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1. Last Glacial Maximum (LGM) climate forcing and ocean dynamical feedback and their implications for estimating climate sensitivity

   https://doi.org/10.5194/cp-17-253-2021  

   Zhu, Jiang ; Poulsen, Christopher J. ( January 2021 , Climate of the Past)

   null (Ed.)

   Abstract. Equilibrium climate sensitivity (ECS) has been directly estimated using reconstructions of past climates that are different than today's. A challenge to this approach is that temperature proxies integrate over the timescales of the fast feedback processes (e.g., changes in water vapor, snow, and clouds) that are captured in ECS as well as the slower feedback processes (e.g., changes in ice sheets and ocean circulation) that are not. A way around this issue is to treat the slow feedbacks as climate forcings and independently account for their impact on global temperature. Here we conduct a suite of Last Glacial Maximum (LGM) simulations using the Community Earth System Model version 1.2 (CESM1.2) to quantify the forcingand efficacy of land ice sheets (LISs) and greenhouse gases (GHGs) in order to estimate ECS. Our forcing and efficacy quantification adopts the effective radiative forcing (ERF) and adjustment framework and provides a complete accounting for the radiative, topographic, and dynamical impacts of LIS on surface temperatures. ERF and efficacy of LGM LIS are −3.2 W m−2 and 1.1, respectively. The larger-than-unity efficacy is caused by the temperature changes over land and the Northern Hemisphere subtropical oceans which are relatively larger than those in response to a doubling of atmospheric CO2. The subtropical sea-surface temperature (SST) response is linked to LIS-induced wind changes and feedbacks in ocean–atmosphere coupling and clouds. ERF and efficacy of LGM GHG are −2.8 W m−2 and 0.9, respectively. The lower efficacy is primarily attributed to a smaller cloud feedback at colder temperatures. Our simulations further demonstrate that the direct ECS calculation using the forcing, efficacy, and temperature response in CESM1.2 overestimates the true value in the model by approximately 25 % due to the neglect of slow ocean dynamical feedback. This is supported by the greater cooling (6.8 ∘C) in a fully coupled LGM simulation than that (5.3 ∘C) in a slab ocean model simulation with ocean dynamics disabled. The majority (67 %) of the ocean dynamical feedback is attributed to dynamical changes in the Southern Ocean, where interactions between upper-ocean stratification, heat transport, and sea-ice cover are found to amplify the LGM cooling. Our study demonstrates the value of climate models in the quantification of climate forcings and the ocean dynamical feedback, which is necessary for an accurate direct ECS estimation. 

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2. Simulating Marine Isotope Stage 7 with a coupled climate–ice sheet model

   https://doi.org/10.5194/cp-16-2183-2020  

   Choudhury, Dipayan ; Timmermann, Axel ; Schloesser, Fabian ; Heinemann, Malte ; Pollard, David ( January 2020 , Climate of the Past)

   null (Ed.)

   Abstract. It is widely accepted that orbital variations areresponsible for the generation of glacial cycles during the latePleistocene. However, the relative contributions of the orbital forcingcompared to CO2 variations and other feedback mechanisms causing thewaxing and waning of ice sheets have not been fully understood. Testingtheories of ice ages beyond statistical inferences, requires numericalmodeling experiments that capture key features of glacial transitions. Here,we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6covering the period from 240 to 170 ka (ka: thousand years before present). Thistransition from interglacial to glacial conditions includes one of thefastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupledatmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate thetransient evolution of Northern Hemisphere and Southern Hemisphere ice sheets duringthe MIS 7–6 period in response to orbital and greenhouse gas forcing. For arange of model parameters, the simulations capture the evolution of globalice volume well within the range of reconstructions. Over the MIS 7–6period, it is demonstrated that glacial inceptions are more sensitive toorbital variations, whereas terminations from deep glacial conditions needboth orbital and greenhouse gas forcings to work in unison. For someparameter values, the coupled model also exhibits a critical North Americanice sheet configuration, beyond which a stationary-wave–ice-sheettopography feedback can trigger an unabated and unrealistic ice sheetgrowth. The strong parameter sensitivity found in this study originates fromthe fact that delicate mass imbalances, as well as errors, are integratedduring a transient simulation for thousands of years. This poses a generalchallenge for transient coupled climate–ice sheet modeling, with suchcoupled paleo-simulations providing opportunities to constrain suchparameters. 

