Norwich: Scientists have warned that 2024 could mark the year when global warming exceeds 1.5 above pre-industrial levels. They attribute these predictions, at least in part, to the emergence of an El Nino event.
An El Nino is declared when the sea surface temperature in large parts of the central or eastern equatorial regions of the Pacific Ocean warms significantly sometimes by as much as 2 degree Celsius. This additional heat in turn warms the atmosphere. During El Nino years, this warming contributes to a temporary rise in the global temperature by a fraction of a degree.
El Nino primarily affects weather in the tropics. Intense downpours that would usually fall on parts of south-east Asia or eastern Australia instead fall on the west coast of South America. This change can cause major drought and flooding on different continents, affecting food production and even weather-dependent sports like cricket. But changes to the weather in these regions can have knock-on effects all over the world. Even thousands of kilometres away in northern Europe, El Nino tends to cause colder and drier winter weather. Yet many factors affect European weather, especially during winter. So care is needed when linking unusual weather events in Europe to El Nino.
What is El Nino?
The Pacific Ocean spans over 13,000 kilometres from its eastern edge on the South American coast to its western margins near Indonesia. The sea surface temperature changes considerably over this vast distance. Normally, the eastern edge of the Pacific Ocean is more than 5 degree Celsius colder on average than the western Pacific. This is primarily due to the upwelling of cold water near South America, a process in which colder water is pulled up from deeper down in the ocean.
However, this temperature contrast flattens or steepens every few years in a natural cycle called the El Nino southern oscillation (Enso). During this cycle, the strength of trade winds that blow westwards across the Pacific can strengthen or weaken, causing more or less cold water to upwell and flow along the equator. We're currently entering a period where the eastern Pacific will be warmer than it usually is an El Nino event.
Forecasts suggest that a part of the equatorial Pacific, regarded as a key indicator of Enso, has a 50 per cent chance of warming by over 1.5 degree Celsius by the start of 2024. La Nina is the opposite phase of the cycle. It is instead characterised by cooler sea surface temperatures in these waters. This year brought an end to three successive La Nia years. The western tropical Pacific region has some of the warmest ocean temperatures on Earth.
