According to five-year analysis, 2011-2015 has seen the warmest five-year period on record, with many extreme weather events influenced by climate change.
WMO secretary general Michel Jarraud said: "The state of the global climate in 2015 will make history for a number of reasons.
"Levels of greenhouse gases in the atmosphere reached new highs and in the Northern hemisphere spring 2015 the three-month global average concentration of CO2 crossed the 400 parts per million barriers for the first time.
"2015 is likely to be the hottest year on record, with ocean surface temperatures at the highest level since measurements began. It is probable that the 1°C Celsius threshold will be crossed."
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