Tropical cyclones have likely increased in intensity over the past 40 years, the report says, a shift that cannot be explained by natural variability alone.
And as global temperatures continue to rise, the report notes, the dangers will increase too. Imagine a dangerous heat wave that in the past has only occurred 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 global warming, these heat waves occur every 5 years and become significantly hotter. They occur almost annually at a temperature of 4 degrees.
Or take the rise in sea level. With a 1.5-degree warming, sea levels are projected to rise another 1 to 2 feet this century, regularly inundating many coastal cities with floods that in the past would only have occurred once a century. But if temperatures continue to rise, the report says, there is a risk that the giant ice sheets in Antarctica and Greenland will destabilize in unpredictable ways, potentially causing sea levels to rise another three feet in the worst-case scenario this century.
Other unpredictable changes may be pending. For example, a crucial ocean circulation system in the Atlantic, which is helping to stabilize the climate in Europe, is slowing down. While the panel concluded with “medium confidence” that the system was unlikely to collapse abruptly this century, it warned that the likelihood of such “low likelihood, high impact” would increase as the planet continued to heat up.
“It’s not that we can draw a sharp line where we’re safe at 1.5 degrees, and at 2 or 3 degrees it’s game over,” said Robert Kopp, a climate scientist at Rutgers University, who was writing the test report. “But every additional warming increases the risks.”
Experts estimate that the current policies of world governments will bring the world to a warming of about 3 degrees Celsius by the end of the century. This has put pressure on countries to make more ambitious commitments that go beyond what they agreed upon in an international climate agreement in Paris in 2015.