Category: Publications-summaries

Wilson, C., Aksenov, Y., Rynders, S., Kelly, S. J., Krumpen, T., & Coward, A. C. (2021). Significant variability of structure and predictability of Arctic Ocean surface pathways affects basin-wide connectivity. Communications Earth & Environment, 2(1), 164. https://doi.org/10.1038/s43247-021-00237-0

Summary

The Arctic Ocean is of central importance for the global climate and ecosystem. It is a region undergoing rapid climate change, with a dramatic decrease in sea ice cover over recent decades. Surface pathways connect the transport of nutrients, freshwater, carbon and contaminants with their sources and sinks. Pathways of drifting material are deformed, due to atmosphere-ocean-ice coupling. Deformation is largest at fine space- and time-scales and is associated with a loss of potential predictability, analogous to weather often becoming unpredictable. However, neither satellite observations nor climate model projections resolve fine-scale processes responsible for this. The authors of this study used a high-resolution ocean model to determine these fine scale physical processes and transport pathways and their interannual variability.

Ziegler, M., Anton, A., Klein, S. G., Rädecker, N., Geraldi, N. R., Schmidt-Roach, S., Saderne, V., Mumby, P. J., Cziesielski, M. J., Martin, C., Frölicher, T. L., Pandolfi, J. M., Suggett, D. J., Aranda, M., Duarte, C. M., & Voolstra, C. R. (2021). Integrating environmental variability to broaden the research on coral responses to future ocean conditions. Global Change Biology, 27(21), 5532–5546. https://doi.org/10.1111/gcb.15840

Summary

Our understanding of the response of reef-building corals to changes in their physical environment is largely based on laboratory experiments, analysis of long-term field data, and model projections. Experimental data provide unique insights into how organisms respond to variation of environmental drivers. However, an assessment of how well experimental conditions cover the breadth of environmental conditions and variability where corals live successfully is missing. In this study the authors compiled and analysed a globally distributed dataset of local seasonal and daily cycle variability of key environmental drivers (temperature (ocean warming), pCO₂ (ocean acidification), and O₂ ocean deoxygenation)) critical for the growth and livelihood of reef-building corals. The authors compared the variability of environmental conditions set in coral experimental studies to current and projected conditions in their natural habitats. The scientists found that annual temperature profiles projected for the end of the 21st century were characterized by distributional shifts in temperatures with warmer winters and longer warm periods in the summer, not just peak temperatures. Furthermore, short-term hourly fluctuations of temperature and pCO₂ may regularly expose corals to conditions beyond the projected average increases for the end of the 21st century. Coral reef sites varied in the degree of coupling between temperature, pCO₂, and dissolved O₂, which warrants site-specific, differentiated experimental approaches depending on the local hydrography and influence of biological processes on the carbonate system and O₂ availability. This study highlights that a large portion of the natural environmental variability at short and long timescales is underexplored in experimental designs, which may provide a path to extend our understanding on the response of corals to global climate change.

Bell, J. D., Senina, I., Adams, T., Aumont, O., Calmettes, B., Clark, S., Dessert, M., Gehlen, M., Gorgues, T., Hampton, J., Hanich, Q., Harden-Davies, H., Hare, S. R., Holmes, G., Lehodey, P., Lengaigne, M., Mansfield, W., Menkes, C., Nicol, S., … Williams, P. (2021). Pathways to sustaining tuna-dependent Pacific Island economies during climate change. Nature Sustainability, 4(10), 900–910. https://doi.org/10.1038/s41893-021-00745-z

Summary

Climate-driven redistribution of tuna threatens to disrupt the economies of Pacific Small Island Developing States (SIDS) and sustainable management of the world’s largest tuna fishery. This study shows that by 2050, under a high greenhouse gas emissions scenario (RCP 8.5), the total biomass of three tuna species in the waters of ten Pacific SIDS could decline by an average of 13% due to a greater proportion of fish occurring in the high seas. The potential implications for Pacific Island economies in 2050 include an average decline in purse-seine catch of 20%, an average annual loss in regional tuna-fishing access fees of US$90 million and reductions in government revenue of up to 13% for individual Pacific SIDS. However, redistribution of tuna under a lower-emissions scenario (RCP 4.5) is projected to reduce the purse-seine catch from the waters of Pacific SIDS by an average of only 3%, indicating that even greater reductions in greenhouse gas emissions, in line with the Paris Agreement, would provide a pathway to sustainability for tuna-dependent Pacific Island economies. An additional pathway involves Pacific SIDS negotiating within the regional fisheries management organization to maintain the present-day benefits they receive from tuna, regardless of the effects of climate change on the distribution of the fish.

