A landmark report slated for release Friday is likely to announce the world’s leading climate scientists are 95 percent certain that recent global warming is largely due to human activities.
But what the report, from the U.N. Intergovernmental Panel on Climate Change (IPCC), will not say is that scientists are 95 percent certain about the severity of the consequences.
This is an important distinction, but one that frequently gets blurred.
For example, in an otherwise well-constructed piece in the Associated Press Tuesday about the significance of the 95 percent certainty, Seth Borenstein writes:
Top scientists from a variety of fields say they are about as certain that global warming is a real, man-made threat as they are that cigarettes kill
Yet the leaked version of the IPCC draft I reviewed never uses the word “threat” to characterize the warming.
In a similar vein, after a recent journal article found 97 percent of scientific studies (which took a position on the issue) agree climate change is manmade, President Obama tweeted: “Ninety-seven percent of scientists agree: #climate change is real, man-made and dangerous.” [bold text is my added emphasis]
— Barack Obama (@BarackObama) May 16, 2013
But the journal article Obama referred to stopped short of analyzing whether studies found climate change to be “dangerous.”
It is one thing for the scientific community to agree that greenhouse gases – whose heat-trapping properties are well-established – are warming the climate. But the question of whether manmade is a “threat” or “dangerous” involves a more complex set of judgments, some of which are scientific, but others which are more subjective, involving risk tolerance as well as geographic, ethical and political considerations.
“Defining what is dangerous interference with the climate system is a complex task that can only be partially supported by science, as it inherently involves normative judgements,” the last IPCC assessment said.
Regarding the potential impacts of climate change, the IPCC will issue a report in the spring which will assess the consequences of warming, discussing potential threats but also benefits and opportunities. How exactly these differing outcomes play out is an open question – which depends, in part, on how effectively we respond to climate change.
In previous reports, the IPCC has stated that greater the rate and magnitude of warming, the greater the risk of unwelcome changes and the more likely the balance of impacts tilts in a negative direction. Its reports on responding to climate change have stressed we can avoid the worst consequences of climate change by reducing greenhouse emissions and implementing coping (adaptation) strategies.
Most working climate scientists surely agree climate change poses risks and, in some to many respects, is threatening. A study of scientists’ views on their level of concern about different impacts and climate change overall would be extremely illuminating. But for now, it’s simplistic and misleading to conflate the impressive level of agreement that warming is manmade with more subjective judgments about what that signifies.