater risk of de-oxygenation in the near future. The OMZ in this basin extends over 1000 m in the water column and covers the coastal waters of countries accounting for more than one-quarter of the world’s population (India, Pakistan, Bangladesh, etc.). Anthropogenic eutrophication, a process that further consumes oxygen, exacerbates the effect of global warming in coastal areas. Eutrophication refers to excessive nutrient enrichment due to waste waters, fertilizers, urbanization, and other factors. In the Indian Ocean, reports of coastal hypoxia events (near-zero O2), or “dead zones,” related to anthropogenic eutrophication have increased exponentially in the past 20 years. Riverine nutrient loadings are also projected to increase due to population growth (Sinha et al. 2017).
Uncertainties in the processes controlling oxygen in the Indian Ocean limit the ability to predict coastal dead zones and their impacts on ecosystems and populations. A major limitation so far was to quantify both natural and anthropogenic processes that incorporate a wide range of spatio-temporal scales. This includes local processes, such as the fine-scale heterogeneities induced by ocean dynamics (Figure 5.1), and global scale processes, such as rainfall changes associated with global warming. The Resplandy group has designed an ocean ecosystem model, run on Princeton’s TIGER research computing cluster, that includes the processes needed to constrain the occurrence of dead zones that were not accounted for in previous studies. This research is examining two key questions: Which natural factors lead to or predispose the coastal Indian Ocean for low oxygen levels? How are these low oxygen levels exacerbated by human activity and lead to coastal dead zones?
Bopp, L., L. Resplandy, A. Untersee, P.L. Mezo, and M. Kageyama, 2017. Ocean (de)oxygenation from the Last Glacial Maximum to the twenty-first century: insights from Earth System models. Phil. Trans. R. Soc. A 375, 20160323. https://doi.org/10.1098/rsta.2016.0323.
Busecke, J.J.M., L. Resplandy, and J.P. Dunne, 2019. The equatorial undercurrent and the oxygen minimum zone in the pacific. Geophysical Research Letters 46, 6716–6725.
Diaz, R. J., and R. Rosenberg, 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926–929.
Levin, L. A., 2018. Manifestation, drivers, and emergence of open ocean deoxygenation. Annu. Rev. Mar. Sci. 10, null.
Resplandy, L., 2018. Will ocean zones with low oxygen levels expand or shrink? Nature 557, 314–315.
Sinha, E., A.M. Michalak, and V. Balaji, 2017. Eutrophication increase during the 21st century as a result of precipitation changes. Science 357.