The 80:20 estimate of land vs. ocean sources of marine pollution: fit for purpose?

An estimation that 80% of ocean pollution comes from land-based sources while 20% comes from ocean-based sources (the "80:20 estimate") has been oft-quoted in the scientific literature, across various media platforms, and in policy documents for decades. The estimation most likely derives from a study published by the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP), which provides authoritative, independent, interdisciplinary scientific information to United Nations (UN) sponsoring organisations with substantial maritime and ocean interests to support the protection, sustainable use and governance of the ocean. In the report (GESAMP 1990), GESAMP provided a rough estimation of relative contributions of all potential pollutants resulting from human activities and entering the ocean, positing that 44% was from land-based discharges and run-off, 33% from atmospheric inputs; 12% from maritime transportation, 10% from ocean dumping, and 1% from offshore production. These estimates represented an educated approximation based on review of select published studies and reports and suggested that ocean pollution derived mainly from land-based sources, while shipping and ocean dumping were lesser sources at that time.