Could reflecting sunlight substitute for reducing carbon dioxide?
Short answer. Partly, and less cleanly than the global averages suggest. Reducing sunlight to offset doubled CO2 diminishes regional and seasonal climate change far more than the mismatch in forcing patterns would imply, and a polar-weighted aerosol distribution does better still. But CO2 changes the hydrological cycle and plant physiology in ways sunlight does not, so the substitution is inherently incomplete — and because the climate responds within years, stopping abruptly would produce rapid warming.
Why the question matters
Solar geoengineering is unusual among climate measures in being cheap enough that cost will not decide how much is done. That makes risk, and the governance of risk, the binding constraint — a single actor could in principle affect the entire planet.
The question of how closely sunlight reduction can imitate a low-CO2 world is not rhetorical. It determines whether the residual climate change left behind is a detail or the main event, and the answer differs sharply between temperature and rainfall.
Because the intervention masks warming rather than removing its cause, the underlying CO2 commitment continues to accumulate behind it — which is what makes the speed of the climate's response a liability as well as a convenience.
What our research finds
- Each 0.1 of interhemispheric difference in stratospheric aerosol optical depth shifts the intertropical convergence zone 1.8° of latitude and cuts northern hemisphere monsoon precipitation by 6.9 per cent (Roose et al., 2023).
- Sulfate aerosols cool more when placed higher in the stratosphere even with the amount held fixed, because heating the layer they occupy alters stratospheric water vapour, tropospheric stability and cloud, and so the effective radiative forcing itself (Krishnamohan et al., 2019).
- Offsetting doubled-CO2 warming needs different forcing from each scheme — stratospheric aerosol is about 15 per cent less efficacious than CO2, marine cloud brightening and cirrus thinning about 10 per cent more — and each leaves a different hydrological cycle (Duan et al., 2018).
- Combining stratospheric aerosol with cirrus cloud thinning restores both global mean temperature and precipitation to preindustrial at once, which neither can do alone — though regional agreement is no better than aerosol alone (Cao et al., 2017).
- Solar geoengineering's deployment will be limited primarily by considerations of risk because it is cheap, whereas carbon dioxide removal's will be limited primarily by cost (Caldeira et al., 2013).
- Within days of a step change in forcing, added CO2 suppresses evaporation and precipitation over ocean and reduces plant transpiration over land, while equivalent added sunlight does neither — differences that exist before any surface warming and so cannot be tuned away (Cao et al., 2012).
- Holding temperature at present-day levels in a doubled-CO2 world raised modelled global yields relative to that unmanaged world by 14 per cent for maize, 21 per cent for wheat, and 8 per cent for rice, because heat stress is removed while CO2 fertilization remains (Pongratz et al., 2012).
- Removing a quadrupled-CO2 forcing raises global precipitation 5.1 per cent within a year while removing an equivalent solar forcing lowers it immediately, because the fast precipitation adjustment is about -4.2 per cent per CO2 doubling but near zero for sunlight (Cao et al., 2011).
- Brightening marine clouds to offset doubled CO2 lowers global precipitation by about 1.3 per cent yet raises runoff over land by 7.5 per cent, because dimming only the ocean drives sinking air over sea and rising air over land (Bala et al., 2011).
- A stratospheric aerosol loading weighted toward the poles rather than the tropics comes closest to reproducing a low-CO2 climate in a high-CO2 world (Ban-Weiss and Caldeira, 2010).
- The apparent difference in hydrological sensitivity between solar and carbon dioxide forcing lies entirely in the fast response; the slow response of the water cycle is the same whichever forcing drives it (Bala et al., 2010).
- Global and Arctic-only insolation reductions fall on the same line, so the global mean temperature response per unit top-of-atmosphere albedo change is relatively insensitive to the latitude at which sunlight is deflected (Caldeira and Wood, 2008).
- The climate responds to reduced insolation within years, so there may be little cost to delaying deployment — and by the same token, abrupt termination would produce rapid warming (Matthews and Caldeira, 2007).
- Geoengineered temperature patterns resemble preindustrial ones but precipitation patterns do not, and carbon sinks strengthen because warming is masked while CO2 continues to rise (Matthews and Caldeira, 2007).
- Reduced solar luminosity largely cancels the regional and seasonal climate change from quadrupled CO2, but leaves tropical cooling and drying, incomplete polar compensation, a still-cold stratosphere and sea ice not fully restored (Govindasamy et al., 2003).
- Reducing solar luminosity to balance doubled CO2 markedly diminishes regional and seasonal climate change, not merely the global mean, despite the two forcings having very different spatial patterns (Govindasamy and Caldeira, 2000).