UC San Diego

A major driver of both the Earth's natural climate variability and current climate change is the net solar input to the Earth system, i.e. the amount of incident solar radiation minus the fraction reflected back to space. Changes in this so-called albedo may have substantial effects on the Earth's climate. I use observations to address aspects of both the Arctic sea ice -albedo feedback and the albedo effects of aerosols on Indian Ocean cumulus clouds. In Chapter 2, I use satellite radiation and sea ice fraction measurements to document the Arctic-wide decrease in planetary albedo and its amplifying effect on global warming. I calculate that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, for an additional 6.4 ± 0.9 W/m² of solar energy input into the Arctic Ocean region. Averaged globally, this albedo decrease corresponds to a forcing 25% as large as that due to the change in CO₂ during this period. I then present results using field measurements made in the northern Indian Ocean, including ground station observations and aircraft measurements. In Chapter 3, I discuss the field operations and data processing. I then present results on the climatology of the Indian Ocean trade cumulus regime, and place these observations in the context of previous studies of trade cumulus clouds. In Chapter 4, I use subsets of the observations to isolate specific effects within the natural variability. I find a positive correlation between aerosol concentration and cloud liquid water for dry cases only, and determine that increased boundary-layer humidity lowering the cloud base is responsible for this effect. Large-scale analysis indicates that the high pollution cases originate with a highly-polluted air mass approaching the observatory from a northwesterly direction; this polluted mass was within the boundary layer and also exhibited higher temperatures and humidity, although the latter was observed to develop rather than disperse along with the air mass. A statistical analysis indicates that the relationship between aerosol and humidity is lagged. While not causation, the nature of this correlation suggests a potential effect of aerosol within this air mass enhancing the atmospheric humidity