Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets

Abstract

Abstract. Ocean deoxygenation is recognized as key ecosystem stressor of the future ocean and associated climate-related ocean risks are relevant for current policy decisions. In particular, benefits of reaching the ambitious 1.5 • C warming target mentioned by the Paris Agreement compared to higher temperature targets are of high interest. Here, we model oceanic oxygen, warming and their compound hazard in terms of metabolic conditions on multi-millennial timescales for a range of equilibrium temperature targets. Scenarios where radiative forcing is stabilized by 2300 are used in ensemble simulations with the Bern3D Earth System Model of Intermediate Complexity. Transiently, the global mean ocean oxygen concentration decreases by a few percent under low forcing and by 40 % under high forcing. Deoxygenation peaks about a thousand years after stabilization of radiative forcing and new steady-state conditions are established after AD 8000 in our model. Hypoxic waters expand over the next millennium and recovery is slow and remains incomplete under high forcing. Largest transient decreases in oxygen are projected for the deep sea. Distinct and near-linear relationships between the equilibrium temperature response and marine O 2 loss emerge. These point to the effectiveness of the Paris climate target in reducing marine hazards and risks. Mitigation measures are projected to reduce peak decreases in oceanic oxygen inventory by 4.4 % • C −1 of avoided equilibrium warming. In the upper ocean, the decline of a metabolic index, quantified by the ratio of O 2 supply to an organism's O 2 demand, is reduced by 6.2 % • C −1 of avoided equilibrium warming. Definitions of peak hypoxia demonstrate strong sensitivity to additional warming. Volumes of water with less than 50 mmol O 2 m −3 , for instance, increase between 36 % and 76 % • C −1 of equilibrium temperature response. Our results show that millennial-scale responses should be considered in assessments of ocean deoxygenation and associated climate-related ocean risks. Peak hazards occur long after stabilization of radiative forcing and new steady-state conditions establish after AD 8000.

A Bayesian Ensemble Data Assimilation to Constrain Model Parameters and Land Use Carbon Emissions

“…The assessment of the performance of a given model version using observational benchmarks has also been actively discussed in the literature (Hoffman et al, 2017;Peng et al, 2014;Kelley et al, 2013;Luo 15 et al, 2012;Blyth et al, 2011;Randerson et al, 2009) and different frameworks have been proposed. Here we employ the Latin Hypercube Sampling (LHS) (McKay et al, 1979) approach, as used successfully in previous studies Battaglia et al, 2016;Steinacher and Joos, 2016;Battaglia and Joos, 2017;Zaehle et al, 2005). It allows simultaneous stratified sampling of a range of parameters, given an appropriate prior parameter distribution, while offering the opportunity to change evaluation metrics a posteriori, thus enabling a sensible incorporation of multiple observational constraints.…”

“…Transient Land uptake (Inversion) Global land uptake from atmospheric inversion (Boden et al, 2017;Battaglia and Joos, 2017) Transient Land uptake (IPCC) Global land uptake in five periods The modeled total and soil carbon distribution between 1982 and 2005 are compared to a data set based on observations (Carvalhais et al, 2014), regridded to the model resolution. The soil carbon map is divided into low and high latitudes regions in order to avoid potential biases from peat areas with very high soil carbon content.…”

“…In addition to uncertain prescribed LULCC forcings and processes in DGVMs, the parametrization of those processes is subject to further uncertainties. We use a Monte-Carlo-like data assimilation approach (Steinacher 5 et al, 2013;Steinacher and Joos, 2016;Battaglia and Joos, 2017) to sample 15 key model parameters and construct a 1000-member model ensemble to investigate this parameter related uncertainty in the DGVM LPX-Bern. Furthermore, we establish a new reference version of the model.…”

Global Warming Can Lead to Depletion of Oxygen by Disrupting Phytoplankton Photosynthesis: A Mathematical Modelling Approach

“…It has been shown that global warming can lead to a significant reduction in the amount of oxygen stocked in the ocean [6], in particular by reducing the oxygen solvability and slowing down the ocean upturning and mixing [7]. If this happens, it could have a devastating effect on marine life [8]. More recently, it has been shown that the effect of warming on the total oxygen budget in the ocean can be even worse, as a sufficiently large increase in the water temperature can disrupt the net oxygen production by phytoplankton's photosynthesis [15,17].…”

“…Somewhat less publicized in the media, but perhaps even more dangerous is the observed decrease in the concentration of dissolved oxygen in the ocean [6]. It is predicted that in the worst case scenario, the amount of ocean oxygen could fall almost twice [7], which would lead to disastrous consequences for the ocean ecosystems by suffocating most of marine life [8].…”