Cloudier Skies

Abstract

Over the period 1901-2012, the average global sea surface temperature has increased by 0.89 °C due to climate change and it is expected to increase consistent with global warming. As a consequence, marine ecosystems have become more susceptible to change in species and ocean chemistry. Due to the increase in sea surface temperature, the ocean pH has decreased in all regions. The main driver for this development is the uptake of approximately 30% of anthropogenic carbon dioxide (CO2) by oceans (IPCC, 2014). In a warming ocean, populations of warm-water species grow and populations of cold-water species decline. Furthermore, declines in coral growth are observed (Poloczanska et al., 2016).

Marine cloud brightening is a proposed way to counteract climate change. This geo-engineering technique brightens clouds by spraying aerosols in the air which act as cloud condensation nuclei in order to form cloud droplets. As a result, the albedo of the brightened cloud increases and more sunlight is reflected, reducing the global mean temperature (Latham et al., 2012). Although it is a potential way to counteract climate change, marine cloud brightening has never been applied on a large scale. Also, little is known about its feasibility.

We determined how physical properties of sea salt aerosols, specifically the aerosol mean geometric radius, the aerosol number concentration and the modal standard deviation of the initial aerosol spectrum, relate to the albedo of the brightened cloud using a numerical cloud parcel model with input parameters (Glassmeier et al., 2020). For six different values of the initial aerosol mean geometric radius, the initial aerosol number concentration and the modal standard deviation of the initial aerosol spectrum, i.e. the variability in aerosol radius, three output variables were generated in time and height. These output variables were the mean radius of cloud droplets, the cloud droplet number and the relative supersaturation. The differences caused by varying the input parameters were interpreted for the output variables. Also, we generated the albedo for the six different values of each input parameter mentioned.

The cloud droplet number depends on the aerosol number concentration. The higher the number, the more cloud droplets are formed. The cloud albedo does not depend on the initial aerosol mean geometric radius. The moment of activation depends on the initial aerosol mean geometric radius and the initial aerosol number concentration. The growth of the cloud droplets is mainly driven by condensation when variability in aerosol radius is small. The higher the initial aerosol number concentration, the higher the albedo of the brightened cloud. When the variability in radius is bigger, collision-coalescence occurs as the spectrum of cloud droplets consists of bigger radii which tend to collide and coalesce. This leads to bigger and heavier droplets which precipitate due to an increased mass, resulting in a decrease in cloud droplet number and a decrease in cloud albedo.

To conclude, three relations between physical properties of sea salt aerosol and the cloud albedo were found. First, the aerosol mean geometric radius at start does not affect the cloud albedo. Secondly, we found that the higher the aerosol number concentration, the higher the cloud albedo. Furthermore, a higher modal standard deviation of the initial aerosol spectrum leads to a lower cloud albedo.

We recommend further research on other types of aerosols, like organic matter and algae, and the effect of external forcing, like wind, on cloud brightening.