The world's largest organization of physicists clarified its position on climate change last week, and it no longer believes, as it did in 2007, that the evidence for global warming is "incontrovertible."

Instead, the American Physical Society (APS) now states that climate change is a "critical issue that poses the risk of significant disruption around the globe." It then discusses uncertainties inherent in climate science and the risk involved in not taking action in a draft statement that was released last week (see box for the 2007 and draft 2015 statements).

To some, the latest version improves on a word—"incontrovertible"—that seems to conflict with the basic nature of science, but to others the change could sow confusion in the minds of the general public.

"A trained scientist understands the broader context, but many segments of the general public may not," said Seth Darling, a nanoscientist at Argonne National Laboratory who has written a book on communicating climate science to the general public. "It does seem to take a step back in terms of the firm tone of the 2007 statement."

What a group of physicists think about climate change matters greatly because climate science is, after all, a branch of physics, and most atmospheric scientists are based in physics departments. And the APS's route to its 2015 statement has been dogged by controversy and its own internal politics, including a coup d'état that led to the resignation of Steve Koonin, the head of the climate statement review committee and a former Obama administration official.

The nadir of all this was reached when Koonin wrote an editorial in The Wall Street Journal that appeared to question climate science.

The process of putting the statement together was "painful," said Robert Rosner, an astrophysicist at the University of Chicago and an APS member involved in the drafting process until December 2014.

That a group of seemingly staid physicists could come to metaphorical blows over a previously accepted climate statement reflects just how politically charged the issue remains.

Controversy over 'incontrovertible'
The roots of the conflict can be traced to 2007, when the APS released a statement on climate change stating, "The evidence is incontrovertible: Global warming is occurring."

According to the Merriam-Webster dictionary, "incontrovertible" means "not able to be denied or questioned."

This term was anathema to scientists. In science, everyone is free to question everything, and that is how the field progresses.

"A few physicists got their knickers in a twist over that one," said Philip Taylor, a professor of physics at Case Western Reserve University and an APS member.

Revision of the statement fell to a six-member drafting committee headed by Koonin, director of New York University's Center for Urban Science and Progress who has previously served as an undersecretary at the Department of Energy and as a chief scientist at BP. Koonin did not respond to ClimateWire's request for comment by deadline.

In January 2014, Koonin organized a symposium in Brooklyn, N.Y., to which he invited six climate experts. Three are well-respected by most scientists.

The other three—John Christy, a climate scientist at the University of Alabama; Judith Curry, a climatologist at the University of Georgia; and Richard Lindzen, an emeritus physicist at the Massachusetts Institute of Technology—are well-respected by climate skeptics and are often challenged by the climate science establishment.

These scientists were invited because the APS committee wanted to rebut a claim that skeptics often make—that mainstream scientists are willfully silencing dissenting voices, Rosner said.

The skeptical scientists got a fair airing at the APS's symposium, where the attendees could separate the wheat from the chaff, Rosner said. The attendees were trained physicists, and at the end, they understood that the skeptical scientists' "critique of science itself was extremely weak," he said.

Rosner's takeaway from the meeting was that Curry, Christy and Lindzen were questioning the presentation of the science, he said.

New wording from the 'POPA'
In February 2014, Koonin's drafting committee briefed the APS's Panel on Public Affairs, colloquially known as POPA, on its activities.

Some POPA members became concerned. They felt the climate statement was too important to be written by physicists who had little or no training in climate science before receiving a one-day crash course from a mixed bag of instructors.

"There was a contingent of folks who were supremely concerned that we would give ammunition to the other side, meaning the deniers," Rosner said.

In the spring of 2014, Koonin's drafting committee produced a statement that attributed equal weight to human influences and natural variability as a driver of climate change, according to a source who declined to be named.

That would be disputed by many climate scientists. The Intergovernmental Panel on Climate Change says that it is extremely likely that human activity has been the dominant cause of global warming since the 1950s.

Koonin quickly lost control of the committee, according to a June POPA meeting's minutes. Elbows came out, and POPA members presented a series of amendments and strong-armed their way into the drafting of the review, according to the minutes.

"Physicists can be emotional," Rosner said, laughing. "Some of us are really passionate."

Koonin resigned shortly thereafter. The larger POPA panel prepared the draft, in its present form, by the fall.

