By Chris Mooney
11 April 2016
(Washington Post) – In 11 days, on Earth Day, world leaders will assemble at the United Nations in New York to sign the Paris climate agreement. That document pledges to hold the planet’s warming to “well below” 2 degrees Celsius above pre-industrial levels, and even to aspire to a 1.5 degree C temperature limit.
The urgency of signing the agreement has been underscored by recent climate news and events, including devastating coral bleaching around the world, newly shattered temperature records and disturbing news about the vulnerability of Arctic permafrost and the Antarctic ice sheet.
But there’s a problem: It is far from clear that, even if governments sign on to the Paris agreement and start implementing it rapidly, they actually know how to limit warming to 2 or 1.5 degrees Celsius. There are a number of problems with thinking that anyone does, argues Glen Peters, a researcher with the Center for International Climate and Environmental Research in Oslo, Norway, in the latest installment of Nature Climate Change.
1. Knowing when we cross the threshold. The first problem is simply with knowing when the world is actually 1.5 degrees C or 2 degrees C above a pre-industrial baseline temperature, often taken to be the average between the years 1850 and 1900 (though this, too, is disputed). Indeed, some have noted that we breached the 1.5 degree threshold in February of 2016, albeit only for the space of a single month (which probably isn’t what scientists have in mind when they think of truly crossing a climate threshold).
The problem is both that it will be hard to define where the actual threshold lies, and also hard to be sure when we’ve crossed it, given differing baselines and periods of analysis, and the fact that temperatures will always fluctuate up and down. And there’s an even bigger problem, which involves so-called “overshoot” scenarios in which the world emits too much and presumably warms up more than 1.5 or 2 degrees Celsius, but then starts to mass-produce “negative emissions” technologies that pull some carbon dioxide back out of the air again.
If this happens, we also will not know, for some time, how long it will actually take to get back to 1.5 or 2 degrees C, moving in the opposite direction, once we’ve overshot. “It may not be known for many decades if 1.5C/2C has been exceeded or successfully avoided,” Peters notes. [more]