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…
From: [email protected] <[email protected]> On Behalf Of Andrew Lockley Sent: Tuesday, November 17, 2020 2:38 PM To: geoengineering <[email protected]> Subject: [geo] Solar geoengineering may not prevent strong warming from direct effects of CO2 on stratocumulus cloud cover 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 Add to Cart ($10) Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 7, 2020 (received for review February 27, 2020) Article Figures & SI Info & Metrics PDF 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 -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CAJ3C-04C%2BAn%3DPFaR%3DSOehEH012HcF1HzVjd4sJEhsW%3Degx9ZKQ%40mail.gmail.com<https://groups.google.com/d/msgid/geoengineering/CAJ3C-04C%2BAn%3DPFaR%3DSOehEH012HcF1HzVjd4sJEhsW%3Degx9ZKQ%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CH2PR04MB6936EF1191B8A6A4FC4F3C168FE20%40CH2PR04MB6936.namprd04.prod.outlook.com.
