Since 2003, the world has had its 10 warmest Septembers on record, with 2018 tying 2017 for fourth place, according to the U.S. National Centers for Environmental Information.

Combined land and ocean temperatures were 1.4 degrees Fahrenheit (0.8 Celsius) above the 20th century average of 59 degrees. The last five Septembers have ranked as the top five warmest in records going back to 1880.

Arctic sea ice shrank to 1.77 million square miles on Sept. 19 — the annual low point — tying 2018 with 2008 and 2010 as the sixth-smallest in the satellite record going back to 1979, according to the National Snow and Ice Data Center. The smallest-ever was reached in 2012.

Many climatologists are studying how open water in the North Pole affects the world’s weather, including allowing for blasts of Arctic air during northern hemisphere winters.