Solar geoengineering may not prevent strong warming from direct effects of CO2 on stratocumulus cloud cover

[Andrew Lockley]

https://www.pnas.org/content/early/2020/11/10/2003730117.short

Solar geoengineering may not prevent strong warming from direct effects of CO2 on stratocumulus cloud cover

 View ORCID ProfileTapio Schneider,  View ORCID ProfileColleen M. Kaul, and Kyle G. Pressel

PNAS first published November 16, 2020; https://doi.org/10.1073/pnas.2003730117

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Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 7, 2020 (received for review February 27, 2020)

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Significance

Solar geoengineering that manipulates the amount of sunlight Earth absorbs is increasingly discussed as an option to counter global warming. However, we demonstrate that solar geoengineering is not a fail-safe option to prevent global warming because it does not mitigate risks to the climate system that arise from direct effects of greenhouse gases on cloud cover. High-resolution simulations of stratocumulus clouds show that clouds thin as greenhouse gases build up, even when warming is modest. In a scenario of solar geoengineering that is sustained for more than a century, this can eventually lead to breakup of the clouds, triggering strong (5°C), and possibly difficult to reverse, global warming, despite the solar geoengineering.

Abstract

Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO2 can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.

global warminggeoengineeringcloud feedback 

[Douglas MacMartin]

This is a really interesting nonlinear mechanism, whereby high levels of CO2 might result in more warming than our models currently project, and with hysteresis (so that once you lose the clouds, you don’t get them back by cooling).  But worth keeping in mind that their simulations were for 1700 ppm (~6x CO2); unclear whether or how much the mechanism might play a role at lower CO2 levels, but if we let CO2 get that high, we’re pretty much screwed anyway.  Doesn’t say that SG would be bad (or good), just says that if we burn every ounce of fossil carbon in the ground, even SG might not save us.  Hopefully we won’t have to test that hypothesis…

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[SALTER Stephen]

Hi All

 

This is a very interesting result and an important warning about the dangers of 1700 parts per million CO2.  It might be possible if difficult to selectively breed more intelligent politicians in the next a hundred years but we would need to know what to do with rejects.

 

I am not quite so worried about scattered stratocumulus clouds because this indicates  a longer life for condensation nuclei.  We want a low dose over a wide area and to avoid high local concentrations that we would get from a moving point source.  The graphs below of the Twomey effect (via Schwartz and Slingo) show how reflectivity changes as a function of nuclei concentration for different different cloud thicknesses and water contents.

 

 

Start on any red or blue curve near the left of the graph.  Move to the right along a thick black line to increase nuclei concentration and then upwards to get back to the curve you chose.  Then repeat moving twice as far in the nuclei per cm3 direction each time to get successive doublings of nuclei concentration.  Each doubling gives almost the same black step increase  in reflectivity except for the very thinnest clouds.  Cloud thickness and water content are less important than nuclei concentration.

 

Breathe safely

 

Stephen

 

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