Climate engineering through artificial enhancement of natural forcings: Magnitudes and implied consequences

Explosive volcanism and solar activity changes have modulated the Earth's temperature over short and century time scales. Associated with these external forcings were systematic changes in circulation. Here, we explore the effect of similar but artificially induced forcings that mimic natural radiative perturbations in order to stabilize surface climate. Injection of sulfate aerosols into the stratosphere, not unlike the effects from large volcanic eruptions, and a direct reduction of insolation, similar to total solar irradiance changes, are tested in their effectiveness to offset global mean temperature rise resulting from a business-as-usual scenario, thereby reducing surface temperatures to conditions associated with committed warming of a year 2000 stabilization scenario. This study uses a coupled Atmosphere-Ocean General Circulation Model to illustrate the character of resulting climate and circulation anomalies when both enhanced greenhouse (A2 scenario) and opposing geoengineering perturbations are considered. First we quantify the magnitude of the required perturbation and compare these artificial perturbations to the natural range of the respective forcing. Then, we test the effectiveness of the "correction" by looking at the regional climate response to the combined forcing. It is shown that widespread warming could be reduced, but overcompensation in the tropics is necessary because sea ice loss in high latitudes cannot be reversed effectively to overcome higher ocean heat content and enhanced zonal winter circulation as well as the continuous IR forcing. The magnitude of new, greenhouse gas-countering anthropogenic forcing would have to be much larger than what natural forcing from volcanoes and solar irradiance variability commonly provide.