This project aims to improve our understanding of how aerosols (small, suspended particles in air) and aerosol / cloud interactions have contributed to historical climate variations in the North Atlantic and Pacific Oceans. It is well known that aerosols affect global climate both directly by (primarily) reflecting incoming sunlight; and indirectly through altering the amount and characteristics of low clouds that also (primarily) reflect sunlight. Two of the major processes include the “cloud brightness” effect (more aerosols tend to make clouds more reflective) and the “cloud lifetime” effect (aerosols tend to inhibit precipitation, and hence allow clouds to last longer). These processes vary regionally and are also a major source of uncertainty (especially aerosol / cloud interactions) in understanding historical climate variations.

This project is motivated by two major findings. First, observations and model simulations show that sulfate (from human emissions) and dust aerosol variations have played an important role in North Atlantic and Pacific climate variability, including variations in temperature and precipitation. Second, these aerosol variations are strongly seasonally dependent, especially for aerosol / cloud interactions in the North Atlantic and Pacific.

This project will advance our understanding of aerosols and aerosol / cloud interactions in the historical evolution of observed and modeled climate variations through three major activities:

1)    Historical observed and modeled North Atlantic and Pacific climate variability will be examined throughout the seasonal cycle, with an emphasis on understanding how seasonality affects long-term variability.

2)    The project will develop a method for estimating how important historical direct aerosol forcing and indirect (aerosol / cloud interactions) forcing have contributed to the seasonal climatic variations described above.

3)    A set of model simulations will be run to directly simulate the effect of sulfate aerosol variations (the primary contributor to historical anthropogenic aerosol variations) in the historical climate record.

This project will advance our understanding of how aerosols have affected seasonal, regional climate variations, as well as informing historical and future regional and seasonal climate predictability.