As President and founder of the Institute for Governance & Sustainable Development (IGSD) in Washington, D.C., and Paris, he currently focuses on fast mitigation strategies to protect the climate, including strategies to reduce short-lived climate pollutants (HFCs, black carbon, ground level ozone, methane), in the context of the need for speed to limit anthropogenic warming to 1.5 °C.

[7] At ELI he also worked with Frederick R. Anderson on NEPA in the Courts: A Legal Analysis of the National Environmental Policy Act (Resources for the Future, 1973).

After returning from Alaska, Zaelke directed the Washington, D.C., office of the Sierra Club Legal Defense Fund and founded their international program.

While still at SCLDF, Zaelke was asked by Sebia Hawkins, then heading the South Pacific Campaign for Greenpeace,[13] to investigate litigation against Japan for whaling, only to find that no action could be brought by an NGO in the International Court of Justice.

In 1989, Zaelke co-founded the Center for International Environmental Law (CIEL) in Washington, D.C., and London, with his late wife Barbara L. Shaw, James Cameron,[14] Philippe Sands and Wendy Dinner.

[18] Zaelke and his colleagues contributed to the scientific foundation for these efforts by co-authoring several papers, including several in the Proceedings of the National Academy of Sciences (2007)[19] and (2009),[20][17] the Review of European Compliance & International Environmental Law,[21] Atmospheric Chemistry and Physics,[22] among others.

[24] Considerably more warming can be avoided from fast implementation and parallel efforts to improve energy efficiency of air conditioners and other cooling equipment.

[25] The Montreal Protocol's 2018 quadrennial Scientific Assessment of Ozone Depletion confirmed that beyond phasing down HFCs, improving the energy efficiency of air conditioners and other cooling equipment has the potential to double the climate benefits of the Kigali Amendment in the near-term.

[24] IGSD continues work to promote the ratification and implementation of the Kigali amendment and improvements to energy efficiency equipment to achieve the full suite of climate benefits available.

"[30] Housman and Zaelke explained that a "mutually reinforcing mechanism of incentives and disincentives at the international level would direct trade and environmental policies to attain sustainability goals.

[31] Zaelke co-chaired, with Nobel Laureate Mario J. Molina, the International Energy Agency (IEA) and UN Environment Program (UNEP) Cooling Emissions and Policy Synthesis Report (2020)[32] and the underlying assessment of the report, the Assessment of Climate and Development Benefits of Efficient and Climate-Friendly Cooling (2020),[33] authored under the guidance of a Steering Committee of leading scholars and government, think tank, and independent experts.

Zaelke also co-chaired with Nobel Laureate Mario J. Molina, and Professor V. Ramanathan at the University of California, San Diego, the Well Under 2 Degrees Celsius: Fast Action Policies to Protect People and the Planet from Extreme Climate Change report (2017), which identified scalable solutions to achieve rapid climate stability, authored by a team of 33 prominent scientists and policy experts.

[36] He has also authored and co-authored many Op-Eds in leading publications including, The San Diego Union-Tribune,[37] The Hill,[38] Project Syndicate,[39] The New York Times,[40], among others.

[41] Zaelke has taught various environmental courses and programs nationally and abroad including:[10] Washingtonian Magazine's list of the experts and advocates, outside the government, who are playing big roles in Washington's policy debates.

"[42] On 24 December 1976, Zaelke married Barbara Lee Shaw (1943 to 2013),[49] who co-founded CIEL and IGSD, and in 2000 founded the Maasai Girls Education Fund (MGEF) in Kenya and the U.S. which she directed until her death in 2013.