Understanding the radiative impact of ship emissions through both bottom-up and top-down approaches

The radiative impact of ship emissions, mostly through iteracting with marine low clouds, is uncertain. Survey of literature shows an almost 1000-fold difference in its magnitude. Here we use detected ship-tracks, bottom-up, and top-down geospatial krigging, top-down, to constrain its magnitude in the Southeast Atlantic. We show that physics derived based on bottom-up approach provides similar estimate of the forcing estimate as the top-down approach. With the derived physics, we further estimate the forcing of the total ship emissions. In particular, the forcing due to the recent IMO 2020 is estimated to play a significant role in driving short-term additional warming. We discuss the policy implications of our estimate in terms of regulations and geoengineering.