Factors contributing to climate change are moving faster than predicted and pushing us toward planetary conditions unlike any humans have ever known—this was one of the salient themes to emerge from this month's meeting of the American Geophysical Union, the world's largest gathering of earth and space scientists. Some scientists think we've already crossed that boundary and are, as Jonathan Foley, director of the University of Minnesota Institute on the Environment, said, "in a very different world than we have ever seen before."

What scientists are now witnessing as the earth responds to increasing levels of atmospheric greenhouse gases presents many of them with a dilemma: How far should they go in expressing their concerns about how government and society are responding to climate change? This question is particularly charged given that efforts to undermine climate science have become part of the political debate on these issues.

Running through the meeting's scientific presentations were formal and informal discussions about the scientist's role in guiding society's response to climate change, including how to effectively communicate the certainties and uncertainties of the science—and how to respond to what Don Wuebbles, University of Illinois professor of atmospheric science and chair of the organization's Global Environmental Change committee, called the "confusionists."

After the meeting, InsideClimate News interviewed several leading climate scientists and a renowned science historian to get a sense of how they are navigating this difficult terrain. All of them have testified before Congress and several were contributing authors on the 2007 report of the Nobel Prize-winning Intergovernmental Panel on Climate Change (IPCC), so all understand the challenges of working in the public spotlight.

When it comes to the certainties of climate science, all of them agree that the scientific literature has established definitively that rising atmospheric carbon dioxide levels have led to global warming and that anthropogenic fossil fuel burning is a major factor in this rise.

"The bottom line is that the climate system is telling us an internally and physically consistent story," said Benjamin Santer, an atmospheric scientist at Lawrence Livermore National Laboratory who delivered the 2011 AGU Stephen Schneider Memorial lecture. "While there will always be sizable scientific uncertainties, there can be no reasonable debate about whether the planet is warming," or that fossil fuel burning is a prime contributing factor, said Santer, an IPCC report contributor, whose work linking climate change with anthropogenic factors has been attacked by climate change skeptics.

The scientists also agreed that—based on what's now being observed as the earth responds to high levels of atmospheric CO 2 and other greenhouse gases—what constitutes a dangerous level of global warming is likely less than what models have anticipated. James Hansen of the NASA Goddard Institute for Space Studies explained that scientists who are examining the fossil record for clues about dramatic ecosystem and climate shifts in previous geologic eras are finding evidence that current changes are happening faster than any comparable changes the earth has previously experienced.

"We will get a very different climate with an increase of 2 degrees Celsius, and that's what international organizations are setting as a goal," Hansen said. "A 2 degrees Celsius increase is actually a prescription for disaster."

Hansen has become one of the most outspoken scientists when it comes to advocating for policies to curb greenhouse gas emissions. While continuing to publish peer-reviewed scientific papers—his most recent paper examines the paleo-climate implications for current climate change—he has urged political leaders to curtail the extraction and use of fossil fuels.

"If you want to succeed as a hard scientist doing original research you do have to be a little careful about public communications," Hansen said, but, "I'm old enough that I'm willing to take the hits I get."

Hansen explained his decision to speak out as "a matter of intergenerational justice." He began voicing his concerns after he "realized governments were not responding to the information that was available."

"This is what we must get the public to understand—that people wouldn't intentionally leave children in a situation guaranteed to have tragic results but that's the pathway we're on," said Hansen. "We have a really sensitive climate system ... and we have only witnessed so far a fraction of the results."

In August, Hansen participated in the Washington D.C. protests against the Keystone XL where he was among the more than 1,000 people arrested.

"The scientific community has to make it very clear that we can't burn all the fossil fuels—but governments are going ahead and setting up the infrastructure for unconventional fuels—tar sands and tar shale," he said. "The science is clear we can't do that without creating a different planet. What the paleo-record tells us is that we have to start ending emissions now."

When scientists become involved in the decision-making process, it's essential that they distinguish between professional knowledge and personal views said Santer, of the Lawrence Livermore lab. "But this is not a reason for being silent," he added.

"The core integrity of being a human being demands you speak out," said Chris Field, a Stanford University professor of biology and director of the Carnegie Institution's Department of Global Ecology who spoke at AGU about climate change impacts on world food security. But "you have to be very careful explaining what is reflective of scientific information and what is filtered through one's values."

Ken Caldeira, a senior scientist at the Carnegie Institution's Department of Global Ecology who has also testified before Congress on climate change, said it is important "to make a distinction between what I think is likely to happen as a scientist and what I think we should do as a citizen."

"It's important to keep science separate from policy prescriptions," said Caldeira, who noted in his AGU presentation that the ongoing burning of conventional fossil fuel resources is producing temperatures that could threaten the last Antarctic ice sheet.

The climate change impacts scientists are watching unfold, plus the attacks on the science itself, are prompting them to speak out in a way they might not have done 10 or 15 years ago, said Naomi Oreskes, a University of California San Diego history professor and co-author of Merchants of Doubt. Scientists once presumed that "if you do the science right, society will respond appropriately," Oreskes said. But over the years, she said, they've learned that this isn't always what happens. Nuclear weapons scientists who reflected publicly on the risks of nuclear science in the 1940s and 1950s were accused of being traitors. Later, the tobacco industry discovered it could defend its product by denying the science.

When it comes to dealing with climate change skepticism or denial, Field pointed out that the scientific method is designed to acknowledge and consider all study results, even those that are outliers. "Skepticism is useful, but climate change skeptics' assertions have very rarely been part of the fabric of science," he said. "Scientists are not just another special interest."

"Never engage in science by assertion," said Santer, sharing advice from the late Stephen Schneider, a Stanford University professor and climate scientist who became famous for his defense of climate science.

The balancing between science and policy isn't likely to get easier any time soon. While thousands of scientists were discussing evidence of climate change at the AGU meeting in San Francisco, the international climate talks in Durban, South Africa were faltering and Congressional lawmakers were drafting legislation to prevent enforcement of energy efficiency standards for incandescent light bulbs, prohibit funding of a presidential climate change adviser, and compel President Obama to make an early decision on construction of the Keystone XL pipeline.

To close the gap between what scientists know and what the general public and policy-makers understand and are prepared to act on, scientists must communicate more effectively with the public, Hansen said. Essential to this communication, the scientists all agreed, is maintaining credibility.

"We do have to follow the scientific method and ... not be the mirror image of the deniers," Hansen said.