Gruber, N., Boyd, P. W., Frölicher, T. L., & Vogt, M. (2021). Biogeochemical extremes and compound events in the ocean. Nature, 600(7889), 395–407. https://doi.org/10.1038/s41586-021-03981-7
The ocean is warming, losing oxygen and being acidified, primarily as a result of anthropogenic carbon emissions. With ocean warming, acidification and deoxygenation projected to increase for decades, extreme events, such as marine heatwaves, will intensify, occur more often, persist for longer periods of time and extend over larger regions. Nevertheless, our understanding of oceanic extreme events that are associated with warming, low oxygen concentrations or high acidity, as well as their impacts on marine ecosystems, remains limited. Compound events—that is, multiple extreme events that occur simultaneously or in close sequence—are of particular concern, as their individual effects may interact synergistically. In this paper authors assess patterns and trends in open ocean extremes based on the existing literature as well as global and regional model simulations. They discuss the potential impacts of individual and compound extremes on marine organisms and ecosystems and propose a pathway to improve the understanding of extreme events and the capacity of marine life to respond to them. The conditions exhibited by present extreme events may be a harbinger of what may become normal in the future. As a consequence, pursuing this research effort may also help to better understand the responses of marine organisms and ecosystems to future climate change.
Policy relevant message
Since the pre-industrial times, marine heatwaves have become 10 x more common and low oxygen extremes have become about 5 x more frequent. Ocean acidity extremes have become essentially near permanent. All three types of extreme events will increase in frequency, magnitude and intensity with continuously rising atmospheric carbon emissions and global temperature.
Tagliabue, A., Kwiatkowski, L., Bopp, L., Butenschön, M., Cheung, W., Lengaigne, M., & Vialard, J. (2021). Persistent Uncertainties in Ocean Net Primary Production Climate Change Projections at Regional Scales Raise Challenges for Assessing Impacts on Ecosystem Services. Frontiers in Climate, 3. https://doi.org/10.3389/fclim.2021.738224
Ocean net primary production (NPP) results from CO2 fixation by marine phytoplankton, catalysing the transfer of organic matter and energy to marine ecosystems, supporting most marine food webs, and fisheries production as well as stimulating ocean carbon sequestration. Thus, alterations to ocean NPP in response to climate change, as quantified by Earth system model experiments conducted as part of the 5th and 6th Coupled Model Intercomparison Project (CMIP5 and CMIP6) efforts, are expected to alter key ecosystem services. Despite reductions in inter-model variability since CMIP5, the ocean components of CMIP6 models disagree roughly 2-fold in the magnitude and spatial distribution of NPP in the contemporary era, due to incomplete understanding and insufficient observational constraints. Projections of NPP change in absolute terms show large uncertainty in CMIP6, most notably in the North Atlantic and the Indo-Pacific regions, with the latter explaining over two-thirds of the total inter-model uncertainty. While the Indo-Pacific has previously been identified as a hotspot for climate impacts on biodiversity and fisheries, the increased inter-model variability of NPP projections further exacerbates the uncertainties of climate risks on ocean-dependent human communities. Drivers of uncertainty in NPP changes at regional scales integrate different physical and biogeochemical factors that require more targeted mechanistic assessment in future studies. Globally, inter-model uncertainty in the projected changes in NPP has increased since CMIP5, which amplifies the challenges associated with the management of associated ecosystem services. Notably, this increased regional uncertainty in the projected NPP change in CMIP6 has occurred despite reduced uncertainty in the regional rates of NPP for historical period. Improved constraints on the magnitude of ocean NPP and the mechanistic drivers of its spatial variability would improve confidence in future changes. It is unlikely that the CMIP6 model ensemble samples the complete uncertainty in NPP, with the inclusion of additional mechanistic realism likely to widen projections further in the future, especially at regional scales. This has important consequences for assessing ecosystem impacts. Ultimately, we need an integrated mechanistic framework that considers how NPP and marine ecosystems respond to impacts of not only climate change, but also the additional non-climate drivers.
Spring, A., Dunkl, I., Li, H., Brovkin, V., & Ilyina, T. (2021). Trivial improvements in predictive skill due to direct reconstruction of the global carbon cycle. Earth Syst. Dynam., 12(4), 1139–1167. https://doi.org/10.5194/esd-12-1139-2021
Numerical carbon cycle prediction models usually do not start from observed carbon states due to sparse observations. Instead, only physical climate is reconstructed, assuming that the carbon cycle follows indirectly. In this study the authors tested such an assumption and found that indirect reconstruction works quite well and that improvements from the direct method are limited, strengthening the current indirect use.
