Climate Model Test Beds: Calibrating Nonlinear ESMs focused on E3SM/ACME

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The sooner that Earth System model correctly account for interactions and feedbacks with ice sheets, the better:

Fyke, J, O Sergienko, M Lofverstrom, S Price, and J Lenaerts.  2018.  "An Overview of Interactions and Feedbacks Between Ice Sheets and the Earth System."  Reviews of Geophysics, doi:10.1029/2018rg000600.

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018RG000600

Abstract: "Ice sheet response to forced changes—such as that from anthropogenic climate forcing—is closely regulated by two‐way interactions with other components of the Earth system. These interactions encompass the ice sheet response to Earth system forcing, the Earth system response to ice sheet change, and feedbacks resulting from coupled ice sheet/Earth system evolution. Motivated by the impact of Antarctic and Greenland ice sheet change on future sea level rise, here we review the state of knowledge of ice sheet/Earth system interactions and feedbacks. We also describe emerging observation and model‐based methods that can improve understanding of ice sheet/Earth system interactions and feedbacks. We particularly focus on the development of Earth system models that incorporate current understanding of Earth system processes, ice dynamics, and ice sheet/Earth system couplings. Such models will be critical tools for projecting future sea level rise from anthropogenically forced ice sheet mass loss."

[jacksmith4tx]

Judith Curry occasionally post a list of climate relate research and news articles and today I spotted something interesting. In this case it was building Artificial Intelligence climate models. AI is one of my favorite areas of science research and especially when it applies to climate change.
A group of private donors have committed to funding a major effort to apply state-of-the-art AI and machine learning to create a high resolution model that will include an experimental cloud simulator in addition to extensive observational data from the atmosphere, oceans, land use and biosphere feedback. One to keep an eye on.

Welcome to the EARTH MACHINE:
http://www.sciencemag.org/news/2018/07/science-insurgents-plot-climate-model-driven-artificial-intelligence

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Newly identified evidence indicates that the Southern Ocean will likely stop absorbing as much CO₂ as it recently has been doing, with continuing anthropogenic radiative forcing:

Title: "How much longer will Southern Ocean slow climate change?"

http://www.newstalkzb.co.nz/news/national/how-much-longer-will-southern-ocean-slow-climate-change/

Extract: "The vast and wild ocean current sucks up more than 40 per cent of the carbon dioxide we produce, acting as a temporary climate-change buffer by slowing down the accumulation of greenhouse gases in our atmosphere.

Yet the same westerly winds that play a critical role in regulating its storing capacity are now threatening its future as a CO2 bank, by bringing deep carbon-rich waters up to the surface.
Many climate models predict that the westerly winds overlying the ocean would get stronger if atmospheric greenhouse gas concentrations continued to risk.

A new international study suggests that in the past, strong westerlies have been linked to higher levels of atmospheric CO2 due to their impact on the Southern Ocean carbon balance.

That meant stronger westerlies could actually speed up climate change if mankind continued to emit as much CO2 as it does today.
…
"Our new records of the Southern Hemisphere westerly winds suggest there have been large changes in wind intensity over the past 12,000 years.

"This is in marked contrast to climate model simulations that predict only relatively small wind speed changes over the same period."
…
Yet, Mikaloff-Fletcher added, sea surface carbon data suggested that there was a reversal of this trend in the early 2000s, when the Southern Ocean began taking up carbon much more quickly, even though the westerlies didn't slow.

"The mechanisms behind this change still aren't fully explained, which makes it hard to predict whether this is a short-term effect or a long-term one," she said.

"The Macquarie study suggests that the sudden increase in Southern Ocean carbon uptake may not persist on longer timescales.""

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Offshore methane seeps from hydrates is a 'difficult but important scientific problem'.  The linked article discusses new research of how underwater audio microphones can continuously monitor the size of methane bubbles escaping from such seeps.  This better enables scientists to differentiate (attribute causality) the influence of various factors (such as earthquakes, or submarine landslides) that change the rate of methane releases from this enormous source of currently sequestered methane:

Title: "Audio Reveals Sizes of Methane Bubbles Rising from the Seafloor"

https://eos.org/articles/audio-reveals-sizes-of-methane-bubbles-rising-from-the-seafloor?utm_source=eos&utm_medium=email&utm_campaign=EosBuzz081018

Extract: "A sensitive underwater microphone captures the sounds of methane, a potent greenhouse gas, escaping into waters off the coast of Oregon. Using this sound, researchers can estimate the bubbles’ sizes.
…
“Earthquakes potentially give new pathways for methane to rise up,” said Tamara Baumberger, a marine geochemist at Oregon State University and the National Oceanic and Atmospheric Administration in Newport, Ore. That gas can have positive and negative effects. Methane seeps are often sites of rich ecosystems that include microbes, crabs, and clams, but the escaping gas can also contribute to ocean acidification, said Baumberger.

And the number of known methane seeps is increasing exponentially, said H. Paul Johnson, a marine geophysicist at the University of Washington in Seattle who was not involved in the research. Therefore, quantifying how much methane is escaping from the seafloor is a “difficult but important scientific problem,” he said, noting that scientists are still working on accurately estimating the number of methane seeps and the rate at which gas is escaping from them.

Audio measurements could fundamentally help."

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The linked reference indicates that: "… climate models suggests that about half of the model uncertainty in the mid-latitude cloud-circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change."

Lipat, B.R., A. Voigt, G. Tselioudis, and L.M. Polvani (2018), "Model uncertainty in cloud-circulation coupling, and cloud-radiative response to increasing CO2, linked to biases in climatological circulation", J. Climate, doi:10.1175/JCLI-D-17-0665.1.

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0665.1

Abstract: "Recent analyses of global climate models suggest that uncertainty in the coupling between mid-latitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud-circulation coupling have remained unclear. Here, we use a global climate model in idealized aquaplanet setup to show that the Southern Hemisphere climatological circulation, which in many models is biased equatorward, contributes to the model differences in the cloud-circulation coupling. For the same poleward shift of the Hadley circulation (HC) edge, models with narrower climatological HCs exhibit stronger mid-latitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced radiative warming results predominantly from a subsidence warming that decreases cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with comprehensive climate models suggests that about half of the model uncertainty in the mid-latitude cloud-circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change."

