Category: Publications-summaries

Hauck, J., Zeising, M., Le Quéré, C., Gruber, N., Bakker, D. C. E., Bopp, L., Chau, T. T. T., Gürses, Ö., Ilyina, T., Landschützer, P., Lenton, A., Resplandy, L., Rödenbeck, C., Schwinger, J., & Séférian, R. (2020). Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget. In Frontiers in Marine Science (Vol. 7, p. 852). https://www.frontiersin.org/article/10.3389/fmars.2020.571720

Summary:

Based on the 2019 assessment of the Global Carbon Project, the ocean took up on average, 2.5 ± 0.6 PgC yr⁻¹ or 23 ± 5% of the total anthropogenic CO₂ emissions over the decade 2009–2018. This sink estimate is based on simulation results from global ocean biogeochemical models (GOBMs) and is compared to data-products based on observations of surface ocean pCO₂ (partial pressure of CO₂) accounting for the outgassing of river-derived CO₂. In this study, the GOBM simulations are evaluated by comparing the simulated surface ocean pCO₂ to observations. Based on this comparison, the simulations are well-suited for quantifying the global ocean carbon sink on the time-scale of the annual mean and its multi-decadal trend, as well as on the time-scale of multi-year variability, despite the large model-data mismatch on the seasonal time-scale. Biases in GOBMs comparison have a small effect on the global mean ocean sink (0.05 PgC yr⁻¹), but need to be addressed to improve the regional budgets and model-data comparison. Additionally, GOBMs and data-products point consistently to a shift from a tropical CO₂ source to a CO₂ sink in recent years. On average, the GOBMs reveal less variations in the sink than the data-based products. Despite the reasonable simulation of surface ocean pCO₂ by the GOBMs, there are discrepancies between the resulting sink estimate from GOBMs and data-products. These discrepancies are within the uncertainty of the river flux adjustment, increase over time, and largely stem from the Southern Ocean. Progress in our understanding of the global ocean carbon sink necessitates significant advancement in modeling and observing the Southern Ocean carbon sink including (i) a game-changing increase in high-quality pCO₂ observations, and (ii) a critical re-evaluation of the regional river flux adjustment.

Reygondeau, G., Cheung, W. W. L., Wabnitz, C. C. C., Lam, V. W. Y., Frölicher, T., & Maury, O. (2020). Climate Change-Induced Emergence of Novel Biogeochemical Provinces. In Frontiers in Marine Science (Vol. 7, p. 657). https://www.frontiersin.org/article/10.3389/fmars.2020.00657

Summary:

The global ocean is commonly partitioned into 4 biomes subdivided into 56 biogeochemical provinces (BGCPs). Each province corresponds to a unique regional environment that shapes biodiversity and constrains ecosystem structure and functions. Biogeochemical provinces are dynamic entities that change their spatial extent and position with climate and are expected to be perturbated in the near future by global climate change. In this study, the changes in spatial distribution of BGCPs from 1950 to 2100 using three earth system models under two representative concentration pathways (RCP 2.6: zero CO₂ emissions by 2100 and RCP 8.5: “no mitigation”) were characterised. Projection of the future distribution of BGCPs also revealed the emergence of new climate that has no analog with past and current environmental conditions. These novel environmental conditions, are named No-Analog BGCPs State (NABS), will cover areas where a substantial proportion of global marine biodiversity presently occurs and with a crucial dependence on seafood production and will expand from 2040 to 2100 at a rate of 4.3 Mkm² per decade (1.2% of the global ocean). The NABS were characterized by very warm mean annual temperatures, high salinity, low oxygen concentration and low net primary production. Most marine species will be physiologically stressed under such conditions, which could impact their survival rate. This study subsequently quantified the potential number of marine species and annual volume of fisheries catches that would experience such novel environmental conditions to roughly evaluate the impact of NABS on ecosystem services. If the global climate is not kept below 2°C warming, NABS areas can be expected to emerge, as early as 20 years from the 2010s. It would affect 19% of the total number of exploited species in 2050 and 59% in 2100 and would cover regions that are currently responsible for 8% of global marine fisheries catch in 2050 and 30% in 2100, under RCP 8.5. These numbers would change to only 15% of exploited species and 5% of total fisheries catches in NABS areas by the end of the 21st century under the RCP 2.6 scenario. Mitigating anthropogenic pressures at a level sufficient to reach the Paris agreement targets would therefore substantially reduce the risk of emergence of large NABS regions in the global ocean, and the dramatic consequences that such large-scale ecological changes would entail for tropical marine biodiversity, associated fisheries and the human communities that they support.

