After three years of dodging Senate inquiries, the Department of Defense, on March 20, 1974, presented a subcommittee of the Senate Committee on Foreign Relations with a briefing on the extensive rainmaking operations in Southeast Asia. The briefing was classified “Top Secret,” but the hearing report was made public on May 19, 1974. In this way the American public officially learned for the first time that the United States had used a new and developing technology in an attempt to slow movement of North Vietnamese troops and supplies along the Ho Chi Minh Trail network. From March of 1967 until July of 1972 the Air Force had rained canisters of silver iodide into clouds, and these in turn were supposed to initiate an increase in rainfall.

So began geoscientist Gordon J. ­MacDonald’s 1975 essay for TR on weather modification. Although the revelation of the Vietnam War program was the occasion for his article, his concerns were more general. MacDonald, who died in 2002, was then a professor at Dartmouth and had served on the President’s Science Advisory Committee under Lyndon B. Johnson. Throughout his career, he was interested in the way the planet changes as a result of both natural processes and human interference. After World War II, it became clear that industrial activity was changing the world’s climate. If humans were inadvertently creating climate change, it followed that they might be able to reverse those effects by modifying the local weather (see “The Geoengineering Gambit”).

Weather modification moved from the realm of magic to an applied science in July of 1946 when Vincent Schaefer, then at the General Electric Laboratories, discovered that dry ice could bring about nucleation of super-cooled water into ice. These laboratory studies were extended by Irving Langmuir and Bernard Vonnegut, who discovered that silver iodide as well as dry ice acted as an effective agent in bringing about the transformation of super-cooled water into ice.

The laboratory work was soon supplemented by field observations. In November of 1946, Schaefer flew into a cloud over Pittsfield, Massachusetts, at an altitude of 40,000 feet and a temperature of -20 °C. After dispatching several pounds of dry ice into the clouds, Schaefer observed draperies of snow falling below the clouds.

Governments took notice. At his inauguration, President John F. Kennedy pledged not only to “explore the stars” but also to “conquer the deserts”; he would later direct his science advisor, Jerome Wiesner, to pursue weather modification for humanitarian aims. But the scientists Wiesner consulted were uncertain how feasible weather modification would be and cautioned that international coöperation would be needed to ensure that the technology would not be used in war. The lure proved great, however, and rainmaking was used as a tactical weapon during the Vietnam War, albeit to minimal effect.

To some, rainmaking may seem relatively innocuous as compared with bombing or napalm. In some sense this is a correct view, but in the broader view the implications for future political stability are immense. We are developing a far more detailed understanding of the earth and its surroundings. …

It may be possible in the future to trigger earthquakes with devastating results from a great distance, or to bring about major climatic changes by triggering the instabilities inherent in the Antarctic icecap. All of these possibilities seem today to be far-fetched. But our history has shown us that if a technology develops, it will be used, unless international agreements can be secured.

MacDonald didn’t have to wait long. As the issue that contained his essay went to press, a proposal from the United States and the Soviet Union to ban the hostile use of environmental modification was put before the United Nations. Three years later, the Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques entered into force. As the search for solutions to global warming grows more desperate, perhaps a new international consensus will be required.