Pathways for avoiding ocean biogeochemical damage: Mitigation limits, mitigation options, and projections

Anthropogenic greenhouse gas emissions cause multiple changes in the ocean and its ecosystems through climate change and ocean acidification. These changes can occur progressively with rising atmospheric carbon dioxide concentrations, but there is also the possibility of large-scale abrupt, and/or potentially irreversible changes, which would leave limited opportunity for marine ecosystems to adapt. Such changes, either progressive or abrupt, pose a threat to biodiversity, food security, and human societies. However, it remains notoriously difficult to determine exact limits of a “safe operating space” for humanity. Here, we map, for a variety of ocean impact metrics, the crossing of limits, which we define using the available literature and to represent a wide range of deviations from the unperturbed state. We assess the crossing of these limits in three future emission pathways: two climate mitigation scenarios, including an overshoot scenario, and one high-emission no-mitigation scenario. These scenarios are simulated by the latest generation of Earth system models and large perturbed-parameter ensembles with two Earth system models of intermediate complexity. Using this comprehensive model database, we estimate when and at which warming level 4 mitigation limits based on expert judgement for 14 different impact metrics are exceeded along with an assessment of uncertainties. We find that under the high-emissions scenario, the two highest limits are exceeded with high confidence for the marine heatwaves’ duration, expansion of ocean areas that are undersaturated with respect to aragonite, decreases in plankton biomass, and loss of Arctic summer sea ice extent. The probability of exceeding a given limit generally decreases clearly under low-emissions scenario. Yet, exceedance of ambitious limits related to Arctic aragonite undersaturation, plankton biomass, and Arctic summer sea ice extent are projected to be difficult to avoid (high confidence) even under the low-emissions scenario. The scenario including a temporary overshoot reduces with high confidence the risk of exceeding mitigation limits by year 2100 related to the marine heatwave duration, metabolic index, plankton biomass, Atlantic meridional overturning circulation, aragonite undersaturation, global deoxygenation, and Arctic summer sea ice extent compared to the high-emissions scenario. Our study highlights the urgent need for ambitious mitigation efforts to minimize extensive impacts and potentially irreversible changes to the world's oceans and ecosystems.