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In page United Nations Environment Programme:

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The position was then held for 17 years (1975–1992) by Dr. Mostafa Kamal Tolba, who was instrumental in bringing environmental considerations to the forefront of global thinking and action. Under his leadership, UN Environment's most widely acclaimed success—the historic 1987 agreement to protect the ozone layer—the Montreal Protocol was negotiated. He was succeeded by Elizabeth Dowdeswell (1992–1998), Klaus Töpfer (1998–2006), Achim Steiner (2006–2016), and Erik Solheim (2016–2018).