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1. Does Disabling Cloud Radiative Feedbacks Change Spatial Patterns of Surface Greenhouse Warming and Cooling?

   https://doi.org/10.1175/JCLI-D-21-0391.1  

   Chalmers, Jason ; Kay, Jennifer E. ; Middlemas, Eleanor A. ; Maroon, Elizabeth A. ; DiNezio, Pedro ( March 2022 , Journal of Climate)

   The processes controlling idealized warming and cooling patterns are examined in 150-yr-long fully coupled Community Earth System Model, version 1 (CESM1), experiments under abrupt CO₂forcing. By simulation end, 2 × CO₂global warming was 20% larger than 0.5 × CO₂global cooling. Not only was the absolute global effective radiative forcing ∼10% larger for 2 × CO₂than for 0.5 × CO₂, global feedbacks were also less negative for 2 × CO₂than for 0.5 × CO₂. Specifically, more positive shortwave cloud feedbacks led to more 2 × CO₂global warming than 0.5 × CO₂global cooling. Over high-latitude oceans, differences between 2 × CO₂warming and 0.5 × CO₂cooling were amplified by familiar linked positive surface albedo and lapse rate feedbacks associated with sea ice change. At low latitudes, 2 × CO₂warming exceeded 0.5 × CO₂cooling almost everywhere. Tropical Pacific cloud feedbacks amplified the following: 1) more fast warming than fast cooling in the west, and 2) slow pattern differences between 2 × CO₂warming and 0.5 × CO₂cooling in the east. Motivated to quantify cloud influence, a companion suite of experiments was run without cloud radiative feedbacks. Disabling cloud radiative feedbacks reduced the effective radiative forcing and surface temperature responses for both 2 × CO₂and 0.5 × CO₂. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on non-cloud feedbacks and circulation. With the exception of the tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds.

   We analyze the processing controlling idealized warming and cooling under abrupt CO₂forcing using a modern and highly vetted fully coupled climate model. We were especially interested to compare simulations with and without cloud radiative feedbacks. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on forcing, non-cloud feedbacks, and circulation. With the exception of the tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds. When combined with estimates of cooling at the Last Glacial Maximum, the findings also help rule out large (4+ K) values of equilibrium climate sensitivity.

    

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2. Climate, cryosphere and carbon cycle controls on Southeast Atlantic orbital-scale carbonate deposition since the Oligocene (30-0 Ma)

   https://doi.org/https://doi.org/10.5194/cp-17-2091-2021  

   Drury, AnnaJoy ; Liebrand, Diederik ; Westerhold, Thomas ; Beddow, Helen M. ; Hodell, David A. ; Wilkens, Roy H. ; Lyle, Mitchell ; Bell, David B. ; Kroon, Dick ; Palike, Heiko ; et al ( October 2021 , Climate of the past)

   Beaufort, Luc (Ed.)

