If one is the loneliest number, two is the most terrifying. Humanity must not pass a rise of 2 degrees Celsius in global temperature from pre-industrial levels, so says the Paris climate agreement. Cross that line and the global effects of climate change start looking less like a grave situation and more like a catastrophe.
The frustrating bit about studying climate change is the inherent uncertainty of it all. Predicting where it's going is a matter of mashing up thousands of variables in massive, confounding systems. But today in the journal Nature, researchers claim they’ve reduced the uncertainty in a key metric of climate change by 60 percent, narrowing a range of potential warming from 3°C to 1.2°C. And that could have implications for how the international community arrives at climate goals like it did in Paris. Bonus: The new numbers paint a not altogether terrifying picture of humanity’s response to a climate crisis. Hell, you might even call it vaguely optimistic.
The metric is called equilibrium climate sensitivity, but don’t let the name scare you. “It's essentially the amount of global warming we would predict if we just doubled the atmospheric carbon dioxide and let the atmosphere and climate come to equilibrium with the carbon dioxide,” says lead author Peter Cox, who studies climate system dynamics at the University of Exeter.
For the past 25 years, the generally accepted range for this potential warming has stood between 1.5 and 4.5°C. Which is a big range when you consider what a one-degree bump can do. Think 5 to 10 percent less rainfall during the dry season in the Mediterranean, southwest North America, and southern Africa. Reach 3°C of warming and Earth will lose 100,000 square miles of wetlands and drylands.