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Essentially, this reference acknowledges that current SLR projections are incomplete, and discusses means to improve those existing SLR projections:

Fyke, J, O Sergienko, M Lofverstrom, S Price, and J Lenaerts.  2018.  "An Overview of Interactions and Feedbacks Between Ice Sheets and the Earth System."  Reviews of Geophysics, doi:10.1029/2018rg000600.

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018RG000600

Abstract: "Ice sheet response to forced changes—such as that from anthropogenic climate forcing—is closely regulated by two‐way interactions with other components of the Earth system. These interactions encompass the ice sheet response to Earth system forcing, the Earth system response to ice sheet change, and feedbacks resulting from coupled ice sheet/Earth system evolution. Motivated by the impact of Antarctic and Greenland ice sheet change on future sea level rise, here we review the state of knowledge of ice sheet/Earth system interactions and feedbacks. We also describe emerging observation and model‐based methods that can improve understanding of ice sheet/Earth system interactions and feedbacks. We particularly focus on the development of Earth system models that incorporate current understanding of Earth system processes, ice dynamics, and ice sheet/Earth system couplings. Such models will be critical tools for projecting future sea level rise from anthropogenically forced ice sheet mass loss."

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Future freshwater exports from the Arctic into the North Atlantic can come several sources including: a) the Beaufort Gyre, b) melting Arctic Sea Ice and c) ice mass loss from the Greenland Ice Sheet.  Furthermore, this Arctic freshwater can follow different pathways, and the cited reference indicates that these different pathways would have different (but significant) impacts on both the North Atlantic Convection and on the AMOC.  This research provide insights into Hansen's ice-climate feedback mechanism:

Wang, He, Sonya Legg, and Robert Hallberg, July 2018: The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation. Journal of Climate, 31(13), DOI:10.1175/JCLI-D-17-0629.1 .

https://journals.ametsoc.org/doi/10.1175/JCLI-D-17-0629.1
&
https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0629.1

Abstract: "This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate."

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The linked report summarizes modeling work for the Arctic System Reanalysis Version 2:

Bromwich, D., A. Wilson, L. Bai, Z. Liu, M. Barlage, C. Shih, S. Maldonado, K. Hines, S.-H. Wang, J. Woollen, B. Kuo, H. Lin, T. Wee, M. Serreze, and J. Walsh, 2018: The Arctic System Reanalysis Version 2. Bull. Amer. Meteor. Soc., 99, 805-828, doi: 10.1175/BAMS-D-16-0215.1.

http://polarmet.osu.edu/PMG_publications/bromwich_wilson_bams_2018.pdf

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The two linked references provide new paleo information that is useful for calibrating state-of-the-art ESMs with regard to the likelihood of future: a) CO₂ fluxes into the atmosphere from deep Pacific Ocean waters and b) changes in the marine nitrogen cycles and the emergence of anoxic marine conditions.

Jianghui Du et al. (2018), "Flushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrations", Nature Geoscience, 1–7; doi: https://doi.org/10.1038/s41561-018-0205-6

http://www.nature.com/articles/s41561-018-0205-6

Extract: "Enhanced overturning in the Pacific Ocean flushed carbon from the abyssal ocean to the atmosphere during the last deglaciation, according to authigenic neodymium isotope data."

&

 Christopher K. Junium, Alexander J. Dickson & Benjamin T. Uveges (2018), "Perturbation to the nitrogen cycle during rapid Early Eocene global warming", Nature Communications 9, 3186; doi: https://doi.org/10.1038/s41467-018-05486-w

http://www.nature.com/articles/s41467-018-05486-w

 Extract: "Studying the PETM, a past period of rapid warming ~56 Ma, could provide insights into ecosystem response under future warming conditions. Here, the authors present stable nitrogen isotope data that reveal a dramatic change in the marine nitrogen cycle and the emergence of anoxic conditions."

[mitch]

There is one other pathway for freshwater to be exported: down. Mixing by storms in the summer time can exchange relatively fresh surface water with saltier Atlantic water at depth. This also increases the heat reservoir needed to be overcome in fall/winter for ice to form. 

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Given the impact (and increasing probability) of a significant freshwater release from the Beaufort Gyre into the North Atlantic, numerous scientists are actively studying related issues including those cited in the links below:

"Beaufort Gyre Exploration Project"

http://www.whoi.edu/page.do?pid=66456

&

Meneghello G. , J. Marshall, M.-L. Timmermans and J. Scott (2018). Observations of seasonal upwelling and downwelling in the Beaufort Sea mediated by sea ice. J. Phys. Oceanogr., 48, 795–805. doi:10.1175/JPO-D-17-0188.1

https://journals.ametsoc.org/doi/10.1175/JPO-D-17-0188.1

Abstract: "We present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003–14 average of 2.3 m yr−1 downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30 m yr−1 downward to 10 m yr−1 upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18 m yr−1 is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed."

&

Meneghello, G., J. Marshall, S. Cole, and M.-L. Timmermans (2018). Observational inferences of lateral eddy diffusivity in the halocine of the Beaufort Gyre. Geophys. Res. Lett., 44, 12 331–12 338, https://doi.org/10.1002/2017GL075126.

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017GL075126

Abstract: "Using Ekman pumping rates mediated by sea ice in the Arctic Ocean's Beaufort Gyre (BG), the magnitude of lateral eddy diffusivities required to balance downward pumping is inferred. In this limit—that of vanishing residual‐mean circulation—eddy‐induced upwelling exactly balances downward pumping. The implied eddy diffusivity varies spatially and decays with depth, with values of 50–400 m2/s. Eddy diffusivity estimated using mixing length theory applied to BG mooring data exhibits a similar decay with depth and range of values from 100 m2/s to more than 600 m2/s. We conclude that eddy diffusivities in the BG are likely large enough to balance downward Ekman pumping, arresting the deepening of the gyre and suggesting that eddies play a zero‐order role in buoyancy and freshwater budgets of the BG."

Note: "Zeroth-order approximation (also 0th order) is the term scientists use for a first educated guess at an answer."

