Ekaterina E. Popova

Early warning systems (EWS) for climate tipping point must support decisions to be useful. Using the potential near-term Subpolar Gyre tipping point as a test case, we develop a pre-elicitation framework linking Earth System science with decision-making. We show that no single EWS can meet diverse decision needs. Instead, the framework identifies shared decision architectures and information requirements, enabling scalable stakeholder co-design without bespoke systems for every user.

The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB's nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the subpolar gyre where it has been projected to abruptly decline within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the gyre may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various Global Warming Levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre‐industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB started to recover toward preindustrial conditions. However, zero emissions after 2.5°C and warmer showed a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5 and 2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB did not occur within 5 centuries under zero emissions, nor did it occur within one century under negative emissions.

The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB’s nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the sub-polar gyre (SPG) where it has been predicted to collapse within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the SPG may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various global warming levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre-industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB starts to recover and is extrapolated to return to preindustrial levels only after several centuries to millennia. However, zero emissions after 2.5°C and warmer shows a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5-2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB under negative emissions does not occur within one century.

Regional effects of marine cold spells (MCS, periods of anomalous cooling), their impact on ecosystem biogeochemistry, and link to salinity extremes remain underexplored. A case in point is North Atlantic's Cold Anomaly (CA) region (known as the “cold blob”), which hits record low temperatures during 2014–16 while most of the global ocean warmed. Using up to 42 years of observations, we characterize the CA as a manifestation of both MCS and Fresh Waves (FW, low salinity extremes) and analyze the surface biogeochemical response. We observe a quasiperiodic pattern of MCS from the 1980s and FW (at least) from the 1990s to early 2020s in the CA region with alternations from cool and freshwater to warm and saline conditions. Since 1990s, the CA region appears to be potentially undergoing MCS and FW compound events that are more frequent and prolonged but less intense than other North Atlantic areas. The 2014‐16 CA was among the most widespread and prolonged MCS and FW events associated with a deeper mixed layer and distinct biogeochemical signature, including elevated nutrients and oxygen, an overall increased chlorophyll‐a and intensified ocean acidification. These results suggest that MCS could mitigate certain climate change effects through cooling and enhanced productivity, while exacerbating others such as ocean acidification. We compare 2014–16 CA region effects with those of Pacific's warm blob, identifying contrasting behaviors from physical processes to biogeochemical impacts and discussing a common atmospheric driver. Our findings emphasize the need to further study ecological responses to MCS in the North Atlantic.

The North Atlantic subpolar gyre (SPG) is a key region for the North Atlantic bloom (NAB), the phytoplankton foundation of the regional food web. The NAB depends on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, representing a potential “tipping point” in the Earth system, and may “collapse” as a result. We investigate changes in winter mixing and the impacts on the SPG and the northern North Atlantic using a spread of future projections from a low‐resolution Earth system model (UKESM) and a high‐warming projection of a high‐resolution ocean‐only configuration of the same model (NEMO‐MEDUSA). Both models project significant declines in the strength of the NAB during the 21st century. In UKESM, this occurs across all projections, but with low spatiotemporal coherence. In NEMO‐MEDUSA, changes in mixed layer depth, nutrients, and chlorophyll concentrations are abrupt and more highly spatiotemporally correlated. We find a >30‐day phenological shift in the peak of the bloom aligned with the timing of this change, which may affect food web dynamics. Defining “collapse” as halving of surface chlorophyll, we find that the NAB collapses this century regardless of future projection. However, the spatial‐temporal coherence of the timing and abruptness of this collapse is greater in our high‐resolution model. Because key physical processes driving biogeochemical responses are poorly represented in low‐resolution models, especially at high latitudes, this suggests that higher resolution may be essential for predicting abrupt and irreversible changes, particularly those involving ecosystem dynamics.

