Have We Crossed a Dangerous Warming Threshold? Here’s What to Know.

Our planet’s hottest January on record also helped global warming pass a different, unwelcome milestone, according to data released on Thursday by the European Union climate monitor: Over the past 12 months, the average temperature worldwide was more than 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, higher than it was at the dawn of the industrial age.

That number carries special significance in the international effort to stop dangerous climate change. Under the 2015 Paris Agreement, nations agreed to try to limit global warming to 1.5 degrees Celsius compared with preindustrial times, or at least to keep it comfortably below 2 degrees Celsius.

The latest temperature data doesn’t mean we’ve already passed that lower limit. Still, it is a potent symbolic reminder that, barring huge changes to the climate or the world economy, we are headed that way in the coming years.

Here’s what to know.

What are the 1.5 and 2 degree targets?

It might be helpful to start with what they aren’t, which is thresholds encoded somewhere in the laws of nature. Instead, they represent warming levels that would bring consequences that are unacceptably difficult for societies to manage, as decided and agreed upon by the nearly 200 nations that signed the Paris accord.

Deadlier hot spells. Higher sea levels. Greater loss of biodiversity. Longer droughts and fiercer storms. Scientists agree that these and other effects of a hotter Earth would increase significantly if warming continued very much beyond recent levels. The temperature targets therefore represent guardrails for humanity to avoid for the sake of our communities, ecosystems and landscapes.

In fact, though, many of these physical consequences of warming are already intensifying as we continue adding heat-trapping gases to the atmosphere by burning fossil fuels. That’s why scientists and diplomats often stress that, even if we someday heat the planet past 1.5 degrees of warming, it will still be worth trying to restrain temperatures from rising beyond 1.6, or 1.7, or 1.8.