You might have heard that 97% of climate scientists agree the world is warming and people are the cause. This level of agreement, known as “consensus”, is often put forward in the climate debate in support of human-caused global warming and action to mitigate it. It was recently popularised on US talk show Last Week Tonight with John Oliver:
The 97% figure comes from a paper by social scientist John Cook at the University of Queensland and colleagues, who quantified this consensus by analysing the abstracts of scientific papers on climate change. Cook estimated 97% of the abstracts supported the idea that recent climate change is man-made.
This to-ing and fro-ing might give the impression that climate science is somehow still under debate, but the scientific evidence that people are causing climate change is overwhelming, and mounting.
But “consensus” means different things to different people — and herein lies the problem.
What does consensus mean?
“Consensus” is understood differently in science compared to politics or society.
Scientists use this word to refer to consilience of multiple lines of evidence that underlie widespread agreement or support a theory.
In the case of climate change, multiple lines of evidence underpin the prevailing view that the climate system is showing decade-on-decade warming over the past 50 years.
In particular, this warming bears temporal and spatial patterns, or “fingerprints”, that point to human causes.
For example, the stratosphere (that part of the atmosphere higher than about 11 km) has been cooling as the lower atmosphere and the ocean warm. This is the pattern we expect from the addition of greenhouse gases and not from, say, changes in the sun’s output.
But in public and especially political discourse, “consensus” tends to imply majority opinion or concurrence. As consensus in this public context is often arrived at by negotiation, saying there’s a scientific “consensus” may imply to the community that prevailing scientific views represent a negotiated outcome. This is the antithesis of science.
Consensus of the non-scientific kind does have a role to play in the climate debate. This includes negotiating whether warming is a “good” or “bad” thing and what, if anything, we should do about it.
These are not scientific questions. These are issues of values, politics, ethics and economics. As a nation and as a global society we need to reach consensus to resolve those questions and to make and implement appropriate public policy.
How science works
Science is based on three main things: data, testability and contestability.
Scientists, for example, don’t “negotiate” with data. We may re-analyse, reject outliers, replicate, recalibrate, but we do not negotiate. If the thermometer reads 25C, we don’t say, “I’d like 30; how about we settle on 27.5?”
Testability is a particular challenge for climate science, because we can’t do a laboratory experiment to test the hypothesis that humans are causing climate change. Most of the impacts we are concerned about are in the future and we have no data for the future. Instead, we use models based on the best understanding we have of physics, chemistry and biology to anticipate possible climate futures.
We are of course currently conducting the experiment that will prove whether humans cause global warming, in the uncontrolled planet-wide release of greenhouse gas emissions. However, by the time we’re in a position to test the result of, say, doubling greenhouse gas concentrations, it may be too late to mitigate the impacts.
That last attribute – contestability — is the antithesis of “consensus”. Indeed, it is the adversarial nature of science that is its real strength. In science you’re right until you’re proven wrong, and theories survive only as long they stand up to challenge.
Alfred Wegener’s theory of continental drift (which sought to explain how the continents are distributed across the earth) was rejected for decades. That was partly because there was no plausible mechanism to explain continents moving, and because most geologists at the time viewed vertical movements as the dominant earth-shaping forces. Wegener had been trained in astronomy and most of his work was in meteorology, so he was an “outsider” in geology.
Geological and geophysical observations in the 1960s and 1970s provided the evidence that the earth’s surface could and had shifted, moving continents in the way Wegener had suggested.
Play the science, not the scientist
The second problem with consensus is who to trust. On the website Skeptical Science Cook and his colleagues note they “decided that researchers who work and publish on climate science are the right group to ask.” Others have also tried to identify expert credibility in climate science.
But the problem with asking “who to believe” is that it ignores the merits, or lack thereof, of the arguments.
Kevin Cowtan and Robert Way from the UK and Canada came up with an innovative application of data analysis to “fill in” temperature data where observations are sparse, especially in polar regions. Their paper suggests a greater rate of global warming over the past two decades than previously estimated; their conclusion is that global warming has slowed but not as much we thought.
But Cowtan is a crystallographer, not (previously) a “climate scientist". There’s a whole world of scientists who may have novel techniques, new insights and compelling arguments for different estimates of warming, or new estimates of climate sensitivity, than adopted by the IPCC and other synthesis studies. Are they afraid to publish these arguments and the data supporting them because they are worried they may be dismissed as “non-climate scientists"?
We need all the contributions we can get from all the disciplines we can access to understand the crucial challenges posed by climate change. We need to open up scientific discourse on climate change — the very purpose of science is to broaden intellectual exploration.
UPDATE: This article was updated August 13 to add the source for the quote “Cook and his colleagues note they ‘decided that researchers who work and publish on climate science are the right group to ask.’”