What It Means When Climate Scientists Say They're Certain
Scientists just made history by declaring, definitively and in unison, that climate change is caused by people. “It is unequivocal that human influence has warmed the atmosphere, ocean, and land,” they wrote in the first comprehensive report from the United Nations Intergovernmental Panel on Climate Change in nearly a decade.
And yet the reaction from many people was: Wait, didn’t we already know that?
Scientists and non-scientists have different understanding of “certainty,” and that goes a long way toward explaining why detractors have had such an easy time casting doubt on the reality of climate change. Bridging that communications gap has never been more crucial. The IPCC’s report makes clear that we have little time left to wind emissions down to zero, which means there’s none to waste on semantic arguments.
Let’s recap. Since the creation of the IPCC in 1988, its volunteer scientists from around the world have been tasked with producing periodic assessments of the state of climate change.
- 1990. The first report found, in effect, that human activities were increasing greenhouse-gas concentrations, and that rising concentration would result in greater warming of the Earth’s surface.
- 1995. The second report linked the two by saying: “The balance of evidence suggests a discernible human influence on the global climate.”
- 2001. The third report made an attribution leap: “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.”
- 2007. The fourth report made the conclusion stronger: “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” (This report was also the debut of the new assessment’s key word: “Warming of the climate system is unequivocal.”)
- 2013. The fifth report: “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
- Now. The coup de grace in the sixth report applied that “unequivocal” framing to the whole human-climate link.
Even though the language has become more assured, the report released Monday is still painted in shades of uncertainty on many impacts of climate change or attempts to mitigate it. That’s not just because scientists prefer to be conservative. It’s also because the best way to express reality is through varying degrees of probability.
In IPCC-speak, “very likely” equals 90% to 95% certainty, “likely” equals 66% to 90%, and so on. At this point, IPCC scientists think they have a 1% to 10% chance of being wrong on some aspects of climate change. That isn’t nothing, but still, that’s scientists talking. If they relied on the confidence levels most of us use, they wouldn’t have had to go beyond the calculations that showed burning coal could change the planet—which were done in 1896.
In climate science especially, certainty is the rarest of commodities. Despite hundreds of years of scientific work, how all of the variables that make weather patterns and ecosystems interact is just too complex for even the most advanced model or computer to predict infallibly.
The scientific method recognizes different levels of knowledge. There are observations, which are plain enough. There are hypotheses, which are provisional, testable explanations of the observations. There are scientific laws, which are hypotheses that have been tested so many times that they appear to be correct.
Then there’s the body of knowledge with the greatest explanatory power, backed by reams of evidence: the “theory”—which is also the discipline’s supreme example of unfortunate word choice. There’s the Big Bang theory, confirmed by satellite observations in the 1990s, which explains the origins of the universe. There’s the Theory of Relativity. There’s [ducks] the Theory of Evolution.
But in public life, the word theory means the opposite of that. It’s the most easily dismissible thing.
It’s in this context that we should consider the IPCC authors’ use of the word “unequivocal.” Scientists aren’t just certain that humans are causing climate change, they’re certain enough to defend their understanding in front of nearly 200 delegates from around the world, some of whom would rather not agree to such a statement.
The nearly 4,000-page report released Monday by the IPCC wasn’t full of surprises. In fact, it was crafted from thousands of scientific studies subjected to peer review over the past eight years. Much of the information it contained could be reasonably taken as immutable fact by a non-scientist. But the rest of us have that luxury because scientists have done the work to build up armors of evidence around their conclusions, then harden them in the fires of robust debate before releasing them out into the world.
The public and policymakers should not be afraid to wield these findings. As Laurence Tubiana, leader of the European Climate Foundation and one of the architects of the Paris Agreement—and, for what it’s worth, not a scientist—concluded: “Scientists did their job, it is time for leaders to do theirs.”
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