According to this organization, the global average surface temperature may even reach the significant milestone of one degree Celsius above the pre-industrial era.
Experts affirmed that this temperature increase was due to the combination of the El Niño weather pattern with man-made global warming.
The 2011-15 period has been the warmest five-year period on record, with many extreme weather events, especially heat waves, influenced by climate change, WMO said.
The state of global climate in 2015 will make history for a number of reasons, WMO secretary-general Michel Jarraud said.
Levels of greenhouse gases in atmosphere reached new highs, and during spring 2015 in the northern hemisphere, the three-month global average concentration of carbon dioxide crossed the 400 parts per million barriers for the first time.