ZURICH—The year 2015 is on course to be the warmest on record, a U.N. agency said on Thursday, attributing the forecast to the effects of the El Niño weather system and what the agency said was human-induced global warming.

The global average surface temperature rose to 14.73 degrees Celsius (58.5 degrees Fahrenheit) by the end of October, roughly 1 degree Celsius above the preindustrial era, the Geneva-based World Meteorological Organization said.

The...