   Abstract. The evolution of the Cenozoic cryosphere from unipolar to bipolar over the past 30 million years (Myr) is broadly known. Highly resolved records of carbonate (CaCO3) content provide insight into the evolution of regional and global climate, cryosphere, and carbon cycle dynamics. Here, we generate the first Southeast Atlantic CaCO3 content record spanning the last 30 Myr, derived from X-ray fluorescence (XRF) ln(Ca/Fe) data collected at Ocean Drilling Program Site 1264 (Walvis Ridge, SE Atlantic Ocean). We present a comprehensive and continuous depth and age model for the entirety of Site 1264 (~316 m; 30 Myr). This constitutes a key reference framework for future palaeoclimatic and palaeoceanographic studies at this location. We identify three phases with distinctly different orbital controls on Southeast Atlantic CaCO3 deposition, corresponding to major developments in climate, the cryosphere and the carbon cycle: (1) strong ~110 kyr eccentricity pacing prevails during Oligocene–Miocene global warmth (~30–13 Ma), (2) increased eccentricity-modulated precession pacing appears after the middle Miocene ClimateTransition (mMCT) (~14–8 Ma), and (3) pervasive obliquity pacing appears in the late Miocene (~7.7–3.3 Ma) following greater importance of high-latitude processes, such as increased glacial activity and high-latitude cooling. The lowest CaCO3 content (92 %–94 %) occurs between 18.5 and 14.5 Ma, potentially reflecting dissolution caused by widespread early Miocene warmth and preceding Antarctic deglaciation across the Miocene Climatic Optimum (~17–14.5 Ma) by 1.5 Myr. The emergence of precession pacing of CaCO3 deposition at Site 1264 after ~14 Ma could signal a reorganisation of surface and/or deep-water circulation in this region following Antarctic reglaciation at the mMCT. The increased sensitivity to precession at Site 1264 between 14 and 13 Ma is associated with an increase in mass accumulation rates (MARs) and reflects increased regional CaCO3 productivity and/or recurrent influxes of cooler, less corrosive deep waters. The highest carbonate content (%CaCO3) and MARs indicate that the late Miocene–early PlioceneBiogenic Bloom (LMBB) occurs between ~7.8 and 3.3Ma at Site 1264; broadly contemporaneous with the LMBB in the equatorial Pacific Ocean. At Site 1264, the onset of the LMBB roughly coincides with appearance of strong obliquity pacing of %CaCO3, reflecting increased high-latitude forcing. The global expression of the LMBB may reflect increased nutrient input into the global ocean resulting from enhanced aeolian dust and/or glacial/chemical weathering fluxes, due to enhanced glacial activity and increased meridional temperature gradients. Regional variability in the timing and amplitude of the LMBB may be driven by regional differences in cooling, continental aridification and/or changes in ocean circulation in the late Miocene. 

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3. The Benthic B/Ca Record at Site 806: New Constraints on the Temperature of the West Pacific Warm Pool and the “El Padre” State in the Pliocene

   https://doi.org/10.1029/2019PA003812  

   White, S. M. ; Ravelo, A. Christina ( September 2020 , Paleoceanography and Paleoclimatology)

   The West Pacific Warm Pool (WPWP)'s response to increasedpCO₂during the Pliocene is a key model validation target. Different temperature proxies show different trends: The foraminiferal Mg/Ca sea surface temperature (SST) record shows Pliocene WPWP temperatures ~1.2°C cooler than today (Wara et al., 2005,https://doi.org/10.1126/science.1112596), whereas a TEX₈₆study finds a cooling trend and claims the Pliocene WPWP was warmer than today (Zhang et al., 2014,https://doi.org/10.1126/science.1246172). We focus on understanding biases in Mg/Ca data as the best way to constrain the temperature of the Pliocene WPWP. The strongest nonthermal controls on foraminiferal Mg/Ca are Mg/Ca of seawater and dissolution. Dissolution, which imparts a cool bias to Mg/Ca temperatures, depends on Δ[CO₃²⁻], the difference from the carbonate ion concentration needed for calcite saturation. Thus, Pliocene proxy discrepancies might stem from varying Δ[CO₃²⁻] over time. To constrain the effect of changing dissolution on the Mg/Ca data, we collected benthic foraminiferal B/Ca data (a proxy for Δ[CO₃²⁻]) from the WPWP spanning 0–5.5 Ma. We find no long‐term trend in Δ[CO₃²⁻], but variations above and below the threshold of foraminiferal dissolution yield an ~0.4°C cold bias when averaged over the middle to early Pliocene. Changes in seawater Mg/Ca create an ~0.6°C cold bias in the Pliocene Mg/Ca data. After accounting for these biases, we find that the Pliocene WPWP was ~0.1°C cooler than the late Holocene, ranging from −0.5°C to +0.5°C including all uncertainties. Our reconstruction shows a much lower east‐west temperature gradient in the Pliocene tropical Pacific than today, supporting a permanent El Niño‐like “El Padre” state.

