The radiative forcing potential of different climate geoengineering options

Abstract

Abstract. Climate geoengineering proposals seek to rectify the Earth's current and potential future radiative imbalance, either by reducing the absorption of incoming solar (shortwave) radiation, or by removing CO 2 from the atmosphere and transferring it to long-lived reservoirs, thus increasing outgoing longwave radiation. A fundamental criterion for evaluating geoengineering options is their climate cooling effectiveness, which we quantify here in terms of radiative forcing potential. We use a simple analytical approach, based on energy balance considerations and pulse response functions for the decay of CO 2 perturbations. This aids transparency compared to calculations with complex numerical models, but is not intended to be definitive. It allows us to compare the relative effectiveness of a range of proposals. We consider geoengineering options as additional to large reductions in CO 2 emissions. By 2050, some land carbon cycle geoengineering options could be of comparable magnitude to mitigation "wedges", but only stratospheric aerosol injections, albedo enhancement of marine stratocumulus clouds, or sunshades in space have the potential to cool the climate back toward its pre-industrial state. Strong mitigation, combined with global-scale air capture and storage, afforestation, and bio-char production, i.e. enhanced CO 2 sinks, might be able to bring CO 2 back to its pre-industrial level by 2100, thus removing the need for other geoengineering. Alternatively, strong mitigation stabilising CO 2 at 500 ppm, combined with geoengineered increases in the albedo of marine stratiform clouds, grasslands, croplands and human settlements might achieve a patchy cancellation of radiative forcing. Ocean fertilisation options are only worthwhile if susCorrespondence to: T. M. Lenton (t.lenton@uea.ac.uk) tained on a millennial timescale and phosphorus addition may have greater long-term potential than iron or nitrogen fertilisation. Enhancing ocean upwelling or downwelling have trivial effects on any meaningful timescale. Our approach provides a common framework for the evaluation of climate geoengineering proposals, and our results should help inform the prioritisation of further research into them.

Carbon farming in hot, dry coastal areas: an option for climate change mitigation

“…These options may be separated into purely technological approaches such as sun shading, increase of surface albedo by whitening of buildings, and carbon capture and storage (CCS) (Boyd, 2008) or biogeoengineering options (see, e.g., Betts, 2007). A comparison of the effectiveness of different proposals can be found in Lenton and Vaughan (2009). However, this analysis disregards feedbacks in the water cycle.…”

Technical Note: Evaluating a simple parameterization of radiative shortwave forcing from surface albedo change

“…A value of 0.854 (Lenton and Vaughan, 2009) for T a was applied in Cherubini et al (2012) and the forcing results using this simple parameterization aligned well with those calculated using a more sophisticated radiative transfer model (Fu-Liou, 2005;Liou, 1992, 1993) that required detailed prescriptions of cloud and aerosol optical properties. Although −R TOA (t) may be applied with K T (t) to compute the incident flux at surface, −R s (t, i), this surface flux could also be taken directly from Nasa SSE/POWER for similar spatial (1×1 degree) and temporal resolutions (monthly, 1983-2005):…”

“…where −R TOA (t, i) is the local incoming extraterrestrial solar flux at TOA in time step t and region i, which is a function of latitude, K T (t, i) is the fraction of local −R TOA that reaches the surface in time step t ("all-sky clearness index") and region i, which may be considered the downwelling transmittance coefficient for the single atmospheric layer, α s (t, i) is the local change in surface albedo in time step t and region i, and T a is a constant denoting the globally averaged annual fraction of upwelling shortwave radiation exiting a clear sky, which is around 80-85 % (Lenton and Vaughan, 2009;Kiehl and Trenberth, 1997). A value of 0.854 (Lenton and Vaughan, 2009) for T a was applied in Cherubini et al (2012) and the forcing results using this simple parameterization aligned well with those calculated using a more sophisticated radiative transfer model (Fu-Liou, 2005;Liou, 1992, 1993) that required detailed prescriptions of cloud and aerosol optical properties.…”

Do we (need to) care about canopy radiation schemes in DGVMs? Caveats and potential impacts

“…To quantify the influence of a factor that changes the balance of incoming and outgoing energy in an Earth-atmosphere system, the concept of radiative forcing (RF) has been developed (IPCC, 2007). Radiative forcing has been widely used to quantify the global impact of regional changes in surface albedo (Hall and Qu, 2006;Pongratz et al, 2009;Lenton and Vaughan, 2009;Bright et al, 2012) and can be calculated in various ways. Here, we applied a simple method to estimate RF Cherubini et al, 2012).…”