Re: hysteresis
Yes, there are such effects. ASLR has posted on many, not a few on this very thread.
sidd
For convenience, the following are some selected extracts from some of my posts in this thread on hysteresis:
Furthermore, the hysteresis loops in Figures 3 & 4 make it clear that once the stratocumulus clouds dissipate (say partially due to a temporary decades-long perturbation like the collapse of the WAIS and associated feedbacks, and partially due to a temporary pulse of methane emission from Arctic thermokarst lakes [as well as a rapid reduction in anthropogenic aerosols]), it is difficult for them to reestablish themselves even at CO2-equivalent levels well below 1,200ppm.
Tapio Schneider , Colleen M. Kaul and Kyle G. Pressel (2019), "Possible climate transitions from breakup of stratocumulus decks under greenhouse warming", Nature Geoscience,
https://doi.org/10.1038/s41561-019-0310-1https://www.nature.com/articles/s41561-019-0310-1Abstract: "Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight. However, as their dynamical scales are too small to be resolvable in global climate models, predictions of their response to greenhouse warming have remained uncertain. Here we report how stratocumulus decks respond to greenhouse warming in large-eddy simulations that explicitly resolve cloud dynamics in a representative subtropical region. In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO2 levels rise above 1,200 ppm. In addition to the warming from rising CO2 levels, this instability triggers a surface warming of about 8 K globally and 10 K in the subtropics. Once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past. Such transitions to a much warmer climate may also occur in the future if CO2 levels continue to rise."
Extract: "The CO2 level at which the instability occurs depends on how largescale dynamics change with climate, which is heuristically parameterized in our simulations and hence is uncertain. In particular, the large-scale subsidence in the troposphere weakens under warming, which lifts the cloud tops and counteracts the instability. Indeed, when we weaken the parameterized large-scale subsidence by 1 or 3% per Kelvin of tropical SST increase (within the range of GCM responses to warming), the stratocumulus instability occurs at higher CO2 levels: around 1,400 ppm with 1% K–1 subsidence weakening, and around 2,200 ppm with 3% K–1 (Fig. 4). The hysteresis when the CO2 levels drop thereafter remains, but it narrows: stratocumulus decks reform once the CO2 levels drop below 500 ppm for a 1% K–1 subsidence weakening, and once they drop below 1,900 ppm for one of 3% K–1.
…
For the future, our results suggest that stratocumulus decks may break up if CO2 levels continue to rise. Equivalent CO2 concentrations around 1,300 ppm—the lowest level at which the stratocumulus instability occurred in our simulations—can be reached within a century under high-emission scenarios. However, it remains uncertain at which CO2 level the stratocumulus instability occurs because we had to parameterize rather than resolve the large-scale dynamics that interact with cloud cover. To be able to quantify more precisely at which CO2 level the stratocumulus instability occurs, how it interacts with large-scale dynamics and what its global effects are, it is imperative to improve the parameterizations of clouds and turbulence in climate models."
If nothing else, the findings of the linked reference could be used to better calibrate state-of-the-art ESM climate change projections w.r.t. 'the role of the Southern Ocean in abrupt transitions and hysteresis' in the MOC:
Sophia K.V. Hines, Andrew F. Thompson, Jess F. Adkins (15 March 2019), "The Role of the Southern Ocean in Abrupt Transitions and Hysteresis in Glacial Ocean Circulation", Paleoceanography and Paleoclimatology,
https://doi.org/10.1029/2018PA003415https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018PA003415Abstract: "High‐latitude Northern Hemisphere climate during the last glacial period was characterized by a series of abrupt climate changes, known as Dansgaard‐Oeschger events, which were recorded in Greenland ice cores as shifts in the oxygen isotopic composition of the ice. These shifts in inferred Northern Hemisphere high‐latitude temperature have been linked to changes in Atlantic meridional overturning strength. The response of ocean overturning circulation to forcing is nonlinear and a hierarchy of models have suggested that it may exist in multiple steady state configurations. Here, we use a time‐dependent coarse‐resolution isopycnal model with four density classes and two basins, linked by a Southern Ocean to explore overturning states and their stability to changes in external parameters. The model exhibits hysteresis in both the steady state stratification and overturning strength as a function of the magnitude of North Atlantic Deep Water formation. Hysteresis occurs as a result of two nonlinearities in the model—the surface buoyancy distribution in the Southern Ocean and the vertical diffusivity profile in the Atlantic and Indo‐Pacific basins. We construct a metric to assess circulation configuration in the model, motivated by observations from the Last Glacial Maximum, which show a different circulation structure from the modern. We find that circulation configuration is primarily determined by North Atlantic Deep Water density. The model results are used to suggest how ocean conditions may have influenced the pattern of Dansgaard‐Oeschger events across the last glacial cycle."
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The linked reference finds that:
"… Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years.
…
We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA – sourced from melting glaciers and thickened sea ice that developed earlier in the LIA – weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet."
These findings support Hansen's ice-climate feedback mechanism.
Thornalley et al. (2018), "Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years", Nature, doi:10.1038/s41586-018-0007-4
https://www.nature.com/articles/s41586-018-0007-4&
E. Gasson, R.M. DeConto, D. Pollard, and R.H. Levy (2016), "Dynamic Antarctic ice sheet during the early to mid-Miocene", Proceedings of the National Academy of Sciences, pp. 201516130, doi: 10.1073/pnas.1516130113
http://www.pnas.org/content/early/2016/02/17/1516130113Significance: "Atmospheric concentrations of carbon dioxide are projected to exceed 500 ppm in the coming decades. It is likely that the last time such levels of atmospheric CO2 were reached was during the Miocene, for which there is geologic data for large-scale advance and retreat of the Antarctic ice sheet. Simulating Antarctic ice sheet retreat is something that ice sheet models have struggled to achieve because of a strong hysteresis effect. Here, a number of developments in our modeling approach mean that we are able to simulate large-scale variability of the Antarctic ice sheet for the first time. Our results are also consistent with a recently recovered sedimentological record from the Ross Sea presented in a companion article."