Global temperatures may temporarily reach 1.5 degrees Celsius above pre-industrial levels

But it does underscore that "we are getting measurably and inexorably closer" to that threshold, said U.N. World Meteorological Organization Secretary-General Petteri Taalas. Every year from 2021 through 2025 is likely to be at least 1 degree Celsius warmer. The report also predicts a 90% chance that at least one of those years will become the warmest year on record, topping 2016 temperatures.

"There's a little bit of up and down in the annual temperatures," said Gavin Schmidt, director of NASA's Goddard Institute for Space Studies in New York City. "But these long term-trends are unrelenting. It seems inevitable that we're going to cross these boundaries," Schmidt said, "and that's because there are delays in the system, there is inertia in the system, and we haven't really made a big cut to global emissions as yet."

Temperature shifts are occurring both on average and in temperature extremes, said Russell Vose, chief of the climatic analysis and synthesis branch at NOAA's National Centers for Environmental Information. Over the next five years, these extremes are "more likely what people will notice and remember," he said. The WMO report predicts an increased chance of tropical cyclones in the Atlantic Ocean, that Africa's Sahel and Australia will likely be wetter, and that the southwest of Northern America is likely to be drier.

The projections are part of a recent WMO effort to provide shorter-range forecasts of temperature, rainfall and wind patterns, to help nations keep tabs on how climate change may be disrupting weather patterns. Looking at marine and land heat waves, ice sheets melting, ocean heat content rising, and species migrating toward colder places, "it's more than just temperature," Vose said. "There are other changes in the atmosphere and in the ocean and in the ice and in the biosphere that all point to a warming world."