Caldeira Lab

Albedo Enhancement of Marine Clouds To Counteract Global Warming: Impacts On the Hydrological Cycle

Govindasamy Bala, Ken Caldeira, Rama Nemani, Long Cao, George Ban-Weiss & Ho-Jeong Shin

Reducing the amount of sunlight absorbed by the earth could counter the effects of global warming. Previous studies have shown that solar radiation "geoengineering" proposals could offset global temperature increases, but could cause a drier land surface. Here we show that increasing sunlight reflections of clouds over the oceans could offset global temperature increases and cause a wetter land surface.

Bala, G., K. Caldeira, R. Nemani, L. Cao, G. Ban-Weiss, and H.-J. Shin, 2011: Albedo enhancement of marine clouds to counteract global warming: impacts on the hydrological cycle. Climate Dynamics, 37, 915–931, doi:10.1007/s00382-010-0868-1.


Recent studies have shown that changes in solar radiation affect the hydrological cycle more strongly than equivalent CO2 changes for the same change in global mean surface temperature. Thus, solar radiation management ‘‘geoengineering’’ proposals to completely offset global mean temperature increases by reducing the amount of absorbed sunlight might be expected to slow the global water cycle and reduce runoff over land. However, proposed countering of global warming by increasing the albedo of marine clouds would reduce surface solar radiation only over the oceans. Here, for an idealized scenario, we analyze the response of temperature and the hydrological cycle to increased reflection by clouds over the ocean using an atmospheric general circulation model coupled to a mixed layer ocean model. When cloud droplets are reduced in size over all oceans uniformly to offset the temperature increase from a doubling of atmospheric CO2, the global-mean precipitation and evaporation decreases by about 1.3% but runoff over land increases by 7.5% primarily due to increases over tropical land. In the model, more reflective marine clouds cool the atmospheric column over ocean. The result is a sinking motion over oceans and upward motion over land. We attribute the increased runoff over land to this increased upward motion over land when marine clouds are made more reflective. Our results suggest that, in contrast to other proposals to increase planetary albedo, offsetting mean global warming by reducing marine cloud droplet size does not necessarily lead to a drying, on average, of the continents. However, we note that the changes in precipitation, evaporation and P-E are dominated by small but significant areas, and given the highly idealized nature of this study, a more thorough and broader assessment would be required for proposals of altering marine cloud properties on a large scale.



Figure: Changes in global and annual mean precipitable water, precipitation (P), evaporation (E) and (P-E) due to increase in CO2 content (2 x CO2 - 1 x CO2), reduction in marine cloud droplet size (2 x CO2 + CCN - 2 x CO2) and a combination of both increased CO2 content and reduced droplet size (2 x CO2 + CCN - 1 x CO2). The hatching indicates regions where the changes are not significant at the 1% level. Significance level is estimated using a Student t test with a sample of 40 annual means and standard error corrected for autocorrelation (Zwiers and von Storch 1995). Precipitation changes are statistically significant at the 1% level over 46 (50), 58 (57), and 37 (31) % of the globe (land area) in the three comparisons, respectively. The corresponding fractions for evaporation are 85 (79), 89 (78) and 47 (44) % indicating higher confidence in evaporation changes. These fractions decrease to 39 (35), 46 (39) and 31 (22) % for (P-E), indicating slightly reduced confidence in regional scale changes in runoff