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Information from the linked reference can be used to help calibrate ESMs to properly account for Hansen's ice-climate feedback mechanism:

Ivanovic RF; Gregoire LJ; Burke A; Wickert AD; Valdes PJ; Ng HC; Robinson LF; McManus JF; Mitrovica JX; Lee L; Dentith JE (2018) Acceleration of northern ice sheet melt induces AMOC slowdown and northern cooling in simulations of the early last deglaciation, Paleoceanography and Paleoclimatology. doi: 10.1029/2017PA003308

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2017PA003308

Abstract
The cause of a rapid change in Atlantic Ocean circulation and northern cooling at the onset of Heinrich Stadial 1 ~18.5 ka is unclear. Previous studies have simulated the event using ice sheet and/or iceberg meltwater forcing, but these idealized freshwater fluxes have been unrealistically large. Here we use a different approach, driving a high‐resolution drainage network model with a recent time‐resolved global paleo‐ice sheet reconstruction to generate a realistic meltwater forcing. We input this flux to the Hadley Centre Coupled Model version 3 (HadCM3) climate model without adjusting the timing or amplitude and find that an acceleration in northern ice sheet melting (up to ~7.5 m/kyr global mean sea level rise equivalent) triggers a 20% reduction in the Atlantic Meridional Overturning Circulation. The simulated pattern of ocean circulation and climate change matches an array of paleoclimate and ocean circulation reconstructions for the onset of Heinrich Stadial 1, in terms of both rates and magnitude of change. This is achieved with a meltwater flux that matches constraints on sea level changes and ice sheet evolution around 19–18 ka. Since the rates of melting are similar to those projected for Greenland by 2200, constraining the melt rates and magnitude of climate change during Heinrich Stadial 1 would provide an important test of climate model sensitivity to future ice sheet melt.

Plain Language Summary
Atlantic Ocean circulation plays a key role in redistributing heat around Earth's surface, and thus has an important influence on our climate. Because of this, sudden shifts in Atlantic Ocean circulation can drive rapid climate changes. One such example is at the onset of “Heinrich Stadial 1”, 18.5 thousand years ago, when geological records show that Atlantic circulation weakened and the Northern Hemisphere cooled while the Southern Hemisphere warmed. At the time, huge ice sheets (several kilometers thick) covered much of North America and northern Europe. Climate model results suggest that the freshwater produced by these melting ice sheets is responsible for weakening the Atlantic Ocean circulation and triggering the abrupt climate changes captured in the geological records. This result helps to elucidate the complex interaction between ice sheets, ocean circulation, and climate, and how these interactions can lead to sudden shifts in climates of the past and, potentially, the future. Indeed, the rate of melting we adopt in the present model is comparable to the melting projected for the Greenland Ice Sheet by 2200.

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Data cited in the linked reference can be used to better calibrate ESMs:

Sebastian G. Mutz et al. (2018), "Estimates of late Cenozoic climate change relevant to Earth surface processes in tectonically active orogens", Earth Surf. Dynam., 6, 271–301, https://doi.org/10.5194/esurf-6-271-2018

https://www.earth-surf-dynam.net/6/271/2018/esurf-6-271-2018.pdf

Abstract. The denudation history of active orogens is often interpreted in the context of modern climate gradients. Here we address the validity of this approach and ask what are the spatial and temporal variations in palaeoclimate for a latitudinally diverse range of active orogens? We do this using high-resolution (T159, ca. 80x80 km at the Equator) palaeoclimate simulations from the ECHAM5 global atmospheric general circulation model and a statistical cluster analysis of climate over different orogens (Andes, Himalayas, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include the Pliocene (PLIO, ~3 Ma), the Last Glacial Maximum (LGM, ~21 ka), mid-Holocene (MH, ~6 ka), and pre-industrial (PI, reference year 1850). The regional simulated climates of each orogen are described by means of cluster analyses based on the variability in precipitation, 2m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate. Results indicate the largest differences in the PI climate existed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO. The LGM climate shows the largest deviation in annual precipitation from the PI climate and shows enhanced precipitation in the temperate Andes and coastal regions for both SE Alaska and the US Pacific Northwest. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen. The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time slice experiments conducted here. Taken together, these results highlight significant changes in late Cenozoic regional climatology over the last ~3 Myr. Comparison of simulated climate with proxy-based reconstructions for the MH and LGM reveal satisfactory to good performance of the model in reproducing precipitation changes, although in some cases discrepancies between neighbouring proxy observations highlight contradictions between proxy observations themselves. Finally, we document regions where the largest magnitudes of late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies that quantify the impact of climate change on denudation and weathering rates.

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The linked reference can be used to better calibrate advanced ESMs:

P. Bakker & M. Prange (03 August 2018), "Response of the Intertropical Convergence Zone to Antarctic Ice Sheet melt", Geophysical Research Letters, https://doi.org/10.1029/2018GL078659

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL078659

Abstract: "Past cooling events in the Northern Hemisphere have been shown to impact the location of the intertropical convergence zone (ITCZ) and therewith induce a southward shift of tropical precipitation. Here we use high‐resolution coupled ocean‐atmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can similarly have led to shifts of the ITCZ, albeit in opposite direction, through large‐scale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere high‐latitude surface air temperature changes and Antarctic Ice Sheet melt rates. This highlights the possibility to use past episodes of anomalous melt rates to better constrain a possible future response of low latitude precipitation to continued global warming and a shrinking Antarctic Ice Sheet."

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Findings of the linked reference can be used to better calibrate advanced ESMs:

Schröder, D., Feltham, D. L., Tsamados, M., Ridout, A., and Tilling, R.: New insight from CryoSat-2 sea ice thickness for sea ice modelling, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-159, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-159/

Abstract. Estimates of Arctic sea ice thickness are available from the CryoSat-2 (CS2) radar altimetry mission during ice growth seasons since 2010. We derive the sub-grid scale ice thickness distribution (ITD) with respect to 5 ice thickness categories used in a sea ice component (CICE) of climate simulations. This allows us to initialize the ITD in stand-alone simulations with CICE and to verify the simulated cycle of ice thickness. We find that a default CICE simulation strongly underestimates ice thickness, despite reproducing the inter-annual variability of summer sea ice extent. We can identify the underestimation of winter ice growth as being responsible and show that increasing the ice conductive flux for lower temperatures (bubbly brine scheme) and accounting for the loss of drifting snow results in the simulated sea ice growth being more realistic. Sensitivity studies provide insight into the impact of initial and atmospheric conditions and, thus, on the role of positive and negative feedback processes. During summer, atmospheric conditions are responsible for 50% of September sea ice thickness variability through the positive sea ice and melt pond albedo feedback. However, atmospheric winter conditions have little impact on winter ice growth due to the dominating negative conductive feedback process: the thinner the ice and snow in autumn, the stronger the ice growth in winter. We conclude that the fate of Arctic summer sea ice is largely controlled by atmospheric conditions during the melting season rather than by winter temperature. Our optimal model configuration does not only improve the simulated sea ice thickness, but also summer sea ice concentration, melt pond fraction, and length of the melt season. It is the first time CS2 sea ice thickness data have been applied successfully to improve sea ice model physics.

