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1. Rapid 20th century warming reverses 900-year cooling in the Gulf of Maine

   https://doi.org/10.1038/s43247-022-00504-8  

   Whitney, Nina M.; Wanamaker, Alan D.; Ummenhofer, Caroline C.; Johnson, Beverly J.; Cresswell-Clay, Nathaniel; Kreutz, Karl J. (August 2022, Communications Earth & Environment)

   Abstract The Gulf of Maine, located in the western North Atlantic, has undergone recent, rapid ocean warming but the lack of long-term, instrumental records hampers the ability to put these significant hydrographic changes into context. Here we present multiple 300-year long geochemical records (oxygen, nitrogen, and previously published radiocarbon isotopes) measured in absolutely-datedArctica islandicashells from the western Gulf of Maine. These records, in combination with climate model simulations, suggest that the Gulf of Maine underwent a long-term cooling over most of the last 1000 years, driven primarily by volcanic forcing and North Atlantic ocean dynamics. This cooling trend was reversed by warming beginning in the late 1800s, likely due to increased atmospheric greenhouse gas concentrations and changes in western North Atlantic circulation. The climate model simulations suggest that the warming over the last century was more rapid than almost any other 100-year period in the last 1000 years in the region. 

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2. Combining shell-based isotope records with numerical model simulations to investigate the time of emergence of widespread warming of the western North Atlantic shelf

   Whitney, N.M.; Wanamaker, A.D.; Ummenhofer, C.C.; Johnson, B.; Cresswell-Clay, N.; and Kreutz, K.J. (October 2021, American Geophysical Union Fall Meeting, December 13-17, 2021, New Orleans, LA)

   Abstract The Gulf of Maine and surrounding western North Atlantic shelf are some of the fastest warming regions of the worlds oceans. The lack of long-term observational records from this area inhibits the ability to assess the timing and initial causes of this warming and consequently accurately predict future changes to this ecologically and economically important region. Here we present oxygen, nitrogen, and radiocarbon isotope data measured in Arctica islandica shells collected in the western North Atlantic to better understand the past temperature and ocean circulation variability of the region over the last 300 years. We combine these results with output from the Community Earth System Model Last Millennium Ensemble simulations to assess the temporal and spatial context of these isotope records. We find that the isotope records capture the end and reversal of a millennium-scale cooling trend in the Gulf of Maine. Last Millennium Ensemble single-forcing simulations indicate that this cooling trend appears to be largely driven by volcanic forcing. The nitrogen and radiocarbon records indicate that ocean circulation is in part driving the reconstructed hydrographic changes, pointing to a potential role of the Atlantic Meridional Overturning Circulation in regulating Gulf of Maine temperatures as suggested by the Last Millennium Ensemble simulations. Both isotope and model results suggest that the Gulf of Maine began to warm in the late 19th century, ultimately driven by increased greenhouse gas forcing. Plain-language Summary The Gulf of Maine, located off of the Eastern Coast of the United States, has experienced significant temperature increases recently. Because the instrumental record only began in 1905, we do not have a good idea of when this warming began and what may have initially caused the warming. Here, we analyze the chemistry of clam shells, which have grown in the Gulf of Maine for hundreds of years, to infer past changes in ocean temperatures and water properties. We combine these results with output from a climate model to reveal that the temperatures reconstructed from the clams shells agree well with the model during the period of overlap. Both the chemical records and the model suggest the Gulf of Maine started warming in the late 1800s as a result of increased atmospheric greenhouse gas concentrations. Before this warming began, the Gulf of Maine region appears to have been cooling. The model suggests that this cooling trend is likely due to the influence of volcanic eruptions. The chemical records from the clam shells also suggest that part of this cooling is likely related to changing ocean circulation patterns. 

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3. Evidence of coherent warming in the northwest Atlantic Ocean from Arctica islandica oxygen isotope records

   Thatcher, D.; Whitney, N.; Walton, A.; Sun, H.; Jarosinski, L.; Wanamaker, A.; LaVigne, M.; Williams, B.; Stewart, J. (May 2023, International Sclerochronology Conference)

   Parts of the northwest Atlantic Ocean, including the Gulf of Maine, along the northeastern coast of the United States, are warming at a rate as much as four times faster than the global ocean, according to instrumental and satellite records. In addition to the longer-term temperature increases, the frequency and severity of marine heat waves have been increasing. Combined, the ecological impacts are numerous and concerning, yet our understanding of past climate in this region is incomplete due to sparse and short-duration instrumental records. Here we present annually resolved oxygen isotope records from the marine bivalve, Arctica islandica, from five locations ranging from Jonesport and Seguin Island in the Gulf of Maine to Long Island, New York, Cape May, New Jersey, and Ocean City, Maryland in the Mid-Atlantic Bight, a span of over 870 km along the Atlantic coast. Several of the isotope records span the last 100 years or more and all records show coherent, substantial warming since at least 1980 CE. The level of warming indicated in the shell oxygen isotopes is comparable to the 0.5 °C per decade (1980-2020 CE) warming also shown in the instrumental record of sea surface temperature from Boothbay Harbor along the central coast in the Gulf of Maine. These five spatially distant isotope records span different oceanographic conditions and dynamics, including water mass sources, yet they all indicate a substantial warming in recent decades, likely related to increased anthropogenic warming. Beyond reconstructing seawater temperature prior to instrumental records, a major goal of this work is to disentangle the global warming signal from these records to better understand the underlying ocean dynamics also influencing these records. 

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4. Oceanographic Variability in the Rapidly Warming Coastal Mid-Atlantic (USA) over the last 200 years: Insights from Shell-Based Geochemistry and Growth Histories

   Sun, H.; Wanamaker, A.; Thatcher, D.; Whitney, N.; Jarosinski, L.; Stewart, J.; Williams, B.; LaVigne, M. (December 2022, American Geophysical Union Fall Meeting)

   Warming in recent decades in the North Atlantic Ocean has been heterogeneous, with locations along the northwestern Atlantic experiencing some of the largest and fastest warming in the last 100 years. This region is important for fisheries but has limited spatial and temporal hydrographic instrumental series extending beyond the past decades, especially along the coastal United States portion of the northwestern Atlantic, thus impacting our understanding of past climatic variability. To provide a longer temporal context for these changes, we constructed a continuous master shell growth chronology spanning the last two centuries and provided geochemical records from the Mid-Atlantic region using the long-lived marine bivalve Arctica islandica. Shells were collected on the outer shelf region off Ocean City, Maryland, in ~ 60 m water depth. This region is sensitive to large-scale North Atlantic Ocean dynamics, including the Atlantic Meridional Overturning Circulation (AMOC) and Gulf Stream eddies. Based on growth histories and shell oxygen isotopes, we provide evidence of hydrographic variability beyond the relatively short instrumental period and evaluate the likely causes for these changes. These data allow us to better characterize recent and past oceanographic changes in the Mid-Atlantic region, synthesize the new results with previously developed paleo-records in the northwestern Atlantic, and provide guidance for the management of fisheries in this region. 

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5. Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

   https://doi.org/10.1073/pnas.2311980121  

   Kirtland_Turner, Sandra; Ridgwell, Andy; Keller, Allison L; Vahlenkamp, Maximilian; Aleksinski, Adam K; Sexton, Philip F; Penman, Donald E; Hull, Pincelli M; Norris, Richard D (June 2024, Proceedings of the National Academy of Sciences)

   Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future. 

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