Will Steffen, climate scientist who warned of a ‘Hothouse Earth,’ dies at 75

Will Steffen, a prominent scientist who has fought climate denial and contributed to several key reports that pushed world leaders to limit the Earth’s warming to 1.5 degrees Celsius (2.7 Fahrenheit) above pre-industrial levels, died on January 29 died in Canberra, Australia. He was 75.

His death was confirmed by his wife of 51 years, Carrie Steffen. She said he died of complications following an operation at Canberra Hospital. Dr. Steffen had been receiving treatment for pancreatic cancer for the past year, but his wife said his death came as a shock because he had recently been in good shape and had begun planning to resume international collaborations.

Dr. Steffen has been a leading advocate for the official recognition of an Anthropocene epoch—a unit of geologic time that marks the beginning of human activities that significantly affect the planet’s climate and ecosystems. A growing number of scientists say it must follow the Holocene, which began 11,700 years ago, after the last great 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 paper that proposed a possible scenario of unstoppable, devastating climate change, described as a “Hothouse Earth.”

The paper explored how a chain of self-sustaining feedback loops could cause significant warming and a large rise in sea levels. For example, the thawing of Arctic permafrost and the subsequent release of methane would heat the Earth, which in turn leads to more permafrost being thawed. Other climate scientists have called the theory important.

We looked at 1,200 possibilities for the planet’s future. This is our best hope.

Dr. Steffen was also part of a research team that included the Swedish scientist Johan Rockström, who 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 explored the collapse of Earth’s biodiversity.

William Lee Steffen was born in Norfolk, Neb., on June 25, 1947. His father was a Lutheran minister and his mother a homemaker. Dr. Steffen studied chemistry at the University of Missouri and received a PhD from the University of Florida in 1975, before moving to Australia to be a postdoctoral student at the Australian National University in Canberra.

He developed a long history of international research collaboration, including contributions to five assessment reports by the UN Intergovernmental Panel on Climate Change, according to the university, where he was an emeritus professor.

From 2004 to 2011, he served as Scientific Advisor to the Australian Government’s Department of Climate Change – a post that spanned a tumultuous period of Australian political history, essentially stemming from how the country should respond to climate change. In 2013, he helped set up the nonprofit Climate Council after a right-wing prime minister disbanded the government-backed independent climate advisory panel of which he was a member.

Dr. Steffen told the Sydney Morning Herald in 2011 that he had received death threats over his views on climate change. “There is no debate in the scientific community about this,” he said, comparing Australia’s climate conversation to a flat-Earth debate. “More than 90 percent of scientists in the area are very clear: the Earth is warming and human activity is the main cause.”

An accomplished rock and ice climber, Dr. Steffen, has scaled mountains around 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 to solve a complex problem.

In his later years, Dr. Steffen championed the cause of young climate activists and provided expert testimony in a number of court cases. In 2021, he gave evidence for a class action lawsuit seeking to force Australia’s environment minister to recognize a duty of care to young people when considering approving a coal mine expansion. (A court ruled in favor of the plaintiffs, but the decision was reversed on appeal.)

He also helped persuade a judge last year to recommend a new open pit coal project in the state of Queensland.

Dr. Steffen is survived by his wife, a daughter, Sonja, 36, and his mother, Dorothy, who is 97 and lives in Franklin, NC.

At the time of his death, Dr. Steffen is working on a memoir that his wife said she hopes to publish posthumously.