    

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4. Arctic and boreal paleofire records reveal drivers of fire activity and departures from Holocene variability

   https://doi.org/10.1002/ecy.3096  

   Hoecker, Tyler J. ; Higuera, Philip E. ; Kelly, Ryan ; Hu, Feng Sheng ( June 2020 , Ecology)

   Boreal forest and tundra biomes are key components of the Earth system because the mobilization of large carbon stocks and changes in energy balance could act as positive feedbacks to ongoing climate change. In Alaska, wildfire is a primary driver of ecosystem structure and function, and a key mechanism coupling high‐latitude ecosystems to global climate. Paleoecological records reveal sensitivity of fire regimes to climatic and vegetation change over centennial–millennial time scales, highlighting increased burning concurrent with warming or elevated landscape flammability. To quantify spatiotemporal patterns in fire‐regime variability, we synthesized 27 published sediment‐charcoal records from four Alaskan ecoregions, and compared patterns to paleoclimate and paleovegetation records. Biomass burning and fire frequency increased significantly in boreal forest ecoregions with the expansion of black spruce, ca. 6,000–4,000 years before present (yr BP). Biomass burning also increased during warm periods, particularly in the Yukon Flats ecoregion from ca. 1,000 to 500 yr BP. Increases in biomass burning concurrent with constant fire return intervals suggest increases in average fire severity (i.e., more biomass burning per fire) during warm periods. Results also indicate increases in biomass burning over the last century across much of Alaska that exceed Holocene maxima, providing important context for ongoing change. Our analysis documents the sensitivity of fire activity to broad‐scale environmental change, including climate warming and biome‐scale shifts in vegetation. The lack of widespread, prolonged fire synchrony suggests regional heterogeneity limited simultaneous fire‐regime change across our study areas during the Holocene. This finding implies broad‐scale resilience of the boreal forest to extensive fire activity, but does not preclude novel responses to 21st‐century changes. If projected increases in fire activity over the 21st century are realized, they would be unprecedented in the context of the last 8,000 yr or more.

    

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5. The latitudinal temperature gradient and its climate dependence as inferred from foraminiferal δ 18 O over the past 95 million years

   https://doi.org/10.1073/pnas.2111332119  

   Gaskell, Daniel E. ; Huber, Matthew ; O’Brien, Charlotte L. ; Inglis, Gordon N. ; Acosta, R. Paul ; Poulsen, Christopher J. ; Hull, Pincelli M. ( March 2022 , Proceedings of the National Academy of Sciences)

   The latitudinal temperature gradient is a fundamental state parameter of the climate system tied to the dynamics of heat transport and radiative transfer. Thus, it is a primary target for temperature proxy reconstructions and global climate models. However, reconstructing the latitudinal temperature gradient in past climates remains challenging due to the scarcity of appropriate proxy records and large proxy–model disagreements. Here, we develop methods leveraging an extensive compilation of planktonic foraminifera δ 18 O to reconstruct a continuous record of the latitudinal sea-surface temperature (SST) gradient over the last 95 million years (My). We find that latitudinal SST gradients ranged from 26.5 to 15.3 °C over a mean global SST range of 15.3 to 32.5 °C, with the highest gradients during the coldest intervals of time. From this relationship, we calculate a polar amplification factor (PAF; the ratio of change in >60° S SST to change in global mean SST) of 1.44 ± 0.15. Our results are closer to model predictions than previous proxy-based estimates, primarily because δ 18 O-based high-latitude SST estimates more closely track benthic temperatures, yielding higher gradients. The consistent covariance of δ 18 O values in low- and high-latitude planktonic foraminifera and in benthic foraminifera, across numerous climate states, suggests a fundamental constraint on multiple aspects of the climate system, linking deep-sea temperatures, the latitudinal SST gradient, and global mean SSTs across large changes in atmospheric CO 2 , continental configuration, oceanic gateways, and the extent of continental ice sheets. This implies an important underlying, internally driven predictability of the climate system in vastly different background states. 

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