Investigation of the surface and circulation impacts of cloud-brightening geoengineering

Projected increases in greenhouse gases have prompted serious discussion on geoengineering the climate system to counteract global climate change. Cloud albedo enhancement has been proposed as a feasible geoengineering approach, but previous research suggests undesirable consequences of globally uniform cloud brightening. The present study uses GFDL's Climate Model version 2G (CM2G) global coupled model to simulate cloud albedo enhancement via increases in cloud condensation nuclei (CCN) to 1000 cm^-3 targeted at the marine stratus deck of the Pacific Ocean, where persistent lowclouds suggest a regional approach to cloud brightening. The impact ofthis regional geoengineering on global circulationand climate in the presence of a 1% annual increase of CO₂ was investigated. Surface temperatures returned to nearpreindustrial levels over much of the globe with cloud modifications in place. In the first 40 years and over the 140-yr mean, significant cooling over the equatorial Pacific, continued Arctic warming, large precipitation changes over the western Pacific, and a westward compression and intensification of the Walker circulation were observed in response to cloud brightening. The cloud brightening caused a persistent La Nin~ a condition associated with an increase in hurricane maximumpotential intensity and genesis potential index, and decreased vertical wind shear between July and November in the tropical Atlantic, South China Sea, and to the east of Japan. Responses were similar with CCN=500 cm^-3.