Policy relevant message:

By 2050, under a high greenhouse gas emissions scenario (RCP 8.5), the total biomass of three tuna species in the waters of ten Pacific Small Island Developing States (SIDS) could decline by an average of 13%. The potential implications for Pacific Island economies in 2050 include an average decline in purse-seine catch of 20%, an average annual loss in regional tuna-fishing access fees of US$90 million and reductions in government revenue of up to 13% for individual Pacific SIDS. Redistribution of tuna under a lower-emissions scenario (RCP 4.5) is projected to reduce the purse-seine catch from the same waters by only 3%, indicating that even greater reductions in greenhouse gas emissions, in line with the Paris Agreement, would provide a pathway to sustainability for tuna-dependent Pacific Island economies. An additional pathway involves Pacific SIDS negotiating within the regional fisheries management organization to maintain the present-day benefits they receive from tuna, regardless of the effects of climate change on the distribution of the fish.

New, A. L., Smeed, D. A., Czaja, A., Blaker, A. T., Mecking, J. V, Mathews, J. P., & Sanchez-Franks, A. (2021). Labrador Slope Water connects the subarctic with the Gulf Stream. Environmental Research Letters, 16(8), 84019. https://doi.org/10.1088/1748-9326/ac1293

Summary

Labrador Slope Water (LSLW) is a relatively fresh and cool water mass north of the Gulf Stream in the North Atlantic. Due to changes in wind stress in the subpolar region these waters are brought into close proximity with the Gulf Stream. Therefore, the Labrador Slope Water offers a new mechanism for decadal variability in the Atlantic climate system, through connecting the subarctic with the Gulf Stream and the Atlantic Meridional Overturning Circulation (AMOC).

Somes, C. J., Dale, A. W., Wallmann, K., Scholz, F., Yao, W., Oschlies, A., Muglia, J., Schmittner, A., & Achterberg, E. P. (2021). Constraining Global Marine Iron Sources and Ligand-Mediated Scavenging Fluxes With GEOTRACES Dissolved Iron Measurements in an Ocean Biogeochemical Model. Global Biogeochemical Cycles, 35(8), e2021GB006948. https://doi.org/10.1029/2021GB006948

Summary

Iron is a key, bio essential micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal marine cycling on a global scale, including removal (scavenging) rates and mechanisms. Iron concentrations in the ocean are affected not only by the source fluxes but also by the presence of ligands, compounds that maintain iron in a dissolved form (more bioavailable) and counteract removal mechanisms (transferring dissolved iron to particulate, less bioavailable form). In this study, the authors used a global dissolved iron (Fe) data set, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical numerical model. The variable ligand parameterization improved the global model-data misfit the most, suggesting that bacteria are an important source of ligands to the ocean. Further parameterization of atmospheric deposition and release of iron from sediments further improved the model most notably in the surface ocean. High scavenging rates were then required to maintain the iron inventory. The model simulates a tight spatial coupling between source inputs and scavenging rates, which may be too strong due to underrepresented ligands near source inputs, contributing to large uncertainties when constraining individual fluxes with dissolved iron concentrations. Model biases remain high and are discussed to help improve global marine iron cycle models.

Pérez, F. F., Olafsson, J., Ólafsdóttir, S. R., Fontela, M., & Takahashi, T. (2021). Contrasting drivers and trends of ocean acidification in the subarctic Atlantic. Scientific Reports, 11(1), 13991. https://doi.org/10.1038/s41598-021-93324-3

Summary

The processes of warming and acidification in the subarctic zone of the North Atlantic are unequivocal in the time-series measurements of the Iceland (IS-TS, 1985–2003) and Irminger Sea (IRM-TS, 1983–2013) stations. Both stations show high rates of acidification with different rates of warming, salinification (water becoming more saline) and stratification (separation of the water column into layers with different densities caused by differences in temperature or salinity or both) linked to regional water circulation. Furthermore, warming contributes to the increase in acidification at the IRM-TS.

Oschlies, A. (2021). A committed fourfold increase in ocean oxygen loss. Nature Communications, 12(1), 2307. https://doi.org/10.1038/s41467-021-22584-4

Summary:

Less than a quarter of ocean deoxygenation that will ultimately be caused by historical CO₂ emissions is already realized, according to millennial-scale model simulations that assume zero CO₂ emissions from year 2021 onwards. About 80% of the committed oxygen loss occurs below 2000 m depth, where a more sluggish overturning circulation will increase water residence times and accumulation of respiratory oxygen demand. According to the model results, the deep ocean will thereby lose more than 10% of its pre-industrial oxygen content even if CO₂ emissions and thus global warming were stopped today. In the surface layer, however, the ongoing deoxygenation will largely stop once CO₂ emissions are stopped. Accounting for the joint effects of committed oxygen loss and ocean warming, metabolic viability representative for marine animals declines by up to 25% over large regions of the deep ocean, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.

