Category: Publications-summaries

Arellano-Nava, B., Halloran, P. R., Boulton, C. A., Scourse, J., Butler, P. G., Reynolds, D. J., & Lenton, T. M. (2022). Destabilisation of the Subpolar North Atlantic prior to the Little Ice Age. Nature Communications, 13(1), 5008. https://doi.org/10.1038/s41467-022-32653-x

Summary

The cooling transition into the Little Ice Age (a period of bitter winters and mild summers that affected Europe and North America between the 14th and 19th centuries) was the last notable shift in the climate system prior to anthropogenic global warming. It is hypothesised that sea-ice to ocean feedbacks sustained an initial cooling into the Little Ice Age by weakening the subpolar gyre circulation; a system that has been proposed to exhibit bistability (two stable states). Empirical evidence for bistability within this transition has however been lacking. Using statistical indicators of resilience in three annually-resolved clam shell proxy records from the North Icelandic shelf, the authors of this study show that the subpolar North Atlantic climate system destabilised during two episodes prior to the Little Ice Age. This loss of resilience indicates a reduced attraction to one stable state, and a system vulnerable to an abrupt transition. The two episodes preceded wider subpolar North Atlantic change, consistent with subpolar gyre destabilisation and the approach of a tipping point, potentially heralding the transition to Little Ice Age conditions.

Burger, F. A., Terhaar, J., & Frölicher, T. L. (2022). Compound marine heatwaves and ocean acidity extremes. Nature Communications, 13(1), 4722. https://doi.org/10.1038/s41467-022-32120-7

Summary

Compound MHW-OAX events, during which marine heatwaves (MHWs) co-occur with ocean acidity extreme (OAX) events, can have larger impacts on marine ecosystems than the individual extremes. Using monthly open-ocean observations over the period 1982–2019, this study shows that globally 1.8 in 100 months (or about one out of five present-day MHW months) are compound MHW-OAX event months under a present-day baseline, almost twice as many as expected for 90th percentile extreme event exceedances if MHWs and OAX events were statistically independent. Compound MHW-OAX events are most likely in the subtropics and less likely in the equatorial Pacific and the mid-to-high latitudes. Additionally, the projected long-term mean warming and acidification trends have the largest effect on the number of MHW-OAX days per year, increasing it from 12 to 265 days per year at 2 °C global warming relative to a fixed pre-industrial baseline. Even when long-term trends are removed, an increase in [H⁺] variability leads to a 60% increase in the number of MHW-OAX days under 2 °C global warming. These projected increases may cause severe impacts on marine ecosystems.

 

Policy relevant message

The projected long-term mean warming and acidification trends have the largest effect on the number of marine heatwaves ocean acidity extreme days (MHW-OAX) per year, increasing it from 12 to 265 days per year at 2 °C global warming relative to a fixed pre-industrial baseline. Even when long-term trends are removed, an increase in [H⁺] variability (acidification) leads to a 60% increase in the number of MHW-OAX days under 2 °C global warming. These projected increases may cause severe impacts on marine ecosystems.

Vaittinada Ayar, P., Bopp, L., Christian, J. R., Ilyina, T., Krasting, J. P., Séférian, R., Tsujino, H., Watanabe, M., Yool, A., & Tjiputra, J. (2022). Contrasting projections of the ENSO-driven CO2 flux variability in the equatorial Pacific under high-warming scenario. Earth System Dynamics, 13(3), 1097–1118. https://doi.org/10.5194/esd-13-1097-2022

 

Summary

The El Niño–Southern Oscillation is the main driver for the natural variability of global atmospheric CO₂. It modulates the CO₂ fluxes in the tropical Pacific with anomalous CO₂ influx during El Niño and outflux during La Niña. This relationship is projected to reverse by half of Earth system models studied here under the business-as-usual scenario. This study shows models that simulate a positive bias in surface carbonate concentrations simulate a shift in the ENSO–CO₂ flux relationship.

