Introduction
An often-quoted estimate holds that 80% of marine pollution originates from land-based sources, while 20% comes from ocean-based sources (the « 80:20 estimate »). This figure has been widely cited in scientific literature, media, and policy documents for decades. It likely originates from a study published by the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP), which provides authoritative and independent interdisciplinary scientific advice to UN agencies with marine and oceanic mandates.
In its 1990 report, GESAMP offered a rough breakdown of the relative contributions of human activity to ocean pollution: 44% from land-based discharges and runoff, 33% from atmospheric inputs, 12% from maritime transport, 10% from ocean dumping, and 1% from offshore production. These estimates were educated approximations based on select studies and reports and suggested that most marine pollution came from land-based sources, while shipping and dumping at sea were minor contributors at the time.
A Frequently Quoted « Statistic »
Since the GESAMP study, a large volume of data has emerged on various marine pollutants, from untreated wastewater to plastics, nutrients, and other chemicals. A quick search for scientific articles on pollution sources almost invariably yields references to the 80% land-based figure, often without a source. When citations do exist, they are usually UN reports that themselves failed to cite the original study. Thus, a vague, decades-old estimate has taken on the aura of scientific fact in academic and popular discourse.
However, as more data has become available and regulations have reduced the entry of certain substances into marine environments, it is time to reassess the accuracy and utility of the 80:20 estimate.
Scientifically, there are two main problems with the 80:20 figure. First, any statement about the proportion of land- versus ocean-based pollution must specify the pollutant in question. The blanket term « pollution » glosses over massive differences in the quantities and impacts of various substances—from tens of millions of metric tons of plastic and nitrogen annually (Jambeck et al., 2015; Tivig et al., 2021) to a few thousand tons of trace metals like mercury (Zhang et al., 2023).
Some pollutants, like untreated wastewater, are clearly land-based, while others—such as ghost fishing gear in the North Pacific Gyre—come largely from ocean-based sources (LeBreton et al., 2018). Treating all pollutants as one undifferentiated whole ignores these key distinctions.
Second, while the 80:20 figure may have served to set a global context and encourage broad awareness, from a policy standpoint, pollution data is far more actionable at smaller geographic scales. It is at the level of specific seas, coastal regions, bays, and estuaries that pollution threats must be quantified and addressed through targeted regulation and governance.
In a time when scientific data is frequently questioned or dismissed, it is essential to clearly articulate the evidence behind our understanding—and acknowledge what lacks rigorous scientific basis. The 80:20 estimate is not based on a globally comprehensive, scientifically validated assessment and should not be cited as such.
Discussion
GESAMP has considered revisiting the 80:20 estimate by conducting a global meta-analysis of data for specific pollutants (e.g., plastics, nitrogen). However, given the limited practical value of a global review for policymaking, such a project would be more academic than actionable. Instead, it would be more useful to build robust scientific knowledge on selected key marine pollutants at regional, national, and local levels (Hatje et al., 2024), focusing on:
- Accurate data (concentrations, flows) of selected pollutants by source, in defined areas and/or timeframes;
- Pollutants of greatest concern due to environmental, human, or animal health impacts;
- Emerging data gaps and concerns related to sources and impacts;
- Temporal and geographic trends in pollution sources and impacts;
- Root causes of key pollutants, including governance or market failures.
It is acknowledged that the 80:20 estimate helped translate science into policy by crystallizing a growing body of marine pollution data into a simple message that raised awareness and spurred action. But despite this communicative value, the broad terminology (« pollution ») and global scale of the estimate render it scientifically weak and of limited policy relevance.
Conclusion
The 80:20 estimate should be retired from scientific usage. Instead, efforts to tackle marine pollution should focus on collecting and analyzing data for priority and emerging pollutants in contaminated areas at smaller geographic scales — regional, national, and local. This more targeted, evidence-based approach will better support effective policy and management responses to the pressing challenge of marine pollution.