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3. Spatially similar surface energy flux perturbations due to greenhouse gases and aerosols

   https://doi.org/10.1038/s41467-018-05735-y  

   Persad, Geeta G. ; Ming, Yi ; Shen, Zhaoyi ; Ramaswamy, V. ( August 2018 , Nature Communications)

   Despite distinct geographic distributions of top-of-the-atmosphere radiative forcing, anthropogenic greenhouse gases and aerosols have been found to produce similar patterns of climate response in atmosphere-and-ocean coupled climate model simulations. Understanding surface energy flux changes, a crucial pathway by which atmospheric forcing is communicated to the ocean, is a vital bridge to explaining the similar full atmosphere-and-ocean responses to these disparate forcings. Here we analyze the fast, atmosphere-driven change in surface energy flux caused by present-day greenhouse gases vs aerosols to elucidate its role in shaping the subsequent slow, coupled response. We find that the surface energy flux response patterns achieve roughly two-thirds of the anti-correlation seen in the fully coupled response, driven by Rossby waves excited by symmetric changes to the land–sea contrast. Our results suggest that atmosphere and land surface processes are capable of achieving substantial within-hemisphere homogenization in the climate response to disparate forcers on fast, societally-relevant timescales.

    

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4. Deglacial Temperature and Carbonate Saturation State Variability in the Tropical Atlantic at Antarctic Intermediate Water Depths

   https://doi.org/10.1029/2023PA004674  

   Oppo, D. W. ; Lu, W. ; Huang, K. ‐F. ; Umling, N. E. ; Guo, W. ; Yu, J. ; Curry, W. B. ; Marchitto, T. M. ; Wang, S. ( September 2023 , Paleoceanography and Paleoclimatology)

   Variations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient andreconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability ofin the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera.

    

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5. Toward New and Independent Constraints on Global Mean Sea‐Level Highstands During the Last Glaciation (Marine Isotope Stage 3, 5a, and 5c)

   https://doi.org/10.1029/2022PA004560  

   Pico, Tamara ( December 2022 , Paleoceanography and Paleoclimatology)

   Estimates of ice volume over the last 120 ka, from marine isotope Stage (MIS) 5d (∼110 ka) through MIS 3 (60–26 ka) are uncertain. Weiss et al. (2022,https://doi.org/10.1029/2021PA004361) offer an innovative new constraint on past sea level using the oxygen isotopes (δ¹⁸O) of planktic (surface and thermocline dwelling) foraminifers to infer the salinity of the Sulu Sea in the Indo‐Pacific Ocean and assess flow through the Karimata Strait (Indonesia) over the last glaciation. Based on the timing of Karimata Strait flooding, the study concludes that local relative sea level in the Karimata Strait was >−8  6 m during MIS 5c (∼100 ka) and >−12  6 m during MIS 5a (∼80 ka), relative to present. For MIS 3, a maximum possible relative sea level of −16  6 m is determined. Here, these results are placed into the context of current knowledge of last glacial sea‐level change and the implications for climate forcings and feedbacks (e.g., global average surface temperature and greenhouse gases) and ice sheet growth are discussed. By tracing past ocean circulation patterns that are modulated by the depth of shallow straits such as the Karimata Strait, Weiss et al. (2022,https://doi.org/10.1029/2021PA004361) provide independent constraints on local sea level, which are essential for improving global mean sea level reconstructions on late Pleistocene glacial‐interglacial cycles.

    

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