Nissen, C., Timmermann, R., Hoppema, M., Gürses, Ö., & Hauck, J. (2022). Abruptly attenuated carbon sequestration with Weddell Sea dense waters by 2100. Nature Communications, 13(1), 3402. https://doi.org/10.1038/s41467-022-30671-3

Summary

Antarctic Bottom Water formation, such as in the Weddell Sea in the Southern Ocean, is an efficient vector for carbon sequestration on time scales of centuries. Possible changes in carbon sequestration under changing environmental conditions are unquantified to date, mainly due to difficulties in simulating the relevant processes on high-latitude continental shelves. The authors of this study use a model setup including both ice-shelf cavities and oceanic carbon cycling and demonstrate that by 2100, deep-ocean carbon accumulation in the southern Weddell Sea is abruptly attenuated to only 40% of the 1990s rate in a high-emission scenario, while the rate in the 2050s and 2080s is still 2.5-fold and 4-fold higher, respectively, than in the 1990s. Assessing deep-ocean carbon budgets and water mass transformations, this decline was attributed to an increased presence of modified Warm Deep Water on the southern Weddell Sea continental shelf, a 16% reduction in sea-ice formation, and a 79% increase in ice-shelf basal melt. Altogether, these changes lower the density and volume of newly formed bottom waters and reduce the associated carbon transport to the abyss.

Policy relevant message:

Under the high emissions scenario, carbon sequestration by Weddell Sea dense water formation will reduce by 40% by 2100.

Köhn, E. E., Münnich, M., Vogt, M., Desmet, F., & Gruber, N. (2022). Strong Habitat Compression by Extreme Shoaling Events of Hypoxic Waters in the Eastern Pacific. Journal of Geophysical Research: Oceans, 127(6), e2022JC018429. https://doi.org/10.1029/2022JC018429

 

Summary

The global ocean is currently losing oxygen. Consequently, marine organisms that require oxygen are increasingly confined to the well-oxygenated surface ocean above the gradually shoaling sub-surface “hypoxic” waters, that is, waters with insufficiently low oxygen concentrations. On top of this long-term trend, internal variability causes the hypoxic waters to intermittently shoal and induce Transient Habitat Reduction Extreme Events (THREEs). THREEs may change biogeochemical processes or alter entire ecosystem structures for weeks to months. To investigate when and where THREEs occur, a simulation of the Eastern Pacific (EP) from 1979 to 2016 was performed. The authors find that EP THREEs are mainly associated with the El Niño-Southern Oscillation, the seasonal cycle, and mesoscale eddies. At low latitudes, THREEs compress the habitat by up to 50%–70% (locally over 80%). Furthermore, 71% of THREEs go along with a shoaling of waters with extremely low pH conditions. Hence, during these THREEs marine organisms face compounding extremes. This study establishes a basis for studying the effects of THREEs in the open ocean. THREEs can provide a window into the future, as the long-term oxygen loss might transform current “extreme” conditions into the future normal state.

 

Policy relevant message

At low latitudes, Transient Habitat Reduction Extreme Events (THREEs) compress the habitat by up to 50%–70% (locally over 80%). Furthermore, 71% of THREEs go along with a shoaling of waters with extremely low pH conditions. Hence, during these THREEs marine organisms face compounding extremes.

Oziel, L., Schourup-Kristensen, V., Wekerle, C., & Hauck, J. (2022). The Pan-Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015). Global Biogeochemical Cycles, 36(6), e2021GB007268. https://doi.org/10.1029/2021GB007268

Summary

Microscopic algae called phytoplankton are the base of the trophic chain, sustaining the entire Arctic Ocean (AO) ecosystem. In the central parts of the AO, multi-year sea-ice used to limit transmission of light in the surface ocean and therefore control phytoplankton growth and primary productivity. However, the massive loss in sea-ice during the last 3 decades allowed more and more light to penetrate the water column, making nutrient availability the main bottom-up control of the AO productivity. A major part of the bio-available nutrients reaching the surface in the central AO are transported with ocean currents from the adjacent North Atlantic and Pacific and from deeper water masses. Using a biogeochemical model resolving processes at high spatial resolution, we were able to quantify the different transport pathways of nutrients with ocean currents and revealed that despite increasing supply along the anticlockwise flowing boundary current, the central AO is still running into more severe nutrient limitation.