Bardon, L. R., Ward, B. A., Dutkiewicz, S., & Cael, B. B. (2021). Testing the Skill of a Species Distribution Model Using a 21st Century Virtual Ecosystem. Geophysical Research Letters, 48(22), e2021GL093455. https://doi.org/10.1029/2021GL093455
Marine plankton communities play a central role within Earth’s climate system, with important processes often divided among different “functional groups.” Changes in the relative abundance of these groups can therefore impact on ecosystem function. However, the oceans are vast, and samples are sparse, so global distributions are not well known. Statistical species distribution models (SDM’s) have been developed that predict global distributions based on their relationships with observed environmental variables. They appear to perform well at summarizing present day distributions, and are increasingly being used to predict ecosystem changes throughout the 21st century. But it is not guaranteed that such models remain valid over time. Rather than wait 100 years to find out, the authors of this study applied a statistical SDM to a complex virtual ocean, and trained it using virtual observations that match real-world ocean samples. This allows them to jump forward to the end-of-century to test the accuracy of our predictions. The SDM performed well at qualitatively predicting “present day” plankton distributions but yielded poor end-of-century predictions. This case study emphasizes both the importance of environmental variable selection, and of changes in the underlying relationships between environmental variables and plankton distributions, in terms of model validity over time.
Cael, B. B., Dutkiewicz, S., & Henson, S. (2021). Abrupt shifts in 21st-century plankton communities. Science Advances, 7(44), eabf8593. https://doi.org/10.1126/sciadv.abf8593
Marine microbial communities are critical in sustaining ocean food webs. However, these communities will change with climate through gradual or foreseeable changes but likely have much more substantial consequences when sudden and unpredictable. Through a complex mathematical model of marine microbial ecosystem, the authors of this study found that climate change–driven shifts over the 21st century are often abrupt, large in amplitude and extent, and unpredictable using standard early warning signals. Phytoplankton (microscopic marine algae) with unique resource needs are prone to abrupt shifts. These abrupt shifts in biomass, biological productivity, and phytoplankton community structure are concentrated in Atlantic and Pacific subtropics. Abrupt changes in environmental variables such as temperature and nutrients rarely precede these ecosystem shifts, indicating that rapid community restructuring can occur in response to gradual environmental changes, particularly in nutrient supply rate ratios.
Drinkwater, K. F., Harada, N., Nishino, S., Chierici, M., Danielson, S. L., Ingvaldsen, R. B., Kristiansen, T., Hunt Jr, G. L., Mueter, F., & Stiansen, J. E. (2021). Possible future scenarios for two major Arctic Gateways connecting Subarctic and Arctic marine systems: I. Climate and physical–chemical oceanography. ICES Journal of Marine Science, 78(9), 3046–3065. https://doi.org/10.1093/icesjms/fsab182
Summary and policy relevant message:
This study reviews recent trends and projected future physical and chemical changes under climate change in transition zones between Arctic and Subarctic regions with a focus on the two major inflow gateways to the Arctic, one in the Pacific (i.e. Bering Sea, Bering Strait, and the Chukchi Sea) and the other in the Atlantic (i.e. Fram Strait and the Barents Sea). Sea-ice coverage in the gateways has been disappearing during the last few decades. Projected higher air and sea temperatures in these gateways in the future will further reduce sea ice, and cause its later formation and earlier retreat. An intensification of the hydrological cycle will result in less snow, more rain, and increased river runoff. Ocean temperatures are projected to increase, leading to higher heat fluxes through the gateways. Increased upwelling at the Arctic continental shelf is expected as sea ice retreats. The pH of the water will decline as more atmospheric CO2 is absorbed. Long-term surface nutrient levels in the gateways will likely decrease due to increased stratification and reduced vertical mixing. Some effects of these environmental changes on humans in Arctic coastal communities are also described in this paper.
