El Nino: what is it and why might this year's be the 'worst ever'?

Meteorologists are predicting that this year's El Nino event might be the most significant on record and is likely to lead to extreme weather events around the world.

The World Meteorological Organisation (WMO) says El Nino, which is already underway, is strengthening and will peak by the end of this year – potentially causing a colder winter in Europe.

The strongest El Nino on record was in 1997-8 and scientists say sea temperatures in parts of the Pacific are now at levels not seen since the late 1990s.

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So what is El Nino and what causes it?

El Nino occurs when temperatures in the Pacific Ocean rise due to a change in the normal wind direction, which causes extreme weather patterns across the world. While it is a naturally occurring event, scientists believe that greenhouse gases and subsequent global warming are intensifying its effects, as the number of El Nino events has increased noticeably in the last few decades. The phenomenon typically lasts up to a year and is part of a complex cycle involving the ocean and the atmosphere which occurs every two to seven years. This year, meteorologists say the surface water temperatures in the east-central tropical Pacific Ocean are likely to exceed 2C above average, "potentially placing this El Nino event among the four strongest events since 1950".

What effect does it have?

El Nino is never exactly the same twice, but it does make certain weather patterns more likely. Australia, Indonesia, and the Philippines typically experience much drier conditions than normal, with droughts, poor harvest and forest fires possible. Monsoons that bring much needed rain to parts of India can become weaker.

In South America, Peru and Ecuador see much heavier rains between January and May, and commercial fishing is often severely affected. "The Peruvian fish industry can be devastated as the change in coastal wind direction prevents nutrient rich water rising leading to huge losses," the Met Office's Adam Scaife told the Daily Mirror.

The United States normally experiences fewer hurricanes but wetter weather becomes more likely in the south during autumn and winter. The warm Pacific air often causes more intense typhoons in Japan and South Korea. "There is also some evidence that El Nino can encourage colder weather in Europe," says BBC meteorologist Chris Fawkes. "It was partly to blame for the very severe winter of 2009, the UK's coldest for decades."

In 1876, the extreme weather caused by El Nino led to the most deadly global famine of the 19th Century. "The combination of extreme droughts and monsoons led to millions of deaths under hellish conditions," wrote historian Cormac O Grada in his 2010 book Famine: A Short History.