Policy relevant message:

The continental slope contributes to the transport of nutrients in the Arctic Ocean. Yet, despite an intensification of ocean dynamics, the Arctic Ocean is still shifting from a light-limited to a nutrient-limited system.

Cael, B. B., Begouen Demeaux, C., Henson, S., Stock, C. A., Taboada, F. G., John, J. G., & Barton, A. D. (2022). Marine Ecosystem Changepoints Spread Under Ocean Warming in an Earth System Model. Journal of Geophysical Research: Biogeosciences, 127(5), e2021JG006571. https://doi.org/10.1029/2021JG006571

 

Summary

Plankton are the backbone of pelagic ocean ecosystems and play important roles in regulating Earth’s climate. Plankton populations and community structures respond to climate change, but much remains unknown about how climate change will influence plankton in the future. In this study scientists looked for rapid changes, or changepoints, in the virtual plankton communities of a global model simulating Earth’s climate over the pre-industrial era, the 20th century, and a projection of 21^st-century climate change. The authors find, for all types of plankton in the model, that the ocean area where changepoints occur expands from the pre-industrial era into the 20th century and again from the 20th to the 21st century. At the same time, hotspot regions, where rapid changes occur at least a few times per century, tend to disappear for all plankton types, and for temperature. Large plankton are more susceptible to changepoints than small plankton, and zooplankton are more susceptible than phytoplankton. The model ecosystem response to climate change is complex and spatially variable but suggests that rapid shifts in plankton communities may become increasingly widespread but less frequent as the climate warms.

 

Policy relevant message

The model ecosystem response to climate change suggests that rapid shifts in plankton communities may become increasingly widespread but less frequent as the climate warms.

Vogt, L., Burger, F. A., Griffies, S. M., & Frölicher, T. L. (2022). Local Drivers of Marine Heatwaves: A Global Analysis With an Earth System Model. Frontiers in Climate, 4. https://doi.org/10.3389/fclim.2022.847995

 

Summary

Marine heatwaves (MHWs) are periods of extreme warm ocean temperatures that can have devastating impacts on marine organisms and socio-economic systems. Despite recent advances in understanding the underlying processes of individual events, a global view of the local oceanic and atmospheric drivers of MHWs is currently missing. In this study, the authors quantified the main local processes leading to the onset and decline of surface MHWs in different seasons. The onset of MHWs in the subtropics and mid-to-high latitudes is primarily driven by net ocean heat uptake associated with a reduction of latent heat loss in all seasons, increased shortwave heat absorption in summer and reduced sensible heat loss in winter, dampened by reduced vertical mixing, especially in summer. In the tropics, ocean heat uptake is reduced and lowered vertical local mixing and diffusion cause the warming. In the subsequent decline phase, increased ocean heat loss to the atmosphere due to enhanced latent heat loss in all seasons together with enhanced vertical local mixing and diffusion in the high latitudes during summer dominate the temperature decrease globally. The processes leading to the onset and decline of MHWs are similar for short and long MHWs, but there are differences in the drivers between summer and winter. Different types of MHWs with distinct driver combinations are identified within the large variability among events. This analysis contributes to a better understanding of MHW drivers and processes and may therefore help to improve the prediction of high-impact marine heatwaves.

Friedlingstein, P., Jones, M. W., O’Sullivan, M., Andrew, R. M., Bakker, D. C. E., Hauck, J., Le Quéré, C., Peters, G. P., Peters, W., Pongratz, J., Sitch, S., Canadell, J. G., Ciais, P., Jackson, R. B., Alin, S. R., Anthoni, P., Bates, N. R., Becker, M., Bellouin, N., … Zeng, J. (2022). Global Carbon Budget 2021. Earth System Science Data, 14(4), 1917–2005. https://doi.org/10.5194/essd-14-1917-2022

 

Summary

The Global Carbon Budget 2021 describes the data sets and methodology used to quantify the emissions of carbon dioxide and their partitioning among the atmosphere, land, and ocean. These living data are updated every year to provide the highest transparency and traceability in the reporting of CO₂, the key driver of climate change.