Shock waves from Wall Street Journal
The public face of the spat played out in The Wall Street Journal. In September 2014, Koonin wrote an editorial where he acknowledged human-caused climate change was happening but wrote that the "impact today of human activity appears to be comparable to the intrinsic, natural variability of the climate system itself."

His article contained some inaccuracies, Raymond Pierrehumbert, a geophysicist at the University of Chicago, said on a phone call from Sweden, where he is on a guest professorship. He said that human influences are not just "comparable" to natural variability as Koonin wrote. Rather, humans have dominated warming since the 1950s.

This may seem like a minor squabble, but Koonin's point is one espoused by Curry and other scientists opposed to man-made dominance of global warming. They assert that the climate is indeed changing, but nature and humans both share the blame. The degree to which humans are dominating nature in shaping the climate, they assert, cannot be known using the tools scientists presently have at their command.

Koonin's editorial shocked the climate science community. Here was a physicist who had served in the Obama administration apparently writing that climate science is uncertain on The Wall Street Journal editorial pages, which some say have served as a Bible for climate deniers.

Rising to Koonin's defense, Rosner said that Koonin's language had been misinterpreted. The language a physicist uses—uncertainty, chance, risk—could be misunderstood by an audience of the type the Journal attracts, he said.

"Everyone knows language matters," Rosner said. "And unfortunately, the technical language that is understood and practiced by physicists is not the same as the language that is used by the public media."

Koonin's main point was that humans, together with nature, are now determining the evolution of the Earth's climate, Rosner said. "That is amazing, and given the difficulties we have in managing our affairs, also very scary," he said.

'Time to move on'
APS released its draft statement to its membership last week and called for comments. Laypeople interviewed by ClimateWire said the draft, which has been split into three parts, did not have the clarity of tone of the 2007 APS statement.

But all scientists interviewed by ClimateWire agreed that the statement is factually accurate.

Michael Mann, a climatologist at Pennsylvania State University, said that the statement remains clear that "climate change is real, it's caused by human activity, and it represents a threat we must deal with."

"It is time to move on from the fake debate over whether climate change is real or poses a risk, and onto the worthy debate about what actions we must take to avoid a climate catastrophe," he said in an email.

APS will finalize its statement later this year, but for those interested in the power of words—rather than of equations—here are the two statements that catalyzed the physicists' unusually strong reactions.

The 2007 statement:

"Emissions of greenhouse gases from human activities are changing the atmosphere in ways that affect the Earth's climate. Greenhouse gases include carbon dioxide as well as methane, nitrous oxide and other gases. They are emitted from fossil fuel combustion and a range of industrial and agricultural processes.

"The evidence is incontrovertible: Global warming is occurring. If no mitigating actions are taken, significant disruptions in the Earth's physical and ecological systems, social systems, security and human health are likely to occur. We must reduce emissions of greenhouse gases beginning now.

"Because the complexity of the climate makes accurate prediction difficult, the APS urges an enhanced effort to understand the effects of human activity on the Earth's climate, and to provide the technological options for meeting the climate challenge in the near and longer terms. The APS also urges governments, universities, national laboratories and its membership to support policies and actions that will reduce the emission of greenhouse gases."

The 2015 draft statement:

On climate change: "Earth's changing climate is a critical issue that poses the risk of significant disruption around the globe. While natural sources of climate variability are significant, multiple lines of evidence indicate that human influences have had an increasingly dominant effect on the climate warming observed since the mid-twentieth century. Although the magnitudes of future effects are uncertain, human influences on the climate are growing. The potential consequences of climate change are great and the policies of the next few decades will determine human influences on the climate for centuries."

On climate science: "As summarized in the 2013 report of the Intergovernmental Panel on Climate Change (IPCC), there continues to be significant progress in climate science. In particular, the connection between rising concentrations of atmospheric greenhouse gases and the increased warming of the global climate system is more certain than ever. Nevertheless, as recognized by Working Group 1 of the IPCC, scientific challenges remain to our abilities to observe, interpret, and project climate changes. To better inform societal choices, the APS urges sustained research in climate science."

On climate action: "The APS reiterates its 2007 call to support actions that will reduce the emissions, and ultimately the concentration, of greenhouse gases, as well as increase the resilience of society to a changing climate. Because physics and its techniques are fundamental elements of climate science, the APS further urges physicists to collaborate with colleagues across disciplines in climate research and to contribute to the public dialogue."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500