Policy relevant message:

The environmental changes that would occur in the global ocean along a “no mitigation” RCP 8.5 scenario would lead to a drastic reorganization of global marine biogeography, associated biodiversity and trophic networks. If the global climate is not kept below 2°C warming, these novel areas can be expected to emerge, as early as 20 years from the 2010s. It would affect 19% of the total number of exploited species in 2050 and 59% in 2100 and would cover regions that are currently responsible for 8% of global marine fisheries catch in 2050 and 30% in 2100, under RCP 8.5. These numbers would change to only 15% of exploited species and 5% of total fisheries catches in these novel areas by the end of the 21st century under the RCP 2.6 scenario (zero CO₂ emissions by 2100). Mitigating anthropogenic pressures at a level sufficient to reach the Paris agreement targets would therefore substantially reduce the risk of emergence of large NABS regions in the global ocean, and the dramatic consequences that such large-scale ecological changes would entail for tropical marine biodiversity, associated fisheries and the human communities that they support.

Burger, F. A., John, J. G., & Frölicher, T. L. (2020). Increase in ocean acidity variability and extremes under increasing atmospheric CO2. Biogeosciences, 17(18), 4633–4662. https://doi.org/10.5194/bg-17-4633-2020

Summary:

Ensemble simulations of an Earth system model reveal that ocean acidity extremes have increased in the past few decades and are projected to increase further in terms of frequency, intensity, duration, and volume extent. The increase is not only caused by the long-term ocean acidification due to the uptake of anthropogenic CO₂, but also due to changes in short-term variability. The increase in ocean acidity extremes may enhance the risk of detrimental impacts on marine organisms.

Policy relevant message:

Ocean acidity extremes have increased in the past few decades and are projected to increase further in terms of frequency, intensity, duration, and volume extent which may enhance the risk of detrimental impacts on marine organisms.

Álvarez-Salgado, X. A., Otero, J., Flecha, S., & Huertas, I. E. (2020). Seasonality of Dissolved Organic Carbon Exchange Across the Strait of Gibraltar. Geophysical Research Letters, 47(18), e2020GL089601. https://doi.org/10.1029/2020GL089601

Summary:

The Mediterranean Sea is a semi enclosed basin connected with the Atlantic Ocean through the Strait of Gibraltar. At this hot spot of ocean circulation, about 0.8 Sv (1 Sv = 106 m³ s⁻¹) of dissolved organic carbon (DOC) rich Atlantic Surface Water enters the Mediterranean Sea and the same volume of DOC poor Mediterranean Overflow Water flows oppositely to the Atlantic Ocean. Both DOC concentrations and water flows are not stationary but vary seasonally. Differences in the amplitude and timing of those seasonal cycles produce a marked bimodal variation in the net DOC flux of Atlantic water that enters the Mediterranean Sea, with minima in late June and late October and maxima in mid‐April and late August. This pattern has been observed for the first time and allowed the authors to better constrain this organic carbon flux, which represents about half of the total input of DOC in the Mediterranean Sea and supports about one third of its net organic carbon demand.