Worldwide, coral reefs are facing risk from climate change. The Western Indian Ocean (WIO) harbours about 16% of global coral reefs with highly reef-dependent local communities. Coastal protection and food security depend on effective conservation management, which requires understanding species abundances. Here, we explore how fish group distribution and abundance across the WIO, categorized by their trophic function, are explained by oceanographic connectivity, sea surface temperature (SST), and chlorophyll a. We designed a proportional oceanographic connectivity metric describing the relative strength of connectivity between all WIO coral reefs and each survey site. We created statistical models for four trophic groups: grazers and detritivores, herbivorous excavators, corallivores, and primary piscivores across 51 sites in the WIO. We show that SST and chlorophyll a are strong predictors of all trophic fish groups and that the proportional oceanographic connectivity metric improved the model predictions significantly for grazers and detritivores and excavators. For excavators, peak abundances were predicted at medium connectivity, and for grazers and detritivores, at low and medium connectivity, suggesting that larvae dispersal predominates at a local scale. Decision making should include connectivity for efficient conservation area prioritization, for which our proportional oceanographic connectivity metric is a valid and useful parameter.

The North Atlantic subpolar gyre is a key region for the North Atlantic phytoplankton bloom (NAB), the foundation of the regional foodweb. The NAB is dependent on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, and the NAB may “collapse” as a result, representing a potential “tipping point” in the Earth system. We investigate change in winter mixing and the impacts on the SPG and the broader northern North Atlantic using 1. a spread of future projections from a low-resolution Earth system model (UKESM) and 2. a single, high-warming projection of a high-resolution ocean-only configuration of the same model (NEMO-MEDUSA). For both models we find significant declines in the strength of the NAB during the 21st century. In UKESM, this occurred across all projections, but with low spatiotemporal coherence. In NEMO-MEDUSA, changes in upper mixed layer depth, surface nutrients and chlorophyll concentrations were noticeably abrupt and more highly spatiotemporally-correlated. We also find a large (>30 day) phenological shift in the peak of the bloom aligned with the timing of this change, which may affect foodweb dynamics. Overall, defining “collapse” as a halving of surface chlorophyll, we find that the NAB collapses by the end of the century regardless of future projection. However, the timing, abruptness and coherence of this collapse differs in high and low resolution models, suggesting the need for higher resolution for prediction of abrupt and irreversible changes, especially those involving ecosystem dynamics.

The North Atlantic subpolar gyre is a key region for the North Atlantic phytoplankton bloom (NAB), the foundation of the regional foodweb. The NAB is dependent on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, and the NAB may “collapse” as a result, representing a potential “tipping point” in the Earth system. We investigate change in winter mixing and the impacts on the SPG and the broader northern North Atlantic using 1. a spread of future projections from a low-resolution Earth system model (UKESM) and 2. a single, high-warming projection of a high-resolution ocean-only configuration of the same model (NEMO-MEDUSA). For both models we find significant declines in the strength of the NAB during the 21st century. In UKESM, this occurred across all projections, but with low spatiotemporal coherence. In NEMO-MEDUSA, changes in upper mixed layer depth, surface nutrients and chlorophyll concentrations were noticeably abrupt and more highly spatiotemporally-correlated. We also find a large (>30 day) phenological shift in the peak of the bloom aligned with the timing of this change, which may affect foodweb dynamics. Overall, defining “collapse” as a halving of surface chlorophyll, we find that the NAB collapses by the end of the century regardless of future projection. However, the timing, abruptness and coherence of this collapse differs in high and low resolution models, suggesting the need for higher resolution for prediction of abrupt and irreversible changes, especially those involving ecosystem dynamics.

Somali upwelling is the fifth largest upwelling globally with high productivity, attracting tuna migratory species. A key control on the upwelling productivity is its interaction with one of the world’s largest oceanic eddies, the Great Whirl inducing a strong downwelling signal. Here, we use satellite-derived observations to determine the Great Whirl impact on the extent of the upwelling-driven phytoplankton bloom. We find that following decreases in upwelling intensity, productivity has declined by about 10% over the past two decades. The bloom extent has also been diminishing with an abrupt decrease around 2006–2007, coinciding with an abrupt increase in the downwelling effect. Absent or weak Great Whirl leads to the occurrence of smaller anticyclonic eddies with a resulting downwelling stronger than when the Great Whirl is present. We suggest that 2006–2007 abrupt changes in the bloom and downwelling extents’ regimes, are likely driven by Indian Ocean Dipole abrupt shift in 2006.