Ancient Clam Shells Show That What Drives El Niño Is Still Unclear

Earth’s path around the Sun may play a role, but other factors are still unknown

Hawaii hurricanes
Two hurricanes, Iselle and Julio, that could hit Hawaii this weekend (seen here in a satellite image captured August 4) may have been influenced by an El Niño developing in the Pacific. NASA/NOAA GOES Project

El Niño has been in the news quite a bit lately: Scientists have predicted that this year will be an El Niño year. That will bring storms to the U.S. West, which might (or might not) save areas such as Texas and California that are parched with drought. El Niño may have contributed to the development of two hurricanes headed towards Hawaii. El Niño may lead to higher prices of coffee and other foodstuffs.

So what is El Niño? El Niño is one half of the El Niño Southern Oscillation, or ENSO—a phenomenon characterized by cycles of abnormally warm water in the equatorial Pacific. In satellite images that measure ocean heat, El Niño shows up as a bright red band stretching from South America to the west.

El Niño occurs when trade winds that blow west across the equatorial Pacific relax or switch direction, causing warm water that usually piles up in the western Pacific to head east. The event is ephemeral: Eventually the trade winds return to normal and eastern Pacific water cools back down. And if normal trade winds get stronger and the water cools even further, a La Niña can develop.

All that ocean heat has a direct effect on weather patterns across the globe. In the United States, especially in winter, an El Niño generally brings rain to California, dry weather to the Pacific Northwest, warm weather to southern Alaska and cool, wet weather to the southeast. Hurricane season lessens, and tornado season is more intense.

Unlike the planet’s seasons, El Niño doesn’t have a regular pattern. It can vary in magnitude and how long it lasts. Some years can be mild (it’s looking likely that 2014’s El Niño will fall into this category). Other years are strong, sometimes with devastating effects. The El Niño of 1997-1998, the last really strong one, brought deadly floods and mudslides to Peru and Ecuador, the strongest hurricane ever seen in the eastern Pacific and record rainfalls to California.

Exactly what ultimately causes winds to shift at the onset of an El Niño isn’t yet clear. One leading theory is that the subtle changes in Earth’s orbit alter the amount of energy the planet receives from the Sun and that influences when and El Niño (or La Niña) forms. Scientists would like to know what drives El Niño not only because it would be nice to better predict when the phenomena occur, but also to have an idea of how everything will change as the planet warms.

But to figure that out, researchers need to know how El Niño has varied in the past. In a study published today in Science, Matthieu Carré of the French National Center for Scientific Research in Montpellier and colleagues reconstructed 10,000 years of past sea surface temperatures from the fossilized shells of Mesodesma donacium clams on the coast of Peru.

People on that coast have been eating these clams for thousands of years, leaving behind mounds of clam shells as high as 10 meters. Soil surrounding the shells were carbon-dated, allowing scientists to estimate their age. The shells themselves hold a signal of the water temperature they were bathed in when they formed, a signal that the scientists extracted by analyzing the ratio of oxygen isotopes found within the shells.

The ENSO cycle has been active over the past 10,000 years, but there’s been some variability in its strength and geography, the clam record reveals. From about 4,000 to 5,000 years ago, for instance, ENSO appears to have dampened. And from 6,700 to 7,500 years ago, cold waters along the coast of Peru skewed the location of warm waters from El Niño.

These fluctuations in broad ENSO patterns don’t match perfectly with known fluctuations in Earth’s orbit or in solar output. What this all means, the researchers say, is that there are multiple factors affecting the ENSO cycle, not just the planet’s wobbly path. For example, melting polar ice may flush the ocean with fresh water and influence ENSO patterns, the authors suggest.

Teasing out the multiple factors that influence ENSO might help scientists to better understand how climate change could affect weather patterns in the future. Unfortunately, though, knowing what causes an El Niño isn’t much help to the people in Hawaii currently staring at maps of two hurricanes headed their way.