This essential picture has not changed in decades. Yet scientists’ confidence has. Experts are more certain than ever that dire consequences are coming. For decades, climate change doubters clung to scientists’ acknowledgment that there is some give in their numbers — in particular, a key measure known as “climate sensitivity,” which refers to how much the planet will warm given a doubling of carbon dioxide in the atmosphere. The U.N. panel had previously offered a wide range of likely scenarios, 1.5 to 4.5 degrees Celsius. Doubters argued that warming might end up reflecting the low end of this range. Why force economic disruption to stave off warming that experts admit might not be as bad as some fear?
This view never took into account the fact that uncertainty works in two directions: Things could also turn out far worse than scientists’ median estimates. Now the United Nations has voided this argument. In its latest report, the panel narrowed its climate sensitivity range to 2.5 to 4 degrees Celsius, ruling out the benign warming scenarios doubters insisted were still possible.
On the current emissions trajectory, global temperatures are likely to rise by 2.1 to 3.5 degrees Celsius, blowing past the 1.5 degree threshold scientists warn humanity should not breach. The experts determined that humans have already added enough heat-trapping gases to the atmosphere to push the planet to the 1.5 degree mark. The presence of other types of air pollution, which reflect sunlight, has tempered the warming that would have otherwise occurred. Humanity’s health depends on cutting this air pollution, but doing so will also reveal the true thickness of the greenhouse gas blanket humans have already draped over the planet.
Scientists are also more confident about the likely consequences. If the world warms by 2 degrees Celsius, extreme temperatures that may have occurred twice a century would likely strike every three or four years instead. What had been once-a-decade droughts would arise about every four years. Various effects would compound, with disasters such as heat waves, droughts and wildfires feeding into one another. Coral reefs would disappear. Truly catastrophic consequences, such as disruption of ocean circulation patterns that govern the temperature of entire continents, are possible.
These findings demand a two-pronged climate strategy designed to plateau warming at 1.5 degrees Celsius. Humans must stop releasing into the air potent short-lived greenhouse gases — among them methane, which wafts from careless oil and gas drilling. The world must also drastically cut the long-term warming drivers, primarily carbon dioxide.
The U.N. panel concludes that this rescue scenario is still possible if governments eliminate net greenhouse gas emissions by mid-century. Time, however, is running out.