Collaborative Research: Evolving Hemispheric Albedo Asymmetry

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The albedo of the earth, meaning the fraction of incident sunlight that earth reflects back to space, is a critical control on the temperature of the planet, as higher albedo means less solar heating of the earth. Planetary albedo is a composite quantity determined by the reflection of sunlight from the relatively dark and thus low albedo ocean surface, lighter and thus higher albedo land surfaces, and the even higher albedos of snow and ice covered surfaces. Clouds add another layer of complexity as bright reflective clouds form intermittently over darker land and ocean. The complexity of planetary albedo is a challenge for efforts to determine the warming effect of greenhouse gas (GHG) increases since warming can cause a decrease in albedo, say due to the replacement of high-albedo sea ice with lower albedo ocean surface, which in turn can substantially enhance the GHG warming.Recent work shows that the albedos of the Northern and Southern Hemispheres are uncannily close: the sunlight received in each hemisphere is the same averaged over the year, and the amount reflected back to space is 99.7 watts per square meter in the Southern Hemisphere and 99.6 in the Northern Hemisphere (the resulting planetary albedo is about 29%). The sameness occurs despite the higher albedo of the Northern Hemisphere due to its larger land area, which yields a difference in hemispheric reflected sunlight of about 6 watts per square meter under clear skies. This difference must be compensated by clouds, and the work of Datseris and Stevens (2021) shows that the compensation is largely occurring over the middle- and high-latitude oceans, as the Southern Ocean is cloudier than the northern Pacific and Atlantic by about 11%. Datseris and Stevens also show that while planetary albedo has decreased appreciably over the past two decades the Northern and Southern Hemispheres have seen equivalent reductions, thereby retaining their "albedo symmetry". There is no theory for why clouds should compensate for surface albedo differences, and it is of course possible that the albedo symmetry of the recent record is a coincidence and not the result of a global-scale adjustment mechanism.Work under this award seeks to determine if an adjustment mechanism exists and if so to develop a theory for it. In addition to observational data from satellite missions and reanalysis products the work takes advantage of large ensembles of model simulations including simulations from the Coupled Model Intercomparison Project and a recent perturbed physics ensemble (PPE) created using the Community Atmosphere Model (CAM, the atmospheric component model of CESM, the Community Earth System Model). Results from analysis of these models are used to design additional simulations created with CAM, CESM, and a simplified version of CESM in which the ocean component model is replaced by a thermodynamic "slab" which simulates ocean heat storage but not ocean circulation. The work is of societal as well as scientific interest given the potentially large role that albedo changes could play in determining how much warming is caused by a given increase in GHG concentrations. A theory of albedo adjustment operating through cloud processes and on a global basis could provide valuable guidance in anticipating the magnitude of future climate change. The project also provides support and training for two graduate students, thereby providing for the future scientific work force in this research area. Undergraduate students are also involved in the project as summer interns.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.