Environment

El Niño is coming

This year is set to be 0.1°C warmer on average than 2014, thanks to an unusually strong El Niño warming the Pacific Ocean and the world's climate.

Last year was the hottest year in recorded history. It continued a trend of every year in the last decade being warmer than the one before. Nine of the ten hottest years in record happened in that same time period. This is despite all the things that traditionally make the world colder – less solar activity and more aerosols in the atmosphere – being dominant in the climactic system.

The odd year out was 1998. This corresponded with a strong El Niño. The phenomenon has a large-scale effect on sea surface temperatures, rainfall and air pressure. It starts when the sea surface temperatures in the central and eastern equatorial regions of the Pacific – to the east of Australia – rapidly increase. This weakens the trade winds that blow across the region, taking the brakes off a warm patch of water that is normally pushed into one area of ocean near Australia.

The water then heads east, towards the Americas, where it warms more of the Pacific. It also starts warming the atmosphere. This then has a knock on effect on local weather systems across the planet – generally warming the planet and leading to less rainfall in the southern hemisphere and more in the northern hemisphere.