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Current (AR5) energy balance methodologies for determining radiative feedback are inaccurate and biased.  The linked reference provide a new framework for improved estimates of radiative feedback.

Cristian Proistosescu et al. (14 May 2018), "Radiative Feedbacks From Stochastic Variability in Surface Temperature and Radiative Imbalance", Geophysical Research Letters, https://doi.org/10.1029/2018GL077678

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL077678

Abstract
Estimates of radiative feedbacks obtained by regressing fluctuations in top‐of‐atmosphere (TOA) energy imbalance and surface temperature depend critically on the sampling interval and on assumptions about the nature of the stochastic forcing driving internal variability. Here we develop an energy balance framework that allows us to model the different impacts of stochastic atmospheric and oceanic forcing on feedback estimates. The contribution of different forcing components is parsed based on their impacts on the covariance structure of near‐surface air temperature and TOA energy fluxes, and the framework is validated in a hierarchy of climate model simulations that span a range of oceanic configurations and reproduce the key features seen in observations. We find that at least three distinct forcing sources, feedbacks, and time scales are needed to explain the full covariance structure. Atmospheric and oceanic forcings drive modes of variability with distinct relationships between temperature and TOA radiation, leading to an effect akin to regression dilution. The net regression‐based feedback estimate is found to be a weighted average of the distinct feedbacks associated with each mode. Moreover, the estimated feedback depends on whether surface temperature and TOA energy fluxes are sampled at monthly or annual time scales. The results suggest that regression‐based feedback estimates reflect contributions from a combination of stochastic forcings and should not be interpreted as providing an estimate of the radiative feedback governing the climate response to greenhouse gas forcing.
Plain Language Summary
Climate sensitivity quantifies the long‐term warming the Earth will experience in response to the additional energy trapped in the system due to greenhouse gases. The physical processes that ultimately determine climate sensitivity—termed climate feedbacks—have been extensively investigated using information from natural variability in Earth's temperature and net energy imbalance. However, a complete physical model for what controls this natural variability has been lacking. We derive such a physical model and calibrate it to a hierarchy of numerical climate simulations of increasing complexity. We are able to answer several outstanding questions about previous estimates of climate feedbacks and sensitivity drawn from natural variability, such as what is the source of this variability, and how the estimates depend on the how the data is analyzed. We find that at least three different mechanisms for natural variability are needed to explain the relationship between temperature and energy imbalance and that none provide direct estimates of climate sensitivity.

See also:

Title: "New Modeling Framework Improves Radiative Feedback Estimates"

https://eos.org/research-spotlights/new-modeling-framework-improves-radiative-feedback-estimates?utm_source=eos&utm_medium=email&utm_campaign=EosBuzz082418

Extract: "A new approach offers insights into the relationship between surface temperature and top-of-atmosphere energy imbalances and improves the understanding of important climate feedbacks.
…
The novel application of the Hasselmann model provides researchers with a new approach to explain the relationship between top-of-atmosphere fluxes and surface temperatures and offers useful insight into the natural variability of radiative feedbacks. The framework also improves estimates of radiative feedback that may currently be inaccurate and biased."

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The linked reference indicates that Arctic Amplification is more sensitive to heat influx from the North Pacific than from the North Atlantic.  Here I note that a collapse of the WAIS would lead to a very large heat flux from the North Pacific into the Artic Ocean:

Summer Praetorius  et al. (2018), "Global and Arctic climate sensitivity enhanced by changes in North Pacific heat flux", Nature Communications, volume 9, Article number: 3124, DOI: https://doi.org/10.1038/s41467-018-05337-8

http://www.nature.com/articles/s41467-018-05337-8

Abstract: "Arctic amplification is a consequence of surface albedo, cloud, and temperature feedbacks, as well as poleward oceanic and atmospheric heat transport. However, the relative impact of changes in sea surface temperature (SST) patterns and ocean heat flux sourced from different regions on Arctic temperatures are not well constrained. We modify ocean-to-atmosphere heat fluxes in the North Pacific and North Atlantic in a climate model to determine the sensitivity of Arctic temperatures to zonal heterogeneities in northern hemisphere SST patterns. Both positive and negative ocean heat flux perturbations from the North Pacific result in greater global and Arctic surface air temperature anomalies than equivalent magnitude perturbations from the North Atlantic; a response we primarily attribute to greater moisture flux from the subpolar extratropics to Arctic. Enhanced poleward latent heat and moisture transport drive sea-ice retreat and low-cloud formation in the Arctic, amplifying Arctic surface warming through the ice-albedo feedback and infrared warming effect of low clouds. Our results imply that global climate sensitivity may be dependent on patterns of ocean heat flux in the northern hemisphere."

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Improved guidance for modelling supraglacial meltwater routing through internally drained catchments on the Greenland Ice Sheet surface:

Yang, K., Smith, L. C., Karlstrom, L., Cooper, M. G., Tedesco, M., van As, D., Cheng, X., Chen, Z., and Li, M.: Supraglacial meltwater routing through internally drained catchments on the Greenland Ice Sheet surface, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-145, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-145/