Policy relevant message:

In the surface layer, the ongoing deoxygenation will largely stop once CO₂ emissions are stopped. The deep ocean, however, will lose more than 10% of its pre-industrial oxygen content even if CO₂ emissions and thus global warming were stopped today, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.

Carracedo, L. I., Mercier, H., McDonagh, E., Rosón, G., Sanders, R., Moore, C. M., Torres-Valdés, S., Brown, P., Lherminier, P., & Pérez, F. F. (2021). Counteracting Contributions of the Upper and Lower Meridional Overturning Limbs to the North Atlantic Nutrient Budgets: Enhanced Imbalance in 2010. Global Biogeochemical Cycles, 35(6), e2020GB006898. https://doi.org/10.1029/2020GB006898

Summary

The North Atlantic Ocean is a major reservoir which absorbs atmospheric carbon dioxide (CO₂) due in part to the extensive plankton (microscopic marine algae) blooms which form there supported by nutrients supplied by ocean circulation. Hence, changes in ocean circulation and/or stratification (separation of the water column into layers with different densities caused by differences in temperature or salinity or both) may influence biological production and carbon export into the deep ocean. In this study, the inorganic nutrient budgets for 2004 and 2010 are evaluated in the North Atlantic based on observations from the transatlantic the Greenland-Portugal section. The water column nutrient budgets were split into upper and lower limbs. The authors found that in 2010 an anomalous circulation led to an enhanced northward transport of more nutrient-rich waters by the upper limb. This anomalous circulation event favoured an enhancement of the nutrient consumption and associated biological CO₂ uptake, which represents a 50% of the mean annual sea–air CO₂ flux in the region. These results indicate that the upper limb modulates the biological carbon uptake, and the lower limb modulates nutrient inventories in the North Atlantic.

Stranne, C., Nilsson, J., Ulfsbo, A., O’Regan, M., Coxall, HK, Meire, L., Muchowski, J., Mayer, LA, Brüchert, V., Fredriksson, J., Thornton, B., Chawarski, J., West, G., Weidner, E., & Jakobsson, M. (2021). The climate sensitivity of northern Greenland fjords is amplified through sea-ice damming. Communications Earth & Environment, 2 (1), 70. https://doi.org/10.1038/s43247-021-00140-8

Summary:

Record-high air temperatures were observed over Greenland in the summer of 2019 and melting of the northern Greenland Ice Sheet was particularly extensive. This study shows through direct measurements, that near surface ocean temperatures in Sherard Osborn Fjord, northern Greenland, reached 4 °C in August 2019, while in the neighboring Petermann Fjord, they never exceeded 0 °C. This disparity in temperature between the two fjords occurred because thick multi-year sea ice at the entrance of Sherard Osborn Fjord trapped the surface waters inside the fjord, which led to the formation of a warm and fresh surface layer. These results suggest that the presence of multi-year sea ice increases the sensitivity of Greenland fjords abutting the Arctic Ocean to climate warming, with potential consequences for the long-term stability of the northern sector of the Greenland Ice Sheet.

Policy relevant message:

The presence of multi-year sea ice increases the sensitivity of Greenland fjords abutting the Arctic Ocean to climate warming, with potential consequences for the long-term stability of the northern sector of the Greenland Ice Sheet.

Lauderdale, J. M., & Cael, B. B. (2021). Impact of Remineralization Profile Shape on the Air-Sea Carbon Balance. Geophysical Research Letters, 48 (7), e2020GL091746. https://doi.org/10.1029/2020GL091746

Summary:

The ocean’s “biological pump” regulates atmospheric carbon dioxide levels and climate by transferring organic carbon produced at the surface by phytoplankton to the ocean interior via “marine snow,” where the organic carbon is consumed and respired by microbes. This surface to deep transport is usually described by a power-law relationship of sinking particle concentration with depth. Uncertainty in biological pump strength can be related to different variable values (“parametric” uncertainty) or the underlying equations (“structural” uncertainty) that describe organic matter export. This study evaluates structural uncertainty using an ocean biogeochemistry model by systematically substituting six alternative remineralization profiles fit to a reference power-law curve. Structural uncertainty makes a substantial contribution, about one-third in atmospheric pCO₂ terms, to total uncertainty of the biological pump, highlighting the importance of improving biological pump characterization from observations and its mechanistic inclusion in climate models.