Post-2020 Changes in Shipping Emissions: Impact on Aerosol-Driven Changes in the Direct and Indirect Forcing of Climate

A major discussion and ongoing concern in the climate science community is to what extent the post-2020 acceleration in global mean surface temperatures reflects simple internal climate variability or whether the acceleration reflects a new and potent change in forcing or feedback. One recent and potentially large change in radiative forcing are new rules promulgated by the International Maritime Organization in 2020 requiring international ships to greatly reduce their sulfur emissions. Thus, it is possible that aerosol related emissions from ships have been substantially reduced — effectively the inverse of what is proposed for marine cloud brightening. In a collaboration between the California Institute of Technology and the University of California, Los Angeles recent airborne in situ observations of the changing chemical composition of ship emissions will be investigated. Simulations using a state-of-the-science chemical transport model will be used to quantify the impact of these changes on global cloudiness and radiative forcing.