Nature has a brief report on a PNAS paper, “Transient climate-carbon simulations of planetary geoengineering” by H. Damon Matthews and Ken Caldeira. BTW, before I get going, look at the sidebar on the right of PNAS – there is a “google scolar” search link built in. The abstract is:
Geoengineering (the intentional modification of Earth’s climate) has been proposed as a means of reducing CO2-induced climate warming while greenhouse gas emissions continue. Most proposals involve managing incoming solar radiation such that future greenhouse gas forcing is counteracted by reduced solar forcing. In this study, we assess the transient climate response to geoengineering under a business-as-usual CO2 emissions scenario by using an intermediate-complexity global climate model that includes an interactive carbon cycle. We find that the climate system responds quickly to artificially reduced insolation; hence, there may be little cost to delaying the deployment of geoengineering strategies until such a time as “dangerous” climate change is imminent. Spatial temperature patterns in the geoengineered simulation are comparable with preindustrial temperatures, although this is not true for precipitation. Carbon sinks in the model increase in response to geoengineering. Because geoengineering acts to mask climate warming, there is a direct CO2-driven increase in carbon uptake without an offsetting temperature-driven suppression of carbon sinks. However, this strengthening of carbon sinks, combined with the potential for rapid climate adjustment to changes in solar forcing, leads to serious consequences should geoengineering fail or be stopped abruptly. Such a scenario could lead to very rapid climate change, with warming rates up to 20 times greater than present-day rates. This warming rebound would be larger and more sustained should climate sensitivity prove to be higher than expected. Thus, employing geoengineering schemes with continued carbon emissions could lead to severe risks for the global climate system.
Nature picked out the bit about precip: it gets drier. The commentary explains this: more CO2 but constant T means more efficient veg use so lower evap so lower (tropical?) ppn. But the commentary also points out that this may only be a model effect.
As for the rest, its fairly obvious: put in stratospheric aerosols and you can reduce T. This isn’t the first paper to do this (though its the first one I’ve noticed); it claims to be the first to do it in a transient rather than 2*CO2 model; but its only an EMIC. And in fact they don’t “put in aerosols”; they just reduce incoming solar to match CO2 forcing. The T response (fig 1) is then fairly small tropical cooling and polar warming. The other “exciting” but bleedin’ obvious result is that if the geoengineering fails then it gets warmer very quickly.
I should probably add… one of the problems with these schemes is to achieve a global balance. For example, there was a scheme to increase the albedo of marine stratocumulus regions. Fine, but if done enough to keep *global* T constant it would have caused massive local decreases whilst allowing massive polar warming. This paper is “cheating” by allowing themselves to do it trivially globally.
Overall: interesting to see people doing this as a thought experiment. Needs a lot of refinement to be more realistic. Would be scary if done in reality.