Abstract. Large volumes of surface meltwater are routed through supraglacial internally drained catchments (IDCs) on the Greenland Ice Sheet surface each summer. Because surface routing impacts the timing and discharge of meltwater entering the ice sheet through moulins, it is crucial for correctly coupling surface energy and mass balance models with subglacial hydrology and ice dynamics. Yet surface routing of meltwater on ice sheets remains a poorly understood physical process. We use high-resolution (0.5m) satellite imagery and a derivative high-resolution (3.0m) digital elevation model to partition the runoff-contributing area of Rio Behar catchment, a moderate-sized (~63km2) mid-elevation (1,207–1,381m) IDC on the southwestern Greenland ablation zone, into meltwater open-channels (supraglacial streams and rivers) and interfluves (small upland areas draining to surface channels, also called hillslopes in terrestrial geomorphology). A simultaneous in-situ moulin discharge hydrograph was previously acquired for this catchment in July 2015. By combining the in-situ discharge measurements with remote sensing and classic hydrological theory, we determine mean meltwater routing velocities through open-channels and interfluves within the catchment. Two traditional terrestrial hydrology surface routing models, the unit hydrograph and rescaled width function, are applied and also compared with a surface routing and lake filling model. We conclude: 1) Surface meltwater is routed by slow interfluve flow (~10−3–10−4m/s) and fast open-channel flow (~10−1m/s); 2) The slow interfluve velocities are physically consistent with shallow, unsaturated subsurface porous media flow (~10−4–10−5m/s) more than overland sheet flow (~10−2m/s); 3) The open-channel velocities yield mean Manning’s roughness coefficient (n) values of ~0.03–0.05 averaged across the Rio Behar supraglacial stream/river network; 4) Interfluve and open-channel flow travel distances have mean length scales of ~100–101m and ~103m respectively; 5) Seasonal evolution of supraglacial stream/river density will alter these length scales and the proportion of interfluves vs. open-channels, and thus the magnitude and timing of meltwater discharge hydrograph received at the outlet moulin. This phenomenon may explain seasonal subglacial water pressure variations measured in a borehole ~20km away. In general, we conclude that in addition to fast open-channel transport through supraglacial streams and rivers, slow interfluve processes must also be considered in ice sheet surface meltwater routing models. Interfluves are characterized by slow overland and/or shallow subsurface flow, and it appears that shallow unsaturated porous-media flow occurs even in the bare-ice ablation zone. Together, both interfluves and open-channels combine to modulate the timing and discharge of meltwater reaching IDC outlet moulins, prior to further modification by en- and sub-glacial processes.

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The linked article discusses the scientific value of a new high resolution elevation model of Antarctica.  Such a tool could be very valuable in verifying/calibrating cliff failure and hydrofracturing models of ice sheets:

Title: "New map of Antarctica shows the icy continent in 'stunning detail'"

https://www.usatoday.com/story/tech/science/2018/09/07/antarctica-new-map-shows-icy-continent-stunning-detail/1224078002/

Extract: "Scientists from Ohio State University and the University of Minnesota have created what they say is the best, most complete and accurate map ever made of the frozen continent at the bottom of the world …
…
“Now we’ll be able to see changes in melting and deposition of ice better than ever before,” Morin said. “That will help us understand the impact of climate change and sea level rise. We’ll be able to see it right before our eyes.”"

[AbruptSLR]

The linked reference provide evidence that the ocean's 'biological carbon pump' is likely a positive feedback on climate change:

F.Boscolo-Galazz et al. (30 August 2018), "Temperature dependency of metabolic rates in the upper ocean: A positive feedback to global climate change?", Global and Planetary Change, https://doi.org/10.1016/j.gloplacha.2018.08.017

https://www.sciencedirect.com/science/article/pii/S0921818118301905

Abstract: "The temperature of seawater can affect marine plankton in various ways, including by affecting rates of metabolic processes. This can change the way carbon and nutrients are fixed and recycled and hence the chemical and biological profile of the water column. A variety of feedbacks on global climate are possible, especially by altering patterns of uptake and return of carbon dioxide to the atmosphere. Here we summarize and synthesize recent studies in the field of ecology, oceanography and ocean carbon cycling pertaining to possible feedbacks involving metabolic processes. By altering the rates of cellular growth and respiration, temperature-dependency may affect nutrient uptake and food demand in plankton and ultimately the equilibrium of pelagic food webs, with cascade effects on the flux of organic carbon between the upper and inner ocean (the “biological carbon pump”) and the global carbon cycle. Insights from modern ecology can be applied to investigate how temperature-dependent changes in ocean biogeochemical cycling over thousands to millions of years may have shaped the long-term evolution of Earth's climate and life. Investigating temperature-dependency over geological time scales, including through globally warm and cold climate states, can help to identify processes that are relevant for a variety of future scenarios."

Extract: "- Heterotrophic respiration rates respond twice as fast as to ocean temperature changes than photosynthesis.

- This may alter the ratio of particulate organic carbon to export production in the ocean, with more carbon sequestered to the deep ocean when it is cooler.

- Temperature dependency of metabolic rates has potential for being a key internal feedback for Earth's climate"

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The linked reference presents findings that can be used to help calibrate Hansen's ice-climate feedback mechanism:

Pepijn Bakker & Matthias Prange (03 August 2018), "Response of the Intertropical Convergence Zone to Antarctic Ice Sheet Melt", Geophysical Research Letters, https://doi.org/10.1029/2018GL078659

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL078659

"Abstract
Past cooling events in the Northern Hemisphere have been shown to impact the location of the intertropical convergence zone (ITCZ) and therewith induce a southward shift of tropical precipitation. Here we use high resolution coupled ocean‐atmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can similarly have led to shifts of the ITCZ, albeit in opposite direction, through large‐scale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes, we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere high‐latitude surface air temperature changes and Antarctic Ice Sheet melt rates. This highlights the possibility to use past episodes of anomalous melt rates to better constrain a possible future response of low latitude precipitation to continued global warming and a shrinking Antarctic Ice Sheet.

Plain Language Summary
Changes in high‐latitude climate can impact the tropical regions through so‐called atmospheric and oceanic teleconnections. Research has mostly focused on past southward shifts in the band of heavy tropical precipitation, called the intertropical convergence zone (ITCZ), linked to large‐scale cooling in the Northern Hemisphere resulting from large‐scale continental ice sheet buildup or a slowdown of the large‐scale Atlantic meridional ocean circulation. Here we use high resolution climate simulations to show that melting of the Antarctic Ice Sheet can similarly lead to northward shifts of the ITCZ and the displacement of the accompanying rain belt. Future melt rates of the Antarctic Ice Sheet are highly uncertain, but our work shows that it might have a nonnegligible impact on the tropical climate. Moreover, we find that because of the apparent linearity of the system under consideration, studying episodes of past changes in the size of the Antarctic Ice Sheet can help us constrain the possible changes in the low latitude hydroclimate."

