The long-term rise in global temperatures, the dominant cause of which is the anthropogenic emission of greenhouse gases, combined with the effects of a developing El Niño, have resulted in unusual global warmth in 2015. The global-average near-surface temperature for 2015 is likely to be the warmest on record according to data sources analysed by the WMO. A preliminary estimate based on data from January to October shows that the global average temperature for the year so far was around 0.73 ± 0.09°C above the 1961-1990 average of 14.0°C and approximately 1°C above the 1850-1899 period, although uncertainties relative to this earlier period are larger and more difficult to estimate.