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The preferred narrative has always been carbon dioxide alone and the inane quest to refine S Arrhenius' estimate of the effect of its doubling (ignoring the intermediate decades with too-hard-to model effects, immense costs of extreme events and model-wrecking tipping points).
Do nothing until the 2050-2100 time frame (except renew our goofy grants). Controversy avoidance, plain and simple -- go along to get along with the beef and oil/gas industries and growth of GDP consumerism. Documented thousands of times in the 19,512 posts by AbruptSLR.
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In the linked 2021 article (see extracts below), Hansen makes it clear that consensus climate science model projections (e.g.: CMIP) significantly overestimate the current mixing of the ocean (i.e.: they assume fast mixing) that result in relatively slow increases in GMSTA, while Hansen's (& his associate's) model projections were tuned to match the observed slower ocean mixing (resulting in a more stratified ocean with a stronger 'pattern effect' that consensus climate scientists do not consider in calculating ECS, but which the MCDS-BN considers when estimating ECS
eff) as illustrated by the first image.
In this article Hansen (whom I agree with, and whose work I based my Maximum Credible Domino Scenario-Bayesian Network, MCDS-BN, projections) expresses his concern that the Southern Ocean Meridional Overturning Circulation (SMOC, that is close related to Antarctic Bottom Water, AABW, formation) will temporarily shutdown circa 2050 (see the second image), which his (& his associate's) 2014 model projections (released in 2015 and published in 2016) would correspond to a marked slowdown of the AMOC circa 2100 (see the third image); due to the bipolar seesaw system (& subsystems) illustrated in the fourth image.
Thus, I concur with Hansen that consensus climate science projection severely underestimate the climate risks of the deep uncertainties associated with the credible interactions between the MOC (including both the AMOC and the SMOC) and possible freshwater fluxes (particularly from ice melting) into the ocean in coming decades.
Title: "
Foreword: Uncensored Science Is Crucial for Global Conservation" by James Hansen 2021
www.columbia.edu/~jeh1/mailings/2021/20210614_ForewordHansen.pdfExtract: "In October 2006 we – Reto Ruedy, Makiko Sato and I – made a model run with meltwater injection from Antarctica and Greenland. The initial ice melt rate was from observations; it then increased with a 10-year doubling time up to a sea level rise of 5 meters. Most of that water could be provided by West Antarctic ice, which rests on bedrock below sea level (Foreword Figure 2). Deep valley outlets on East Antarctica (Greenbaum et al., 2015) and Greenland (Catania et al., 2020) expose additional ice to contact with ocean water.
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Within several decades the North Atlantic and Southern Ocean Overturning Circulations (dubbed AMOC and SMOC) had shut down. In a hot, warming world, sea ice around Antarctica held steady and then expanded northward.
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I had a suspicion about a problem in ocean models. When we doubled atmospheric CO2, we found that global surface temperature after 100 years had only achieved 60 percent of its final warming. Could mixing of heat into the ocean really slow down the surface response that much? Such a long delay was not expected by the legendary Jule Charney (Hansen, 2022f).
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Response function information might spur more focus on ocean mixing and on observations to test the reality of ocean mixing in all models. Such a focus on the key (real world and model) physics is analogous to how Jule Charney focused his famous investigation of climate sensitivity (Charney et al., 1979). Charney would have jumped eagerly on the issue of ocean mixing and climate response time, but he died young, in 1981.
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Argo floats (Argo, 2021) dive to a two-kilometer depth, rise to the surface while making measurements, and radio the data to a satellite. Precise ocean temperatures measured by the Argo floats were the data needed to define Earth’s energy imbalance. That imbalance is important: it defines how much additional global warming is in the pipeline and it thus informs us about actions needed to stop further global warming.
Accurate determination of Earth’s energy imbalance meant that we had two major “knowns” about the climate system, the other being observed global warming in the past century. There are three major unknowns: climate sensitivity to a forcing, the net climate forcing, and the delay of surface temperature change caused by ocean mixing of heat.
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Our paper also confirmed the suspicion that ocean models mixed heat into the deep ocean too efficiently, but it did not tell us why. Did models create an artificial diffusion of heat via their finite differencing approximation of the equations of motion? Did the approximations used to represent mixing on scales smaller than the model’s grid cause too much mixing? Did the coarse vertical resolution of ocean models cause excessive downward mixing?
Whatever the reason(s), excessive mixing makes it difficult to maintain a low-density ocean surface layer fed by meltwater. Therefore, SMOC and AMOC shut down more readily in the real world than in models. SMOC is more important than AMOC because SMOC shutdown accelerates Antarctic ice melt and sea level rise. The high sensitivity of SMOC implies that sea level rise could run out of control within the next few decades.
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In early 2014 we reran climate simulations with our latest climate model; results were similar to those in 2006.
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Prior analyses of ocean circulation focused on AMOC That focus is understandable. Shutdown of AMOC yields large climate change in the North Atlantic with downstream impact on Europe. The reduced northward ocean heat transport also warms the Southern Ocean – an interhemispheric “seesaw” effect (Stocker, 1998). However, the research community and IPCC concluded that AMOC would not shut down this century; it would only slow down somewhat more than it has already (IPCC, 2019).
