Tethys is an online knowledge management system that provides the marine renewable energy (MRE) and wind energy communities with access to information and scientific literature on the environmental effects of devices. Named after the Greek titaness of the sea, the goal of the Tethys database is to promote environmental stewardship and the advancement of the wind and marine renewable energy communities. The website has been developed by the Pacific Northwest National Laboratory (PNNL) in support of the U.S. Department of Energy (DOE) Water Power Technologies Office and Wind Energy Technologies Office. Tethys hosts information and activities associated with two international collaborations known as OES-Environmental and WREN, formed to examine the environmental effects of marine renewable energy projects and wind energy projects, respectively.

As industry, academia, and government seek to develop new renewable energy sources from moving water and wind, there exists an opportunity to gather potential environmental effects of these technologies. Tethys aims to evaluate and measure these effects to ensure that aquatic and avian animals, habitats, and ecosystem functions are not adversely affected, nor that important ocean and land uses are displaced. While these studies are presently scattered among different organizations, Tethys creates a centralized hub where this information can be found. Each document is labeled with an environmental stressor and receptor which categorize the type of potential harm and the affected area of the environment. The categories and the technology types covered are listed below:

OES-Environmental, formerly known as Annex IV, is a collaborative project among member nations of the International Energy Agency (IEA) Ocean Energy Systems (OES) to examine environmental effects of ocean energy projects and research. There is currently a wide range of ocean energy technologies and devices in development around the world; the few data that exist on environmental effects of these technologies are dispersed among different countries and developers. While the US Department of Energy is the operating agent, currently (as of September 2024) 16 out of 23 nations from OES are involved: Australia, Canada, China, Denmark, France, India, Ireland, Japan, Mexico, Monaco, Portugal, Singapore, Spain, Sweden, the United Kingdom, and the United States. There have been four phases of this initiative:

Phase 1 Annex IV (2010-2013) brought together seven Ocean Energy Systems (OES) nations, led by the US, to establish a smart, searchable, public, online knowledge base that features information on environmental effects of marine renewable energy (MRE). Annex IV also engaged numerous academic, scientific, and commercial organizations and institutions through gathering of information for Tethys and engagement in webinars and other online activities, and facilitated sharing of research findings through workshops and online meetings. The culmination of phase 1 was a report, Environmental Effects of Marine Energy Development around the World: Annex IV Final Report, that summarized the state of understanding of environmental effects around wave and tidal devices, focused on three case studies:

Annex IV also began to collect metadata on project sites that were performing environmental baseline studies and/or monitoring studies and relevant research studies that were underway. Over 150 forms were collected, providing details on nearly every project that has been deployed to date and on the most current research being conducted. All of this metadata is hosted in Tethys along with associated reports and publications.

During phase 2 (2013-2016), Annex IV brought together 13 nations, again led by the US. This three-year period continued to expand the collection of scientific papers, reports, and metadata on MRE development in relation to environmental effects. Phase 2 also placed a strong emphasis on bringing together the community interested in environmental effects, which includes researchers, developers, regulators, and stakeholders. This was achieved through quarterly webinars on topics of interest delivered by experts in the field; online expert forums that allowed small groups of researchers to discuss and collaborate on technical topics that continue to complicate monitoring of effects; and workshops, conference support and participation that brought additional focus to environmental topics of mutual concern. Key activities during this phase included a partnership with the European Wave and Tidal Energy Conference (EWTEC 2015) in Nantes, France during September 2015, that brought enhanced focus and researcher participation on environmental issues to this well-regarded conference. Additionally, the preparation of the Annex IV 2016 State of the Science Report: Environmental Effects of Marine Renewable Energy Development Around the World synthesized the knowledge of environmental researchers and information sources to assess progress that has been made and questions that remain to be answered about environmental impacts of MRE devices. A draft of the State of the Science report was made available for OES Executive Committee members and selected peer reviewers in November 2015, and was released for public review during the International Conference on Ocean Energy (ICOE) in Edinburgh UK in February 2016. The final State of the Science report was released in April 2016 in Washington DC., along with a number of derivative products aimed at getting the essential information of the report to varied audiences who might not consider reading the full report.

During phase 3 (2016-2020), Annex IV continued to pursue efforts to identify and bring clarity to the importance of key MRE device interactions with the marine environment. Following release of the 2016 State of the Science report, the Annex IV community put considerable effort into ensuring that the report itself and its key findings were made accessible and easily discovered. Many webinars, conference presentations (including EWTEC 2017, Asian Wave and Tidal Energy Conference (AWTEC) 2018, Australian Ocean Renewable Energy Symposium 2018, Environmental Interactions with Marine Renewables (EIMR) 2018, and Marine Energy Technology Symposium 2018), and other outreach opportunities helped spread the messages of the report and the Annex IV/OES role in furthering understanding of environmental effects to facilitate consenting.

Other activities carried out by Annex IV during phase 3 included two workshops on collision risk to marine mammals, seabirds, and fish from tidal stream and river turbines, and two workshops on the gathering and use of social and economic data needed for consenting processes. Ongoing work to collect, curate, and make accessible existing information on MRE environmental effects on Tethys has expanded the platform and reached ever growing audiences. The phase culminated in the release of the 2020 State of the Science report, which reflects the most current and pertinent published information about interactions of marine renewable energy devices and associated infrastructure with the animals and habitats that make up the marine environment. During this phase, Annex IV was also renamed to OES-Environmental.