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The linked reference discusses how lessons learned from studying paleo 'hyperthermal' events from the past 300 million years can be used to help calibrate models of our current 'Anthropocene hyperthermal' event.  One such lesson learned is that many key negative feedback mechanisms occur over periods longer than one thousand years, while many key positive forcing mechanisms occur over shorter time frames; thus the observed effective climate sensitivity from paleo-hyperthermal events likely underestimate the effective climate sensitivity for our current 'Anthropogenic hyperthermal' event:

Gavin L. Foster, Pincelli Hull, Daniel J. Lunt, James C. Zachos (3 September 2018), "Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past", Philosophical Transactions of the Royal Society A, DOI: 10.1098/rsta.2017.0086

http://rsta.royalsocietypublishing.org/content/376/2130/20170086

Extract: "These modern rates of carbon emission likely dwarf the rate seen during the onset of the PETM by a factor of 10 or so. If humanity's fossil fuel use is not tackled rapidly through the development of a low-carbon economy, we face the possibility of emitting as much carbon as was released during the PETM but in a fraction of the time (0.5 versus 50–100 thousand years). The magnitude of atmospheric CO2 change (and hence the magnitude of warming, anoxia and ocean acidification) that occurs following any carbon addition to the Earth system is a function of rate, due to the time scales of a number of key negative feedbacks. Why the Palaeozoic hyperthermals are associated with significantly greater extinction rate is currently not known. However, a consensus is emerging that it is the extreme heat and anoxia that are the likely ‘kill mechanisms’ [50]. Given that the rate of carbon addition during our ‘anthropogenic hyperthermal’ eclipses that of the PETM, at the very least we are likely looking at a potential future with a more severe impact of life on Earth than any climate change event in the last 56 Myr. Exactly how severe, however, remains perhaps one of the most pressing of the ‘unknown unknowns’."

[AbruptSLR]

The linked reference finds that only the most up-to-date climate models correctly reproduce the rate of observed expansion of the Earth's tropics.  I note that the most up-to-date climate models tend to indicate higher climate sensitivity than those used to determine AR5's likely range of climate sensitivity values:

Title: "Tropics are widening as predicted by climate models, research finds"

https://phys.org/news/2018-09-tropics-widening-climate.html

Extract: "A new paper co-authored by Indiana University Bloomington researcher Paul Staten, however, finds that the most up-to-date models and the best data match up reasonably well.

"If we compare the observed trends of how the tropics have widened to modeling trends, it's actually not outside of what the models predict," said Staten, assistant professor of atmospheric sciences in the College of Arts and Sciences.

Staten is an affiliated researcher with the IU Environmental Resilience Institute, which was established under Prepared for Environmental Change, the second initiative funded by the university's Grand Challenges Program.

The paper, "Re-examining Tropical Expansion," was published in the journal Nature Climate Change. Additional authors include Jian Lu of the Pacific Northwest National Laboratory, Kevin Grise of the University of Virginia, Sean Davis of the National Oceanic and Atmospheric Administration in Colorado and Thomas Birner of Ludwig Maximilians University Munich in Germany."

[AbruptSLR]

The linked reference indicates that Arctic Amplification is inevitable due to atmospheric heat and moisture transport from the tropics to the Arctic; while the degree of Arctic Amplification depends on the details of the actual transport mechanisms, which are still not fully understood yet (and which requires more calibration efforts).

Armour, Kyle C., Nicholas Siler, Aaron Donohoe, and Gerard H. Roe. 2018. “Meridional Atmospheric Heat Transport Constrained by Energetics and Mediated by Large-scale Diffusion.” EarthArXiv. August 30. doi:10.31223/osf.io/c4tqx.

https://eartharxiv.org/c4tqx

Abstract: "Meridional atmospheric heat transport (AHT) has been investigated through three broad perspectives: dynamic perspective, linking AHT to the poleward flux of moist static energy (MSE) by atmospheric motions; an energetic perspective, linking AHT to energy input to the atmosphere by top-of-atmosphere radiation and surface heat fluxes; and a diffusive perspective, representing AHT in terms down-gradient energy transport. It is shown here that the three perspectives provide complementary diagnostics of meridional AHT and its changes under greenhouse-gas forcing. When combined, the energetic and diffusive perspectives offer prognostic insights: anomalous AHT is constrained to satisfy the net energetic demands of radiative forcing, radiative feedbacks, and ocean heat uptake; in turn, the meridional pattern of warming must adjust to produce those AHT changes, and does so approximately according to diffusion of anomalous MSE. The relationship between temperature and MSE exerts strong constraints on the warming pattern, favoring polar amplification. These conclusions are supported by use of a diffusive moist energy balance model (EBM) that accurately predicts zonal-mean warming and AHT changes within comprehensive general circulation models (GCMs). A dry diffusive EBM predicts similar AHT changes in order to satisfy the same energetic constraints, but does so through tropically-amplified warming -- at odds with the GCMs' polar-amplified warming pattern. The results suggest that polar-amplified warming is a near-inevitable consequence of a moist, diffusive atmosphere's response to greenhouse-gas forcing. In this view, atmospheric circulations must act to satisfy net AHT as constrained by energetics."

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The linked reference provides data for calibrating climate change models w.r.t. the amounts and distribution of carbon exports in a Northern Hardwood Forest.  Their findings indicate that associated carbon emissions into the atmosphere should increase in coming decades due to projected increases in surface temperatures in these latitudes:

Oscar E. Senar et al. (2018), "Catchment‐Scale Shifts in the Magnitude and Partitioning of Carbon Export in Response to Changing Hydrologic Connectivity in a Northern Hardwood Forest", Journal of Geophysical Research: Biogeoscience, Vol 123, Issue 8, https://doi.org/10.1029/2018JG004468

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JG004468

Abstract
The capacity of forest soils to store organic carbon is influenced by changing hydrologic connectivity. We hypothesized that hydrologic connectivity, the water‐mediated transfer of matter and energy between different landscape positions, controls the partitioning between aquatic and atmospheric soil carbon fates. Results from a 5‐year study of a northern hardwood forested catchment indicated that hydrologic connectivity affected both the magnitude and fate of carbon export. Atmospheric carbon export was the major export pathway from the catchment; its rate was regulated by topographic position (i.e., uplands, ecotones, and wetlands) but enhanced or supressed through changes in soil moisture and hydrologic connectivity. Wetter soil conditions reduced CO2 flux from the ecotones and wetlands where microbial respiration was oxygen‐limited, whereas drier soil conditions that decreased hydrologic connectivity increased CO2 flux by relieving the oxygen limitation. In contrast, aquatic carbon export was a minor export pathway from the catchment and was driven by hydrologic connectivity, with less carbon export during relatively low discharge years. Past trends suggest a shift to a warmer climate and changes in the timing, duration, and intensity of hydrologic connectivity that are leading to an increase in annual atmospheric carbon export but a decrease in annual aquatic carbon export, despite the intensification of autumn storms. The increase in atmospheric carbon export creates a positive feedback for climate warming that will further disrupt hydrologic connectivity and aquatic carbon export, with consequences for downstream streams and lakes.

