Beyond “catastrophic” climate change

A paper published in the Proceedings of the National Academy of Sciences makes the case for a new category of climate change so risky that it goes beyond “dangerous” and even “catastrophic.” The authors call the new category “unknown.”

It doesn’t have the ring of, say, DEFCON 1, but “unknown” signifies a threat level at which climate change could pose an existential risk to everyone on Earth. It would be so hot that billions of humans would need air conditioning or massive relocations to survive.

The Paris Agreement calls for limiting global average warming to “well below” 2 degrees Celsius, and aspires to keep it below 1.5 degrees. The authors of the PNAS paper, Yangyang Xu of Texas A&M University and Veerabhadran Ramanathan of the Scripps Institution of Oceanography, classify any warming beyond that level as “dangerous.” (The paper was edited by MIT environmental scientist and Bulletin Science and Security Board member Susan Solomon.) The authors propose two additional categories: Warming beyond 3 degrees would be “catastrophic,” and warming beyond 5 degrees would be “unknown.”

Xu and Ramanathan calculate a 50 percent probability that warming will cross the dangerous threshold within three decades if greenhouse gas emissions are not curbed, and a 5 percent probability of reaching catastrophic levels. That may sound low, but as Ramanathan told the San Diego Union-Tribune, “you would not get on an airplane if you thought there was a 5 percent chance that it was going to crash.”

What are the odds that humanity will venture, or rather blunder, into the unknown? If emissions remain unchecked, the two scientists project a 50 percent probability that the global population will be subjected to catastrophic risks by the end of the century, and a 5 percent probability of being fully in the unknown category.

Xu and Ramanathan recommend a “three-lever strategy” to limit warming: reducing carbon dioxide emissions to a net of zero; reducing emissions of short-lived but potent “super pollutants” such as methane and hydrofluorocarbons; and extracting and sequestering as much as 1 trillion tons of carbon dioxide from the atmosphere by the end of the century. “Basically, for a safe climate, all three levers . . . must be deployed as soon as possible,” they write.

If that doesn’t happen, it may be too late to avoid catastrophe. Or whatever comes after that.