Abstract
AbstractProposals to geoengineer Earth's climate by cirrus cloud thinning (CCT) potentially offer advantages over solar radiation management schemes: amplified cooling of the Arctic and smaller perturbations to global mean precipitation in particular. Using an idealized climate model implementation of CCT in which ice particle fall speeds were increased 2×, 4×, and 8× we examine the relationships between effective radiative forcing (ERF) at the top of atmosphere, near‐surface temperature, and the response of the hydrological cycle. ERF was nonlinear with fall speed change and driven by the trade‐off between opposing positive shortwave and negative longwave radiative forcings. ERF was −2.0 Wm−2 for both 4× and 8× fall speeds. Global mean temperature decreased linearly with ERF, while Arctic temperature reductions were amplified compared with the global mean change. The change in global mean precipitation involved a rapid adjustment (~ 1%/Wm2), which was linear with the change in the net atmospheric energy balance, and a feedback response (~2%/°C). Global mean precipitation and evaporation increased strongly in the first year of CCT. Intensification of the hydrological cycle was promoted by intensification of the vertical overturning circulation of the atmosphere, changes in boundary layer climate favorable for evaporation, and increased energy available at the surface for evaporation (from increased net shortwave radiation and reduced subsurface storage of heat). Such intensification of the hydrological cycle is a significant side effect to the cooling of climate by CCT. Any accompanying negative cirrus cloud feedback response would implicitly increase the costs and complexity of CCT deployment.
Highlights
Geoengineering Earth’s climate by intentional reduction of cirrus cloud coverage and optical thickness, referred to here as cirrus cloud thinning (CCT), has been suggested as a way to moderate climate change due to increased greenhouse gas concentrations [Mitchell and Finnegan, 2009]
Using an idealized climate model implementation of CCT in which ice particle fall speeds were increased 2×, 4×, and 8× we examine the relationships between effective radiative forcing (ERF) at the top of atmosphere, near-surface temperature, and the response of the hydrological cycle
We address the following question: How does precipitation respond to different changes in ice particle fall speed and which physical processes support any increase in global mean precipitation? We investigate whether effective radiative forcing (ERF) [Myhre et al, 2013] changes linearly with the scale of CCT, and what are the relationships between changes in global mean cloud cover, global temperature, Arctic temperature, and precipitation?
Summary
There were contrasting responses in global mean climate to CCT. Cloud cover decreased and the changes were greater with greater fall speeds (Figure 1a). Global mean near-surface temperature decreased (Figure 1b), but the changes were nonlinear with the fall speed change. Arctic temperature changes were amplified compared to global mean temperature changes and were greater with accelerated fall speed. In contrast to temperature changes, global mean precipitation increased with increasing fall speed change (Figure 1c) and the precipitation increase was strongly anticorrelated with the change in cloud cover (correlation coefficient À1.0)
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