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Currently, mankind has managed to warm the oceans with a trend of approximately 0.04 °C per decade for the upper 700 meters. Or, less than half a degree per century. The warming is much less than that on deeper levels of the oceans. Thus, it will take a fairly long time to reach anything like the conditions during the Early Eocene. It's on the scale of several millenia.
I'm curious, given this background, what is the relevance of your mentioning of the year 2100 in your comment?
What is your percieved timescale for Earth to develop an Early Eocene equable climate, given the amount of warming of the oceans involved?
I have posted many times on the numerous risks/factors/cascade-of-feedbacks that could push the Northern Hemisphere in to an equable pattern (hothouse pattern) by 2100, so those who are interested can type the word 'equable' into the search engine to learn more about my thinking on this topic. Nevertheless, as a 'lite' summary of key considerations of this possibility I present the following:
1. The primary characteristic of an equable climate (as occurred during the Eocene) is that ocean heat energy is conveyed directly from the tropical oceans (particularly the Tropical Pacific) poleward (& particularly to the Arctic). In this regard, Schneider et al. (2019) cites that the future risk of losing marine stratocumulus clouds (which currently produce a negative feedback) which would result in an abrupt increase in GMSTA. While Schneider et al. (2019) showed that an increase of atmospheric carbon dioxide concentration to about 1,200ppm, would result in such a loss of marine stratocumulus cloud, I previously pointed out in Reply #652 (see also Replies: #633, #642 & #650), the risk of abruptly losing the marine stratocumulus clouds would also occur if the equatorial SST increases from about 27C to about 32C. Note that that there is not need for the entire ocean to warm-up in order for the tropical ocean SST (particularly the Tropical Pacific) to increase from 27C to 32C this century; which could occur for a variety of reasons such as:
a) An abrupt collapse of much of the WAIS due to a collapse of the 'Big Ear' subglacial cavity and an associated collapse of the Thwaites Ice Tongue say circa 2030 could slow the MOC and/or;
b) A cascade of bipolar seesaw feedbacks, such as a surge of ice mass loss from Southern Greenland marine terminating glaciers like Jakobshavn (say between now and 2030) triggering an accelerated ice mass loss from the Eastern Antarctic Peninsula marine glaciers triggering a freshening of the Southern Ocean surface waters leading to increased upwelling of warm CDW across Antarctic continental shelves (see the first image); leading to an accelerated collapse of key Antarctic ice shelves leading to a partial collapse of the WAIS; which would slow the MOC and/or;
c) An abrupt release of relatively fresh water from the Beaufort Gyre (say due to a rainfall event on significant areas of Arctic Sea Ice circa 2030) flowing into the North Atlantic; which would slow the MOC; and/or;
d) The Eastern Tropical Pacific is predisposed to warming due to radiative forcing; which would likely be the case if ECS is roughly 5C (as indicated by many CMIP6 models) as indicated by the second attached image from the Ringberg 2015 workshop.
If indeed the tropical ocean SST does indeed warm up to 32C due to slowing of the MOC, then hysteresis loops shown in the third and fourth attached images [Figures 3 & 4 from Schneider et al. (2019)] make it clear that once the stratocumulus clouds dissipate and equable atmospheric condition in the Northern Hemisphere could occur.
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/s41561-019-0310-12. Even though a slowing of the MOC associated with freshwater hosing events might only last for decades; it is possible that an equable condition could be stabilized even without the CO2equiv reading 1,200pm due to possible future cascades of other tipping points, including:
a) Methane feedbacks such as thermokarst lakes or rapid permafrost degradation, and/or;
b) A rapid decrease of anthropogenic aerosol emissions due say to a socio-economic collapse between 2050 to 2070 and/or;
c) A marked increase of rainfall at high latitudes as is forecast to occur with global warming
3. The linked reference (Pistone et al 2019) calculates the radiative heating of a sea ice free Arctic Ocean during the sunlit part of the year and assuming constant cloudiness they '… calculate a global radiative heating of 0.71 W/m2 relative to the 1979 baseline state. This is equivalent to …' hastening global warming by an estimated 25 years. If the Northern Hemisphere were to flip into an equable pattern this century, this would lead to a sea ice free Arctic Ocean during the sunlit part of the year (particularly due to rainfall on the Arctic Sea Ice); which (together with bipolar seesaw interaction between the GIS and the AIS) might well be sufficient to maintain an equable climate pattern even after the multidecadal pulse of planetary energy imbalance associated with glacial ice mass loss from the GIS & the AIS.
Kristina Pistone et al. (20 June 2019), "Radiative Heating of an Ice‐free Arctic Ocean", Geophysical Research Letters,
https://doi.org/10.1029/2019GL082914https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL082914?af=R&
4. The linked reference (Massoud et al 2019) indicates that using consensus science (CMIP5) analyses the frequency of Atmospheric Rivers (ARs) will increase in frequency by about 50% and in intensity by about 25% by 2100. As an AR rainfall event on the GIS would greatly accelerate the bipolar seesaw mechanism, this might likely serve to maintain equable conditions for centuries past 2100:
E.C. Massoud et al. (12 October 2019), "Global Climate Model Ensemble Approaches for Future Projections of Atmospheric Rivers", Earth's Future,
https://doi.org/10.1029/2019EF001249https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019EF001249?af=RAbstractAtmospheric rivers (ARs) are narrow jets of integrated water vapor transport that are important for the global water cycle, and also have large impacts on local weather and regional hydrology. Uniformly‐weighted multi‐model averages have been used to describe how ARs will change in the future, but this type of estimate does not consider skill or independence of the climate models of interest. Here, we utilize information from various model averaging approaches, such as Bayesian Model Averaging (BMA), to evaluate 21 global climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Model ensemble weighting strategies are based on model independence and atmospheric river performance skill relative to ERA‐Interim reanalysis data, and result in higher accuracy for the historic period, e.g. RMSE for AR frequency (in % of timesteps) of 0.69 for BMA vs 0.94 for the multi‐model ensemble mean. Model weighting strategies also result in lower uncertainties in the future estimates, e.g. only 20‐25% of the total uncertainties seen in the equal weighting strategy. These model averaging methods show, with high certainty, that globally the frequency of ARs are expected to have average relative increases of ~50% (and ~25% in AR intensity) by the end of the century.
Plain Language SummaryAtmospheric rivers (ARs) are storms of integrated water vapor transport that are important for the global water cycle, and also have large impacts on local weather and regional hydrology. An increase in the frequency of ARs is expected to occur by the end of the century throughout most of the globe. Usually, these types of assessments of future climate change rely on simple (i.e. equally‐weighted) multi‐model averages and do not consider the skill or independence of the climate models of interest. Here, we utilize information from various model averaging approaches to constrain a suite of 21 global climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The weighted model combinations are fit to reanalysis data (ERA‐Interim) and are useful because they provide higher skill as well as lower uncertainties compared to equal weighting. This work supports the claim that AR frequency will increase in the future by about ~50% (and intensity will increase by ~25%) globally by the end of the century.
Edit: If what I wrote above is not clear, let me note that for the NH atmosphere to transition to an equable pattern, there is no need for the entire ocean to warm as much as what occurred during the Eocene, there is only a need for the average surface temperature of the tropical oceans to warm by 5C from 27C to 32C.
Edit2: Furthermore, there is no need for the atmospheric C02 concentration to increase to 1,200ppm.