Cheung, W. W. L., Frölicher, T. L., Lam, V. W. Y., Oyinlola, M. A., Reygondeau, G., Sumaila, U. R., Tai, T. C., Teh, L. C. L., & Wabnitz, C. C. C. (2021). Marine high temperature extremes amplify the impacts of climate change on fish and fisheries. Science Advances, 7(40), eabh0895. https://doi.org/10.1126/sciadv.abh0895
Extreme temperature events have occurred in all ocean basins in the past two decades with detrimental impacts on marine biodiversity, ecosystem functions, and services. However, global impacts of temperature extremes on fish stocks, fisheries, and dependent people have not been quantified. This study, using a mathematical model, projected that, on average, when an annual high temperature extreme occurs in an exclusive economic zone, 77% of exploited fishes and invertebrates therein will decrease in biomass while maximum catch potential will drop by 6%, adding to the decadal-scale mean impacts under climate change. The net negative impacts of high temperature extremes on fish stocks are projected to cause losses in fisheries revenues and livelihoods in most maritime countries, creating shocks to fisheries social-ecological systems particularly in climate-vulnerable areas. This study highlights the need for rapid adaptation responses to extreme temperatures in addition to carbon mitigation to support sustainable ocean development.
Policy relevant message:
When an annual high temperature extreme occurs in an exclusive economic zone, 77% of exploited fishes and invertebrates therein will decrease in biomass while maximum catch potential will drop by 6%, adding to the decadal-scale mean impacts under climate change. This study highlights the need for rapid adaptation responses to extreme temperatures in addition to carbon mitigation to support sustainable ocean development.
Tivig, M., Keller, D. P., & Oschlies, A. (2021). Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth system model. Biogeosciences, 18(19), 5327–5350. https://doi.org/10.5194/bg-18-5327-2021
Nitrogen is one of the most important elements for life in the ocean. A major source is the riverine discharge of dissolved nitrogen. While global models often omit rivers as a nutrient source, the authors of this study included nitrogen from rivers in the Earth system computational model. They found that additional nitrogen affected marine biology not only locally but also in regions far off the coast. Depending on regional conditions, primary production was enhanced or even decreased due to internal feedbacks in the nitrogen cycle. This study highlights the importance of incorporation of riverine nitrogen input in the earth system models.
Blenckner, T., Ammar, Y., Müller-Karulis, B., Niiranen, S., Arneborg, L., & Li, Q. (2021). The Risk for Novel and Disappearing Environmental Conditions in the Baltic Sea. Frontiers in Marine Science, 8, 1398. https://doi.org/10.3389/fmars.2021.745722
Future climate biogeochemical projections indicate large changes in the ocean with environmental conditions not experienced at present referred to as novel or disappearing conditions. These climate-induced changes will most likely affect species distribution through changes in growth, behaviour, evolution, dispersal, and species interactions. However, the future risk of novel and disappearing environmental conditions in the ocean is poorly understood, in particular for the compound effects of climate and nutrient management changes. The authors of this study mapped the risk of the occurrence of future novel and disappearing environmental conditions and analysed the outcome of climate and nutrient management scenarios for the Baltic Sea, and the potential consequences for three species. Overall, the future projections show, as expected, an increase in environmental novelty over time. The future nutrient reduction management that improves the eutrophication status of the Baltic Sea contributes to large novel and disappearing conditions. The authors show the consequences of novel and disappearing environmental conditions for three species under different scenarios. Through their comprehensive analysis of environmental novelty and disappearing conditions for a marine system, they found the urgent need to include novelty and disappearing projection outputs in Earth System Models. The results of this study further illustrate that adaptive management is needed to account for the emergence of novelty related to the interplay of multiple drivers. Overall, the analysis provides strong support for the expectation of novel ecological communities in marine systems, which may affect ecosystem services, and needs to be accounted for in sustainable future management plans of our oceans.
Policy relevant message:
Adaptive management is needed to account for the emergence of novelty related to the interplay of multiple drivers. The novel ecological communities in marine systems are to be expected to emerge, which in turn may affect ecosystem services, and needs to be accounted for in sustainable future management plans of our oceans.
Henson, S. A., Cael, B. B., Allen, S. R., & Dutkiewicz, S. (2021). Future phytoplankton diversity in a changing climate. Nature Communications, 12(1), 5372. https://doi.org/10.1038/s41467-021-25699-w
The future response of marine ecosystem diversity to continued anthropogenic forcing is not well understood. Phytoplankton are a diverse set of organisms (microscopic marine algae) that form the base of the marine ecosystem. This study finds that the community structure becomes increasingly unstable in response to climate change over the 21st century. This implies a loss of ecological resilience with likely knock-on effects on the productivity and functioning of the marine environment.