The year 2018 is set to become the fourth hottest on record amid mounting signs of global warming, the World Meteorological Organization (WMO) said Thursday in Geneva.

The past four years saw the four highest annual average temperatures since measurements began in the mid-19th century, according to the UN agency.

The hottest 20 years all occurred in the past 22 years.

The WMO estimates the likelihood of an El Nino occurring within the next three months at 75 to 80 per cent.

This weather phenomenon that occurs every few years involves fluctuations in ocean surface temperatures in the Pacific.

It leads to droughts in tropic regions and reduces the ability of forests to absorb carbon dioxide.