Andrew, This is no contradiction between the Keith et al's commentary and this paper. Keith et al.'s paper is about stocks and this JGR paper is about the rate of flow of carbon between the atmosphere and the land biosphere (flux). The stocks and fluxes can behave very differently. The cooling caused by SRM reduces the rate of fluxing of carbon between the atmosphere and plants but overall it helps to build the carbon stocks in biomass and soils and hence reduce the atmospheric CO2.
Another good example for stocks and fluxes behaving very differently is the change in precipitation (flux) and atmospheric water vapor (stock) under global warming. It is well established now that precipitation increases at the rate of 2-3% per deg warming while water vapor increases at the rate of about 7% per deg warming. Best, Bala On Wed, Apr 29, 2020 at 10:18 AM Andrew Lockley <andrew.lock...@gmail.com> wrote: > Poster's note: this has the opposite sign to other work on the subject eg > https://keith.seas.harvard.edu/publications/solar-geoengineering-reduces-atmospheric-carbon-burden > > https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JD031883 > > Journal of Geophysical Research: AtmospheresVolume 125, Issue 9 > Research Article > A Model‐Based Investigation of Terrestrial Plant Carbon Uptake Response to > Four Radiation Modification Approaches > Lei Duan Long Cao Govindasamy Bala Ken Caldeira > First published:04 April 2020 > https://doi.org/10.1029/2019JD031883 > > Abstract > A number of radiation modification approaches have been proposed to > counteract anthropogenic warming by intentionally altering Earth's > shortwave or longwave fluxes. While several previous studies have examined > the climate effect of different radiation modification approaches, only a > few have investigated the carbon cycle response. Our study examines the > response of plant carbon uptake to four radiation modification approaches > that are used to offset the global mean warming caused by a doubling of > atmospheric CO2. Using the National Center for Atmospheric Research > Community Earth System Model, we performed simulations that represent four > idealized radiation modification options: solar constant reduction, sulfate > aerosol increase (SAI), marine cloud brightening, and cirrus cloud thinning > (CCT). Relative to the high CO2 state, all these approaches reduce gross > primary production (GPP) and net primary production (NPP). In high > latitudes, decrease in GPP is mainly due to the reduced plant growing > season length, and in low latitudes, decrease in GPP is mainly caused by > the enhanced nitrogen limitation due to surface cooling. The simulated GPP > for sunlit leaves decreases for all approaches. Decrease in sunlit GPP is > the largest for SAI which substantially decreases direct sunlight, and the > smallest for CCT, which increases direct sunlight that reaches the land > surface. GPP for shaded leaves increases in SAI associated with a > substantial increase in surface diffuse sunlight, and decreases in all > other cases. The combined effects of CO2 increase and radiation > modification result in increases in primary production, indicating the > dominant role of the CO2 fertilization effect on plant carbon uptake. > > Plain Language Summary > A number of radiation modification approaches have been proposed to > intentionally alter Earth's radiation balance to counteract anthropogenic > warming. However, only a few studies have analyzed the potential impact of > these approaches on the terrestrial plant carbon cycle. Here, we simulate > four idealized radiation modification approaches, which include direct > reduction of incoming solar radiation, increase in stratospheric sulfate > aerosols concentration, enhancement of marine low cloud albedo, and > decrease in high‐level cirrus cloud cover, and analyze changes in plant > photosynthesis and respiration. The first three approaches cool the earth > by reducing incoming solar radiation, and the last approach allows more > outgoing thermal radiation. These approaches are designed to offset the > global mean warming caused by doubled atmospheric CO2. Compared to the high > CO2 world, all approaches will limit plant growth due to induced surface > cooling in high latitudes and will lead to reduced nitrogen supply in low > latitudes, leading to an overall reduction in the plant carbon uptake over > land. Different approaches also produce different changes in surface direct > and diffuse sunlight, which has important implications for plant > photosynthesis. Relative to the unperturbed climate, the combined effects > of enhanced CO2 and radiation modifications leads to an increase in plants' > primary production. > > -- > 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 geoengineering+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/geoengineering/CAJ3C-04U16sw-Z9QCLEVXqs5Z0r0MRzGN0dfr-A8%2By_eaaYiag%40mail.gmail.com > <https://groups.google.com/d/msgid/geoengineering/CAJ3C-04U16sw-Z9QCLEVXqs5Z0r0MRzGN0dfr-A8%2By_eaaYiag%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: gb...@iisc.ac.in; bala....@gmail.com Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- 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 geoengineering+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CAD7fhVm5%2BJSa_a_pMqMtphMbeJMwV01aJde%2BmuH97myRrQ-aOw%40mail.gmail.com.
