The radiative forcing potential of different climate geoengineering options

This paper looks at various wheezes for interfering with global warming – apparently bio-char might be the a good bet, seeding the oceans with iron filings is likely a waste of time.
T. M. Lenton1,2 and N. E. Vaughan1,2
Atmos. Chem. Phys. Discuss., 9, 1–50, 2009
© Author(s) 2009.

Climate geoengineering proposals seek to rectify the Earth’s current radiative imbalance,
either by reducing the absorption of incoming solar (shortwave) radiation, or by
removing CO2 from the atmosphere and transferring it to long-lived reservoirs, thus
5 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
the global energy balance and pulse response functions for the decay of CO2 perturbations.
This aids transparency compared to calculations with complex numerical
10 models, but is not intended to be definitive. Already it reveals some significant errors in
existing calculations, and it allows us to compare the relative effectiveness of a range
of proposals. By 2050, only stratospheric aerosol injections or sunshades in space
have the potential to cool the climate back toward its pre-industrial state, but some
land carbon cycle geoengineering options are of comparable magnitude to mitigation
15 “wedges”. Strong mitigation, i.e. large reductions in CO2 emissions, combined with
global-scale air capture and storage, afforestation, and bio-char production, i.e. enhanced
CO2 sinks, might be able to bring CO2 back to its pre-industrial level by 2100,
thus removing the need for other geoengineering. Alternatively, strong mitigation stabilising
CO2 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 sustained on a millennial timescale and phosphorus addition probably has greater
long-term potential than iron or nitrogen fertilisation. Enhancing ocean upwelling or
downwelling have trivial effects on any meaningful timescale. Our approach provides
25 a common framework for the evaluation of climate geoengineering proposals, and our
results should help inform the prioritisation of further research into them.

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