Antonio Guterres: Climate disruption has begun
According to the World Meteorological Organization (WMO), Earth has recorded its hottest Northern Hemisphere summer ever recorded this year.
August was about 1.5 degrees Celsius warmer than the pre-industrial average, which is the warming threshold the world is trying to avoid.
The world's oceans - more than 70 percent of the Earth's surface - were the hottest ever recorded, nearly 21 degrees Celsius.
Scientists blame increasingly warmer human-caused climate change with an extra push from an El Nino, which is a temporary warming of parts of the Pacific Ocean that changes weather around the world.
"The dog days of summer aren't just barking, they're biting," United Nations Secretary-General Antonio Guterres said in a statement. "Climate disruption has begun."
Scientists have used tree rings, ice cores and other indicators to estimate that temperatures are now warmer than they have been in about 120,000 years.
So far, September's daily temperatures are higher than those previously recorded for this time of year, according to the University of Maine's Climate Reanalyzer.
While the world's air and oceans were setting records for heat, Antarctica continued to set records for low amounts of sea ice, the WMO said.