Our conclusions differed dramatically. We found that SMOC is more important than AMOC because of its effect on future sea level rise. For business-as-usual scenarios used by IPCC, we found that SMOC would shut down by midcentury (Foreword Figure 4). AMOC would also shut down this century and would not recover for centuries. Our approach to the problem also differed greatly from that of IPCC. While IPCC relies heavily on ice sheet models, our approach was based on empirical information from the real world.
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How can real-world ice melt be so much faster than in the ice sheet models that IPCC relies on? Ice sheet modeling is hard. Ice sheet processes occur on spatial scales ranging from microscale freeze-thaw effects that cause pot-holes in our streets to continental-scale “rivers” of ice that discharge icebergs to the ocean. However, as argued in my “slippery slope” paper (Hansen, 2005b), the crucial amplifying feedbacks are probably interactions between ice sheets and oceans abutting against them. Our global climate model results in Ice Melt revealed such specific amplifying feedbacks.
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Shutdown of SMOC is a powerful feedback The shutdown can spur disintegration of the West Antarctic ice sheet (Foreword Figure 2). Our climate model correctly locates deep water formation along the Antarctic coast at places such as the Weddell Sea coast (section 3.8.5 in Ice Melt), which supports use of our model to study the SMOC feedback. That capability is absent in many CMIP (Climate Model Intercomparison Project) models used in the IPCC assessment (Heuze et al., 2015).
SMOC already slowed in our climate simulations by the late 20th century (Foreword Figure 4, which is Fig. 32 in Ice Melt) due to growing freshwater injection from Antarctica. Ocean current measurements are too sparse to accurately monitor SMOC, but sufficient for Purkey and Johnson (2012) to conclude that the real-world SMOC did slow during that period.
SMOC is an escape valve for ocean heat. As relatively warm water reaches the surface near Antarctica (see Foreword Figure 2), heat escapes to the air and space – especially in winter. The salty water cools there to high density and sinks, but as increasing light meltwater is added, the rate of sinking water decreases. As this surface escape valve for heat closes, that heat warms the deeper ocean, with maximum warming at 1-2 km depth. That’s the depth of ice shelf grounding lines, the part of the ice shelf that exerts strongest restraining force on landward ice [Fig. 14 of Jenkins and Doake (1991)]. West Antarctic ice shelves thus have begun to melt more rapidly (Rignot and Jacobs, 2002) and the ice streams feeding them have accelerated (Rignot, 2008).
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Menviel et al. (2010) used a simplified Earth system model to show that collapse of the West Antarctic ice sheet would cause expansion of sea ice on the Southern Ocean, suppression of Antarctic Bottom Water formation, and warming of the Southern Ocean at depth. Fogwill et al. (2015) used a high-resolution atmosphere-ocean model to investigate effects of increasing freshwater flux from West Antarctica today, finding that increased ocean stratification reduced bottom water formation and increased ocean temperature at depth. Fogwill et al. submitted their paper on almost the same date in 2015 that we submitted our paper. They concluded, however, that they saw no significant atmospheric response to the freshwater injection. We found a significant accompanying atmospheric feedback.
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Precipitation provides an amplifying feedback for sea level rise in our model, but a diminishing feedback in the climate models that IPCC has reported and relied on. Their models yield a large reduction of sea ice around Antarctica and increasing snowfall over the continent as Earth warms. This increased snowfall causes sea level to fall, thus at least partially offsetting sea level rise from ice sheet dynamical mass loss (Foreword Figure 2).
In our climate model described in Ice Melt, increasing meltwater cools the Southern Ocean surface enough to offset greenhouse gas warming. Indeed, the sea surface in the western portion of the Southern Ocean, where two-thirds of increased freshwater injection is occurring (Rignot et al., 2013), already has cooled while the rest of the planet has warmed (Fig. 31 in Ice Melt).
If high fossil fuel emissions continue, SMOC will shut down during the next few decades and sea ice in the Southern Ocean will expand several million square kilometers, according to our climate simulations (Foreword 4b). These effects should begin to emerge this decade from the “noise” level of unforced and unpredictable climate variability.
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Before the ink had dried on our Ice Melt paper, Antarctic sea ice cover plummeted (Foreword Figure 4b) to its lowest level in 40 years of satellite data (Parkinson, 2019). Antarctic Bottom Water (AABW) formation – the engine of SMOC – increased (Silvano et al., 2020). So, was the slowdown of SMOC over the prior few decades only temporary? Will Antarctic sea ice decrease now like Arctic sea ice, as predicted by IPCC models?
No, surely not. On the contrary, data that have accumulated since we submitted our paper in 2015 allow improved assessment of the basic time scales of the climate change problem. These time scales are central to our reframing of the ice melt problem and they are at the heart of our disagreement with conclusions of IPCC. One merit of our approach is the role of empirical data, which will allow continual, easily understandable, evaluations as climate response unfolds."