See also:

Title: "Hydrology Dictates Fate of Carbon from Northern Hardwood Forests"

https://eos.org/research-spotlights/hydrology-dictates-fate-of-carbon-from-northern-hardwood-forests?utm_source=eos&utm_medium=email&utm_campaign=EosBuzz101218

Extract: "Future climate predictions project a trend toward higher temperatures and prolonged periods of disconnected hydrology. The authors suggest that under those circumstances, northern hardwood forests will initially increase atmospheric carbon emissions from upland and ecotone habitats, with an eventual decrease as water becomes limited. The reduction in hydrologic connectivity will also result in less aquatic carbon transport downstream."

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The linked reference provides paleo-evidence that prior to the PETM, many of the Earth's carbon cycles became destabilized (i.e. weakened negative feedbacks and greater sensitivity to small shocks).  This is not good news, but this information may be useful in helping to better calibrate state-of-the-art Earth System Models:

Armstrong McKay, D. I. and Lenton, T. M.: Reduced carbon cycle resilience across the Palaeocene–Eocene Thermal Maximum, Clim. Past, 14, 1515-1527, https://doi.org/10.5194/cp-14-1515-2018, 2018.

https://www.clim-past.net/14/1515/2018/

Abstract. Several past episodes of rapid carbon cycle and climate change are hypothesised to be the result of the Earth system reaching a tipping point beyond which an abrupt transition to a new state occurs. At the Palaeocene–Eocene Thermal Maximum (PETM) at  ∼ 56Ma and at subsequent hyperthermal events, hypothesised tipping points involve the abrupt transfer of carbon from surface reservoirs to the atmosphere. Theory suggests that tipping points in complex dynamical systems should be preceded by critical slowing down of their dynamics, including increasing temporal autocorrelation and variability. However, reliably detecting these indicators in palaeorecords is challenging, with issues of data quality, false positives, and parameter selection potentially affecting reliability. Here we show that in a sufficiently long, high-resolution palaeorecord there is consistent evidence of destabilisation of the carbon cycle in the  ∼ 1.5Myr prior to the PETM, elevated carbon cycle and climate instability following both the PETM and Eocene Thermal Maximum 2 (ETM2), and different drivers of carbon cycle dynamics preceding the PETM and ETM2 events. Our results indicate a loss of resilience (weakened stabilising negative feedbacks and greater sensitivity to small shocks) in the carbon cycle before the PETM and in the carbon–climate system following it. This pre-PETM carbon cycle destabilisation may reflect gradual forcing by the contemporaneous North Atlantic Volcanic Province eruptions, with volcanism-driven warming potentially weakening the organic carbon burial feedback. Our results are consistent with but cannot prove the existence of a tipping point for abrupt carbon release, e.g. from methane hydrate or terrestrial organic carbon reservoirs, whereas we find no support for a tipping point in deep ocean temperature.

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The linked reference proves that "… regression-based feedback estimates reflect contribution from a combination of stochastic forcing and should not be interpreted as providing an estimate of the radiative feedback governing the climate response to greenhouse gas forcing."  For example it is not adequate to talk about atmospheric contributions to GMSTA without considering that the oceans (with a slower response time) have been warming for at least 250 years:

Cristian Proistosescu et al. (14 May 2018), "Radiative Feedbacks From Stochastic Variability in Surface Temperature and Radiative Imbalance", Geophysical Research Letters, https://doi.org/10.1029/2018GL077678

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL077678

Abstract
Estimates of radiative feedbacks obtained by regressing fluctuations in top‐of‐atmosphere (TOA) energy imbalance and surface temperature depend critically on the sampling interval and on assumptions about the nature of the stochastic forcing driving internal variability. Here we develop an energy balance framework that allows us to model the different impacts of stochastic atmospheric and oceanic forcing on feedback estimates. The contribution of different forcing components is parsed based on their impacts on the covariance structure of near‐surface air temperature and TOA energy fluxes, and the framework is validated in a hierarchy of climate model simulations that span a range of oceanic configurations and reproduce the key features seen in observations. We find that at least three distinct forcing sources, feedbacks, and time scales are needed to explain the full covariance structure. Atmospheric and oceanic forcings drive modes of variability with distinct relationships between temperature and TOA radiation, leading to an effect akin to regression dilution. The net regression‐based feedback estimate is found to be a weighted average of the distinct feedbacks associated with each mode. Moreover, the estimated feedback depends on whether surface temperature and TOA energy fluxes are sampled at monthly or annual time scales. The results suggest that regression‐based feedback estimates reflect contributions from a combination of stochastic forcings and should not be interpreted as providing an estimate of the radiative feedback governing the climate response to greenhouse gas forcing.

Plain Language Summary
Climate sensitivity quantifies the long‐term warming the Earth will experience in response to the additional energy trapped in the system due to greenhouse gases. The physical processes that ultimately determine climate sensitivity—termed climate feedbacks—have been extensively investigated using information from natural variability in Earth's temperature and net energy imbalance. However, a complete physical model for what controls this natural variability has been lacking. We derive such a physical model and calibrate it to a hierarchy of numerical climate simulations of increasing complexity. We are able to answer several outstanding questions about previous estimates of climate feedbacks and sensitivity drawn from natural variability, such as what is the source of this variability, and how the estimates depend on the how the data is analyzed. We find that at least three different mechanisms for natural variability are needed to explain the relationship between temperature and energy imbalance and that none provide direct estimates of climate sensitivity.

