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Eccentric orbits

El Niño could mean 2015 is even hotter than last year’s scorcher

El Niño means drought in Australia – and floods in America. Len Matthews, CC BY-NC-SA

It’s confirmed: 2014 produced the highest global temperatures since records began in the 1880s. As if that’s not cause enough for concern, this year threatens to see the return of El Niño, which like some enraged climate-driven Godzilla, could emerge from the depths of the South Pacific and lay waste to entire regions.

While the effects can be felt around the world, it is nations bordering the Pacific that are most affected by this natural phenomenon which puts parts of the Earth’s climate into reverse. Rain that would have fallen in northern Australia and Southeast Asia falls instead on the west coast of the Americas. Messing with the hydrological cycle can cause both major droughts and floods on different continents. El Niño can at the same cause crops to fail because of lack of rainfall while on the other side of the world wash away entire communities.

El Niño and La Niña are the two opposing phases of the El Niño Southern Oscillation (ENSO) which is the name given to the phenomenon of regular and sometimes large annual variations in sea surface temperatures, air pressure and rainfall. El Niño is characterised by significant warming of portions of the Pacific while La Niña sees lower temperatures in these waters.

During an El Niño event, sea surface temperatures in the central and eastern equatorial regions of the Pacific increase significantly. This weakens the trade winds that blow westwards across the South Pacific. One result of this is that the warm waters that were previously concentrated into an area of the south west Pacific towards Australasia spread out eastwards across the whole ocean. This delivers a pulse of heat from the seas to the atmosphere.

Click to zoom in. Delphine Digout, Grid-Arendal

It is no coincidence that previous temperature records have been broken during El Niño years. One of the most significant El Niño events in the 20th century happened over 1997-98 and 1998 was, until last week, the hottest year ever. It hasn’t escaped people’s attention that 2014 snatched the title even without a helping hand from an El Niño.

This year we may well find out what El Niño-assisted temperatures might be like as a number of meteorological agencies are giving about a 60% chance for El Niño over the northern hemisphere winter in 2015, perhaps persisting until spring. This estimate is based on something called the Oceanic Niño Index which is a measurement of temperature anomalies in three regions of the equatorial Pacific. If this is greater than 0.5°C for three consecutive months then alarm bells start ringing.

That temperature change may not sound much, but a lot of energy is required to increase the temperature of billions of litres of water by even half a degree. Water stores an immense amount of heat compared with air such that it takes 1,000 times more energy to heat a cubic metre of water by 1°C as it does the same volume of air. Next time you boil the kettle, watch the electricity meter whizz round for a sense of this energy cost.

The bottom of the oceans are cold, approximately 4°C three kilometres down. That represents a massive heat sink in which to hide extra energy from the surface. Since the 1970s, more than 90% of the additional heat due to higher greenhouse gas levels has been absorbed into the oceans. Some have argued that avoiding certain changes in ocean heat content is a more useful safeguard against dangerous climate change than the currently employed 2°C threshold of surface warming.

Given their importance to climate dynamics, understanding what is going on in the oceans is vital if we are to produce useful scenarios for future climate change. Research published in the journal Science last year proposed the hypothesis that more heat is being drawn down into the ocean depths rather than warming the Earth’s surface, perhaps explaining the global warming “hiatus” observed since 1998. The record breaking 1997-98 El Niño may even have been an important driver of this large scale change in ocean currents.

Does that mean there are reasons for optimism? Could a form of negative feedback be operating whereby higher surface temperatures lead to more heat being transferred from the surface to the ocean depths? Would this produce a braking effect on temperature increases? I think it’s fair to say this would have to be pure speculation. Climate-ocean dynamics are too complex to be able to discern such a process right now.

What is certain is that there are large changes occurring in the amount of energy in the Earth’s oceans. One way or another this will have an effect on atmospheric processes and us surface dwellers.

It’s also worth remembering that since 1998 there has been a steady increase in both sea levels and ocean acidity while glaciers have continued to retreat.

Muir Glacier 1941 and 2004: now you see it, now you don’t. NSIDC, CC BY-SA

The El Niño Southern Oscillation brings climate change into focus because it can produce such large and sudden changes in the weather. Current assessments are that an El Niño this year would likely be quite weak and nothing like the titan of 1997-98. The beast may continue to slumber. But unless there have been dramatic and long lasting changes to the El Niño Southern Oscillation, it will inevitably rise up and issue a roar that will be heard around the world.

For now, satellites peer down and monitor surface temperatures while arrays of buoys sample a range of parameters under the water. Fingers crossed they don’t detect a monstrous shape forming any time soon.

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