A group of former NASA employees, including astronauts, has called on the agency to stop making "unproven and unsubstantiated remarks" regarding global climate change — specifically that human activities are driving global warming.
"We believe the claims by NASA and (Goddard Institute for Space Studies), that man-made carbon dioxide is having a catastrophic impact on global climate change are not substantiated," write the 49 signatories in a letter to NASA administrator Charles Bolden. [Read the Full Letter]
This is the most recent objection by skeptics who challenge the reality of human-caused global warming. For decades, climate scientists have warned that humans are changing the composition of our atmosphere, warming the planet and, as a result, face rising sea levels, more extreme weather and other consequences. The concept is now well established in scientific literature and attempts to address it are the subject of ongoing international talks.
The letter originates from members of the Johnson Space Center Chapter of the NASA Alumni League, according to Walter Cunningham, a former astronaut who flew the Apollo 7 in 1968 and one of the letter's signatories.
In its four paragraphs, the letter deliberately avoids delving into its signatories' reasons for doubting climate-change science, Cunningham told LiveScience. "It's really trying to get NASA to back off from taking political positions on science." [10 Climate Change Myths Busted]
Human-caused global warming is "a very open issue right now," he said.
However, that is not how the scientific establishment sees it. NASA Chief Scientist Waleed Abdalati responded to the letter to say that if the work of NASA scientists — who may use the agency's space-based observation tools to study climate change — withstands the scrutiny of other scientists through peer review, then it encourages them to share their results with the public.
"If the authors of this letter disagree with specific scientific conclusions made public by NASA scientists, we encourage them to join the debate in the scientific literature or public forums rather than restrict any discourse," Abdalati said in the statement.
For his part, Cunningham said the peer-review process — in which scientists in relevant fields scurtinize research — Abdalati mentions is rigged against those who challenge the standard view of global warming.
In spite of its wide acceptance among climate scientists and the scientific community, global-warming skeptics include some scientists. For instance, William Happer, a physicist from Princeton University who is not involved in the NASA letter, recently penned an editorial to the Wall Street Journal stating that the world has not warmed as the concept of carbon dioxide-driven global warming predicts it would.
Like Happer, Cunningham has a background in physics, he is not a climate scientist. According to his NASA astronaut profile, Cunningham has a master's in physics from the University of California, Los Angeles, and completed doctoral work as well. Of the 46 signatories who listed their positions, eight are astronauts and others identify themselves as having worked in a variety of positions, including within NASA's science or engineering directorates.
The letter calls out the leadership of NASA's Goddard Institute for Space Studies (GISS) in New York, which is headed by James Hansen, a high-profile climate scientist, who has been an important force propelling the prospect of catastrophic, human-caused climate change into the public eye. Recently, Hansen has increasingly turned to activism, and was arrested at a protest in August. Hansen could not be reached for comment by deadline.
Gavin Schmidt, also a climate scientist at GISS, takes issue with statements by global warming skeptics, such as, "It is clear that the science is NOT settled," as the letter reads.
"Dealing with the future always involves dealing with uncertainty — and this is as true with climate as it is with the economy," Schmidt writes in a post at RealClimate.org. "Science has led to a great deal of well-supported concern that increasing emissions of CO2 (in particular) are posing a substantial risk to human society."
Among those who study climate, issues such as the existence of the greenhouse effect, increasing atmospheric carbon dioxide and other greenhouse gases over the past century, their human origin, and warming over the 20th century are no longer subject to fundamental debate, he writes.
The claims in the letter are too vague to be clear, Schmidt told LiveScience in an email, but "If any of signatories are ever in New York, I would be happy to discuss with them the science that gets done at GISS."
This article was provided by LiveScience.com, a sister site of SPACE.com. You can follow LiveScience senior writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.