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The linked reference finds that the 'atmospheric dynamics feedback' is positive, rather than negative as some researchers had previously assumed.  This implies that TCR & ECS are higher than some researchers previously thought:

MICHAEL P. BYRNE and TAPIO SCHNEIDER (2018), "Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications", Journal of Climate, 31 (, DOI: 10.1175/JCLI-D-17-0470.1

http://climate-dynamics.org/wp-content/uploads/2017/07/Byrne-Schneider-2018.pdf

Abstract: "The regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the ‘‘atmospheric dynamics feedback.’’ Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models.  A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed."

[vox_mundi]

Earth's oceans have absorbed 60 percent more heat than previously thought

For each year during the past quarter century, the world's oceans have absorbed an amount of heat energy that is 150 times the energy humans produce as electricity annually, according to a study led by researchers at Princeton and the Scripps Institution of Oceanography at the University of California-San Diego. The strong ocean warming the researchers found suggests that Earth is more sensitive to fossil-fuel emissions than previously thought.

The researchers reported in the journal Nature Nov. 1 that the world's oceans took up more than 13 zettajoules—which is a joule, the standard unit of energy, followed by 21 zeroes—of heat energy each year between 1991 and 2016. The study was funded by the National Oceanic and Atmospheric Administration and the Princeton Environmental Institute.

 First author Laure Resplandy, an assistant professor of geosciences and the Princeton Environmental Institute, said that her and her co-authors' estimate is more than 60 percent higher than the figure in the 2014 Fifth Assessment Report on climate change from the United Nations Intergovernmental Panel on Climate Change (IPCC).

"Imagine if the ocean was only 30 feet deep," said Resplandy, who was a postdoctoral researcher at Scripps. "Our data show that it would have warmed by 6.5 degrees Celsius [11.7 degrees Fahrenheit] every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4 degrees Celsius [7.2 degrees Fahrenheit] every decade.

 ... "The result significantly increases the confidence we can place in estimates of ocean warming and therefore helps reduce uncertainty in the climate sensitivity, particularly closing off the possibility of very low climate sensitivity," Keeling said. 

Resplandy, L., Keeling, R.F., et.al., Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition, Nature (2018)

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The linked reference verifies that ECS increased with continued warming and that "… cloud albedo feedbacks cause an abrupt transition in climate for warming atmospheres …".

E. T. Wolf et al. (22 October 2018), "Evaluating Climate Sensitivity to CO2 Across Earth's History", JGR: Atmospheres, https://doi.org/10.1029/2018JD029262

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JD029262

Abstract
CO2‐driven changes to climate have occurred during many epochs of Earth's history when the solar insolation, atmospheric CO2 concentration, and surface temperature of the planet were all significantly different than today. Each of these aspects affects the implied radiative forcings, climate feedbacks, and resultant changes in global mean surface temperature. Here, we use a three‐dimensional climate system model to study the effects of increasing CO2 on Earth's climate, across many orders of magnitude of variation, and under solar inputs relevant for paleo, present, and future Earth scenarios. We find that the change in global mean surface temperature from doubling CO2 (i.e. the equilibrium climate sensitivity) may vary between 2.6 K and 21.6 K over the course of Earth's history. In agreement with previous studies, we find that the adjusted radiative forcing from doubling CO2 increases at high concentrations up to about 1.5 bars partial pressure, generally resulting in larger changes in the surface temperature. We also find that cloud albedo feedbacks cause an abrupt transition in climate for warming atmospheres, that depends both on the mean surface temperature and the total solar insolation. Climate sensitivity to atmospheric CO2 has probably varied considerably across Earth's history.

Plain Language Summary
It is evident that climate sensitivity to changing CO2 varies if the amount of solar energy received by Earth is different, if the starting CO2 amount is different, or if the mean temperature of the planet is significantly different.

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I provide the following linked reference to those who are interesting in better understanding the flow of currents thru the Bering Strait into the Arctic:

Dmitrenko, I.A., Kirillov, S.A., Myers, P.G., Forest, A., Tremblay, B., Lukovich, J.V., Gratton, Y., Rysgaard, S. and Barber, D.G., 2018. Wind-forced depth-dependent currents over the eastern Beaufort Sea continental slope: Implications for Pacific water transport. Elem Sci Anth, 6(1), p.66. DOI: http://doi.org/10.1525/elementa.321

https://www.elementascience.org/articles/10.1525/elementa.321/

Abstract
Pacific water contributes significantly to the Arctic Ocean freshwater budget. Recent increases in Arctic freshwater flux, also affected by the Pacific-derived Arctic water, impact the Atlantic overturning circulation with implications for global climate. The interannual variability of the Pacific water outflow remains poorly understood, partly due to different branches of the Pacific water flow in the Arctic Ocean. The shelfbreak current over the Beaufort Sea continental slope transports ~50% of the Pacific-derived water eastward along the Beaufort Sea continental slope towards the Canadian Archipelago. The oceanographic mooring deployed over the eastern Beaufort Sea continental slope in October 2003 recorded current velocities through depths of 28–108 m until September 2005. Data analysis revealed that these highly energetic currents have two different modes of depth-dependent behaviour. The downwelling-favourable wind associated with cyclones passing north of the Beaufort Sea continental slope toward the Canadian Archipelago generates depth-intensified shelfbreak currents with along-slope northeastward flow. A surface Ekman on-shore transport and associated increase of the sea surface heights over the shelf produce a cross-slope pressure gradient that drives an along-slope northeastward barotropic flow, in the same direction as the wind. In contrast, the upwelling-favourable wind associated with deep Aleutian Low cyclones over the Alaskan Peninsula and/or Aleutian Island Arc leads to surface-intensified currents with along-slope westward flow. This northeasterly wind generates a surface Ekman transport that moves surface waters offshore. The associated cross-slope pressure gradient drives an along-slope southwestward barotropic flow. The wind-driven barotropic flow generated by upwelling and downwelling is superimposed on the background bottom-intensified shelfbreak current. For downwelling, this flow amplifies the depth-intensified background baroclinic circulation with enhanced Pacific water transport towards the Canadian Archipelago. For upwelling, the shelfbreak current is reversed, which results in surface-intensified flow in the opposite direction. These results are supported by numerical simulations.