Fontela, M., Pérez, F. F., Carracedo, L. I., Padín, X. A., Velo, A., García-Ibañez, M. I., & Lherminier, P. (2020). The Northeast Atlantic is running out of excess carbonate in the horizon of cold-water corals communities. Scientific Reports, 10 (1), 14714. https://doi.org/10.1038/s41598-020-71793-2

Summary:

The oceanic uptake of atmospheric carbon dioxide (CO₂) emitted by human activities alters the seawater carbonate system. The increase of atmospheric CO₂ leads to an increase in ocean anthropogenic carbon (C_ant) and a decrease in carbonate that is unequivocal in the upper and mid-layers (0–2,500 m depth). In the mid-layer, the carbonate content in the Northeast Atlantic is maintained by the interplay between the northward spreading of recently conveyed Mediterranean Water with excess of carbonate and the arrival of subpolar-origin waters close to carbonate undersaturation. In this study the authors examined the chemical status of the Northeast Atlantic by means of a high-quality database of carbon variables based on the GO-SHIP A25 section (1997–2018). A progression to undersaturation with respect to carbonate could compromise the conservation of the habitats and ecosystem services developed by benthic marine calcifiers inhabiting the mid-layer depth range, such as the cold-water corals (CWC) communities. The authors also stressed that for each additional ppm in atmospheric pCO₂ the waters surrounding CWC communities lose carbonate at a rate of − 0.17 ± 0.02 μmol kg⁻¹ ppm⁻¹. The accomplishment of global climate policies to limit global warming below 1.5–2 ℃ will avoid the exhaustion of excess carbonate in the Northeast Atlantic.

Policy relevant message:

Increasing amount of atmospheric CO₂ causes alterations of oceanic carbon system, which leads to destructions of cold-water corals. The accomplishment of global climate policies to limit global warming below 1.5–2 ℃ is critical to avoid further alterations of carbon system in the Northeast Atlantic.

Rodgers, K. B., Schlunegger, S., Slater, R. D., Ishii, M., Frölicher, T. L., Toyama, K., Plancherel, Y., Aumont, O., & Fassbender, A. J. (2020). Reemergence of Anthropogenic Carbon Into the Ocean’s Mixed Layer Strongly Amplifies Transient Climate Sensitivity. Geophysical Research Letters, 47 (18), e2020GL089275. https://doi.org/10.1029/2020GL089275

Summary:

In this work, the authors demonstrate that the net ocean uptake of anthropogenic carbon is strongly sensitive to perturbations in the CO₂ buffering capacity of surface ocean waters. This result is closely connected to the process of reemergence of anthropogenic carbon from the ocean interior to the surface mixed layer. In this paper, they used an ocean circulation‐carbon cycle model to identify an upper limit on the impact of reemergence of anthropogenic carbon into the ocean’s mixed layer on the cumulative airborne fraction of CO₂ in the atmosphere. They find under an RCP8.5 emissions pathway (with steady circulation) that the cumulative airborne fraction of CO₂ has a sevenfold reduction by 2100 when the CO₂ buffering capacity of surface seawater is maintained at preindustrial levels. The results indicate that the effect of reemergence of anthropogenic carbon into the mixed layer on the buffering capacity of CO₂ amplifies the transient climate sensitivity of the Earth system.

Séférian, R., Berthet, S., Yool, A., Palmiéri, J., Bopp, L., Tagliabue, A., Kwiatkowski, L., Aumont, O., Christian, J., Dunne, J., Gehlen, M., Ilyina, T., John, J. G., Li, H., Long, M. C., Luo, J. Y., Nakano, H., Romanou, A., Schwinger, J., … Yamamoto, A. (2020). Tracking Improvement in Simulated Marine Biogeochemistry Between CMIP5 and CMIP6. Current Climate Change Reports, 6(3), 95–119. https://doi.org/10.1007/s40641-020-00160-0

Summary:

Increasing availability of ocean biogeochemical data, as well as an improved understanding of the underlying processes, allows advances in the marine biogeochemical components of the current generation of ESMs. The present study scrutinizes the extent to which marine biogeochemistry components of ESMs have progressed between the 5th and the 6th phases of the Coupled Model Intercomparison Project (CMIP).

