Tropical cyclones have likely become more intense over the past 40 years, the report said, a shift that cannot be explained by natural variability alone.
And as global temperatures keep rising, the report notes, so will the hazards. Consider a dangerous heat wave that, in the past, would have occurred just once in a given region every 50 years. Today, a similar heat wave can be expected every 10 years, on average. At 1.5 degrees Celsius of global warming, those heat waves will strike every 5 years and be significantly hotter. At 4 degrees of warming, they will occur nearly annually.
Or take sea level rise. At 1.5 degrees of warming, ocean levels are projected to rise another 1 to 2 feet this century, regularly inundating many coastal cities with floods that in the past would have occurred just once a century. But if temperatures keep increasing, the report said, there is a risk that the vast ice sheets in Antarctica and Greenland could destabilize in unpredictable ways, potentially adding another three feet of sea-level rise this century in the worst case.
Further unpredictable changes may be in store. For example, a crucial ocean circulation system in the Atlantic Ocean, which helps stabilize the climate in Europe, is now starting to slow down. While the panel concluded with “medium confidence” that the system was unlikely to collapse abruptly this century, it warned that if the planet keeps heating up, the odds of such “low likelihood, high impact outcomes” would rise.
“It’s not like we can draw a sharp line where, if we stay at 1.5 degrees, we’re safe, and at 2 degrees or 3 degrees it’s game over,” said Robert Kopp, a climate scientist at Rutgers University who helped write the report. “But every extra bit of warming increases the risks.”
Experts have estimated that current policies being pursued by world governments will put the world on track for roughly 3 degrees Celsius of warming by the end of the century. That has ramped up pressure on countries to make more ambitious pledges, beyond what they agreed to under an international climate agreement struck in Paris in 2015.
Brad Plumer and Henry Fountain