Planet Earth has never, in recorded history, had a hotter summer than 2015, NOAA, NASA, and Japan Meteorological Agency (JMA) data indicate.
Temperatures soared about 1.5 degrees warmer than the long-term average, passing 1998 and 2014, which were the previous hottest summers on record depending on the dataset. These records date back to the late 1800s.
The record summer (using the Northern Hemisphere definition) was comprised of the hottest June, July and August periods in NOAA’s and JMA’s analyses (in NASA’s analysis, June and July were hottest on record, while August was narrowly second hottest, behind 2014).
Six of the first eight months this year have ranked warmest on record according to NOAA (the only exceptions being January and April, which ranked third-warmest).