Cumulative human impact assessments in the open oceans (or ‘high seas’) evaluate the combined effect of multiple human stressors on marine ecosystems. Because open ocean areas are far from land, land-based pollution, coastal development and other nearshore pressures have essentially no direct impact on them. However, many stressors have global scope, and are interacting in the high seas to create patterns of total impact different from the impact from any individual stressor.

Within the Food and Agricultural Organisation (FAO) reporting regions for the high seas, overall cumulative impact is highest in the northeast and central Atlantic and the northwest and west central Pacific. It is lowest in the Arctic. Cumulative impact in all regions is driven by climate change variables (particularly sea surface temperature and ocean acidification) and commercial shipping. In addition, commercial fishing of demersal fish (which live and feed on or near the bottom of seas) and pelagic fish (which live in the open water) have higher impacts in a few regions, most notably the western central Pacific.

Most resource management and conservation efforts to date have focused on single issues such as fisheries management, climate change, or pollution. Although each of these issues is important on its own, it is only possible to understand the full nature of human impacts on the oceans by combining these various stressors into an assessment of their combined impacts. This is also the only way to effectively manage for each of the individual pressures.

Cumulative human impact assessments provide a comprehensive overview that can inform strategies to target or prioritize key pressures and restore or mitigate heavily impacted regions. These assessments also identify places that may require additional protection to preserve relatively unimpacted conditions.

Cumulative human impact assessments measure and map the intensity of human stressors. They then estimate the potential impact of those stressors on different marine ecosystems by accounting for differing vulnerability of each ecosystem type to each stressor.

In this assessment, 19 different human stressors were assessed, including land-based, climate change, fishing, and commercial activity stressors. Only 11 of these stressors existed in the high seas. In the high seas, habitat data were available for five different habitats and their expected vulnerability to each of the stressors. The habitats included surface and deep pelagic habitats, seamounts, and hard- and soft-bottom deep benthic habitats including the sediment surface and some sub-surface layers.

Data for all stressors and habitats were mapped at 1km² resolution, and then the single-stressor and cumulative impact scores were calculated and summarized by each of the 18 high seas regions, following FAO reporting regions. These regions include western and eastern halves of the north, central and southern Atlantic and Pacific Oceans; western and eastern Indian Ocean; Pacific, Atlantic and Indian regions of the Southern Ocean, and the Arctic.

Stressor data come from a variety of sources, including satellites measuring UV, sea surface temperature and commercial shipping. Regionally-reported data were down-scaled to finer resolutions based on various modeling approaches including commercial fishing and sea level rise. Data for all stressors came from the most recent available year, generally 2012 or 2013.

Habitat data was based on the best available maps for deep-sea habitats, noting that pelagic habitats exist everywhere in the high seas.

A lot of what happens in the high seas is difficult to track and monitor, most notably the exact location and catch of commercial fish and the location and condition of benthic habitats. Small-scale patterns of human impact within high seas regions are thus uncertain estimates. At the scale of entire high seas regions, however, results for individual and cumulative impacts from human stressors should be fairly robust. Compared to coastal waters, relatively few gaps exist for potential pressures because relatively few activities occur so far from land.

At regional and global scales, pressures related to climate change are having the largest impact on each high seas ecosystem. High seas management must account for, and ideally mitigate, impacts from climate change. Commercial fishing and shipping represent the other two key pressures to high seas regions, but at lower overall impact compared to climate change, at least at the large scale of assessment.

The framework for calculating and mapping expected cumulative human impacts can consider different climate and human use scenarios. When considering different potential actions for managing the high seas, those at international and regional policy levels should consider the direct effect of the actions, such as reduced climate emissions from climate change policy or more sustainable fisheries from better fisheries management. It is also important to consider how these actions would interact with other policy measures to change overall conditions in different high seas regions.