Will Steffen, a prominent scientist who battled climate denialism and contributed to key reports that pushed world leaders to limit the Earth’s warming to 2.7 Fahrenheit (1.5 degrees Celsius) above preindustrial levels, died Jan. 29 at a hospital in Canberra, Australia. He was 75.
Dr. Steffen was a leading advocate for the official recognition of an Anthropocene epoch — a unit of geologic time marking the start of human activity significantly affecting the planet’s climate and ecosystems. A growing number of scientists say this should follow the Holocene, which began 11,700 years ago, after the last major ice age. (Anthropocene is derived from Greek and means the “recent age of man.”)
He was also the lead author of a widely circulated 2018 scientific essay that proposed a possible scenario of unstoppable, devastating climate change, described as a “Hothouse Earth.”
The paper explored how a chain of self-supporting feedback loops could begin causing significant warming and a major rise in sea levels. For instance, the thawing of Arctic permafrost and the subsequent release of methane would warm the Earth, in turn leading to more permafrost thawing. Other climate scientists called the theory important.
In addition, Dr. Steffen was part of a research team that included Swedish scientist Johan Rockström and that described the “planetary boundaries” that govern the safe existence of humans on Earth. Their research became the basis for a Netflix documentary narrated by David Attenborough that examined Earth’s biodiversity collapse.
William Lee Steffen was born in Norfolk, Neb., on June 25, 1947. His father was a Lutheran minister and his mother a homemaker.
He was a 1970 chemical engineering graduate of what is now the Missouri University of Science and Technology in Rolla and received a doctorate in chemistry from the University of Florida in 1975. He moved to Australia for postdoctoral work at Australian National University in Canberra.
Dr. Steffen conducted extensive work on international research collaborations. He contributed to five assessment reports by the U.N. Intergovernmental Panel on Climate Change, according to the university in Canberra, where he was an emeritus professor.
From 2004 to 2011, he was science adviser to the Australian government’s Department of Climate Change — a posting that included a turbulent period of Australian political history, substantially stemming from how the country should respond to climate change.
In 2013, he helped set up the Climate Council nonprofit after a right-wing prime minister dissolved the government-backed independent climate advisory panel on which he was a member.
Dr. Steffen told the Sydney Morning Herald in 2011 that he had received death threats because of his climate change views. “There is no debate in the scientific community about this,” he said, likening Australia’s conversation on climate to a flat-Earth debate. “Well over 90 percent of scientists in the area are quite clear: The Earth is warming and human activity is the major cause.”
In addition to his wife of 51 years, the former Carrie DeBoy, survivors include his mother; and a daughter.
A skilled rock and ice climber, Dr. Steffen scaled mountains all over the world, including in steep ascents in New Zealand, the Canadian Rockies and Nepal. He likened climbing to a science and said it gave him the buzz of resolving a complex problem.
In his later years, Dr. Steffen championed the cause of young climate activists, providing expert testimony in court cases.
In 2021, he provided evidence to a class-action lawsuit that attempted to force Australia’s environment minister to acknowledge a duty of care to young people in considering approving an expansion of a coal mine. A court ruled in favor of the plaintiffs, but the decision was overturned on appeal.
He also helped last year to persuade a judge to recommend against a new open-cut coal project in Queensland state.