Robson, J., Aksenov, Y., Bracegirdle, TJ, Dimdore-Miles, O., Griffiths, PT, Grosvenor, DP, Hodson, DLR, Keeble, J., MacIntosh, C., Megann, A., Osprey, S., Povey, AC, Schröder, D., Yang, M., Archibald, AT, Carslaw, KS, Gray, L., Jones, C., Kerridge, B.,… Wilcox, LJ (2020). The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6. Journal of Advances in Modeling Earth Systems, 12 (9), e2020MS002126. https://doi.org/10.1029/2020MS002126

Summary:

The North Atlantic climate system plays an important role in regulating Earth’s climate, and variability within the Atlantic can have important impacts on society. However, we do not understand all the linkages between different parts of the North Atlantic. Furthermore, climate simulations, which are an essential tool for improving our understanding, have shortcomings that can affect their utility. New developments in Earth System climate simulations could remedy these shortcomings. The extent to which the addition of complex Earth system developments have changed or improved the simulation of the physical climate is still unknown. In this paper, a multidisciplinary evaluation of the North Atlantic climate in simulations made with the UK’s Earth System Model, UKESM1 is presented. The simulations made with UKESM1 capture many aspects of the North Atlantic climate and that human activities have a large impact on the North Atlantic in UKESM1. Nevertheless, some shortcomings of the simulations, many of which are like those seen in physical climate simulations are highlighted. The authors stressed the importance of further development of both the physical and Earth system components to improve climate simulations in the future.

Frölicher, T. L., Aschwanden, M. T., Gruber, N., Jaccard, S. L., Dunne, J. P., & Paynter, D. (2020). Contrasting Upper and Deep Ocean Oxygen Response to Protracted Global Warming. Global Biogeochemical Cycles, 34 (8), e2020GB006601. https://doi.org/10.1029/2020GB006601

Summary:

It is well established that the ocean is currently losing dissolved oxygen (O₂) in response to ocean warming (the solubility of O₂ decreases with increasing seawater temperature resulting in less O₂ available to marine life). However, the long‐term equilibrium response of O₂ to a warmer climate is neither well quantified nor understood. In this study, multimillennial global warming simulations with a comprehensive Earth system model was used to show that the equilibrium response in ocean O₂ differs fundamentally from the ongoing transient response. The deep ocean is better ventilated and oxygenated compared to preindustrial conditions, even though the deep ocean is substantially warmer. In contrast, O₂ in most of the upper tropical ocean is substantially depleted. This study emphasizes the millennial‐scale impact of global warming on marine life, with some impacts emerging many centuries or even millennia after atmospheric CO₂ has stabilized.

Policy relevant message:

The impact of global climate change on marine life not only is already clearly visible and well recorded, but the millennial‐scale impact of global warming will also emerge many centuries or even millennia after atmospheric CO₂ has stabilized.

Lam, V. W. Y., Allison, E. H., Bell, J. D., Blythe, J., Cheung, W. W. L., Frölicher, T. L., Gasalla, M. A., & Sumaila, U. R. (2020). Climate change, tropical fisheries and prospects for sustainable development. Nature Reviews Earth & Environment, 1(9), 440–454. https://doi.org/10.1038/s43017-020-0071-9

Summary:

Tropical fisheries substantially contribute to the well-being of societies in both the tropics and the extratropics, the latter through ‘telecoupling’ — linkages between distant human–natural systems. Tropical marine habitats and fish stocks, however, are vulnerable to the physical and biogeochemical oceanic changes associated with rising greenhouse gases. These changes to fish stocks, and subsequent impacts on fish production, have substantial implications for the UN Sustainable Development Goals. In this Review, the effects of climate change on tropical marine fisheries are synthesised, highlighting the socio-economic impacts to both tropical and extratropical nations, and discuss potential adaptation measures. Driven by ocean warming, acidification, deoxygenation and sea-level rise, the maximum catch potential of tropical fish stocks in some tropical exclusive economic zones is projected to decline by up to 40% by the 2050s under the RCP8.5 emissions scenario, relative to the 2000s. Climate-driven reductions in fisheries production and alterations in fish-species composition will subsequently increase the vulnerability of tropical countries with limited adaptive capacity. Thus, given the billions of people dependent on tropical marine fisheries in some capacity, there is a clear need to account for the effects of climate change on these resources and identify practical adaptations when building climate-resilient sustainable-development pathways.