Sorry- CDR is imperfect solution?

[Greg Rau]

https://www.science.org/doi/10.1126/sciadv.adg1801

“ A poleward shift of the Hadley cell (HC) edge in a warming climate, which contributes to the expansion of drought-prone subtropical regions, has been widely documented. The question addressed here is whether this shift is reversible with CO2 removal. By conducting large-ensemble experiments where CO2 concentrations are systematically increased and then decreased to the present-day level, we show that the poleward-shifted HC edge in a warming climate does not return to its present-day state when CO2 concentrations are reduced. While the Southern Hemisphere HC edge remains poleward of its present-day state, the Northern Hemisphere HC edge ends up farther equatorward of its present-day state. Such hemispherically asymmetric HC edge changes are closely associated with the changes in vertical wind shear in the subtropical atmosphere, which result from the long adjustment time of the ocean response to CO2 removal. Our findings suggest that CO2 removal may not guarantee the recovery of the subtropical dryness associated with the HC changes.”

GR So what happened during glacial/interglacial CO2 changes (about 190<—>280ppm)?

[Shannon A. Fiume]

Can anyone with more experience with CESM 1.2.2, or more current model ensembles tell us if it/they can output that data, when CMIP6 1pctCO2-cdr is run, or more ideally something that restores in a shorter timeframe (model anthropogenic increases: truncated SSP 1 1.9 to 430ppm, 435ppm, etc, SSP 1 1.9, coupled to CDR: remove cumulative anthropogenic emissions)?

Greg, Oddly they didn't run exactly CMIP6 1pctCO2-cdr, they started from 367 ppm instead of pre-industrial (278ppm) returned it to about 367 ppm over the 140 years. On the ramp up, they quadrupling CO2 to 1468 ppmv following the 1% yearly increase from 367 ppm. This is more in style to SSP 5 8.5, it's not anywhere close to a pathway that avoids tipping points.

(CMIP6 1pctCO2-cdr https://view.es-doc.org/?renderMethod=id&project=cmip6&id=6a465822-ac43-4b3e-beb4-388d291860d4&version=1)

I'm surprised they were allowed to publish without noting what type of modeling experiment was used in the abstract. In the abstract the authors don't qualify the conditions for CO₂ additions necessitating CO₂ removal, and many non-climate scientists are left to assume this applies to CO₂ removal as a whole, rather than something that would happen if we allowed something catastrophic such as unabated fossil fuel use like in SSP 5 8.5.

~~sa

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[Ken Caldeira]

The northern hemisphere, because it has more land, warms up and cools down faster than the southern hemisphere.

Thus, in warming to a stationary state, the northern hemisphere is warmer than the southern hemisphere.

In cooling from a stationary state, the northern hemisphere is cooler than the southern hemisphere.

Therefore, one should expect some asymmetry.

I think the final statement of their abstract is overstated.

Our findings suggest that CO2 removal may not guarantee the recovery of the subtropical dryness associated with the HC changes.

Should be qualified:

Our findings suggest that while the climate system is in a transient state, adjusting to the effects of CO2 removal, ....

To view this discussion on the web visit https://groups.google.com/d/msgid/CarbonDioxideRemoval/0552a3ec-fce0-c557-f8ff-a8af021fe63a%40autofracture.com.

[Bruce Melton -- Austin, Texas]

All is not lost. This work has a quite long time period with a peak CO2 at 2140. Modeling is all about the scenarios. One of the fundamental tenets of system collapse is the point of no return - commonly referred to as the "tipping point" in academic literature. I would speculate the reason this modeling says that poleward expansion of the Northern Hemisphere Hadley Cell is mostly irreversible is the amplifying feedback from desertification, though they mention AMOC degradation as well.  Restoration is not just about returning to the Holocene, it is about returning to the Holocene in time frames that matter to the point of no return. Hansen tells us system collapses can survive overshoot, if the perturbation is removed before the point of no return where he specifically cites expansion of the subtropics (Hansen 2008).

&&& not sure about the Hadley Cell during glacial pulses...

Hot in Austin,

B

Hansen 2008 – Evolutionary boundaries of our Earth systems in the Holocene, Tipping and The Point of No Return… (First use of "point of no return.") We define: (1) the tipping level, the global climate forcing that, if long maintained, gives rise to a specific consequence, and (2) the point of no return, a climate state beyond which the consequence is inevitable, even if climate forcings are reduced. A point of no return can be avoided, even if the tipping level is temporarily exceeded. Ocean and ice sheet inertia permit overshoot, provided the climate forcing is returned below the tipping level before initiating irreversible dynamic change… Points of no return are inherently difficult to define, because the dynamical problems are nonlinear. Existing models are more lethargic than the real world for phenomena now unfolding, including changes of sea ice [65], ice streams [66], ice shelves [36], and expansion of the subtropics [67, 68]."
Hansen et. al., Target Atmospheric CO2 - Where should humanity aim?, Open Atmospheric Science Journal, August 2008.
https://openatmosphericsciencejournal.com/contents/volumes/V2/TOASCJ-2-217/TOASCJ-2-217.pdf

RESULTS (Kim 2023 from Greg's previous email)

HC edge changes under a changing CO₂ pathway

The atmospheric CO₂ concentrations are first increased and then decreased in a simple and symmetric pathway (black line in Fig. 1A). Starting from the present-day (PD) state [~367 parts per million volume (ppmv)], CO₂ concentrations are increased at a rate of 1% year⁻¹ in the ramp-up (RU) period until quadrupled and then decreased at the same rate in the ramp-down (RD) period until PD CO₂ concentrations are reached (see Materials and Methods). CO₂ concentrations are then maintained during the remaining period, which is referred to as the restoring (RST) period.

Bruce Melton PE
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