El Niño and La Niña May Influence the Climate More than Greenhouse Gases

The familiar El Niño and La Niña cycles are known to cause drastic fluctuations in global temperature, along with often catastrophic climatic effects in tropical regions of the Pacific Ocean. What is less well known is that the powerful ocean oscillations have been a feature of our climate for at least 20,000 years – that is, since before the most recent ice age ended.

A 2005 study established a complete record of El Niño events in the southeastern Pacific, by examining marine sediment cores drilled off the coast of Peru. The cores contain an El Niño signature in the form of tiny, fine-grained stone fragments, washed into the sea by multiple Peruvian rivers following floods on the continent caused by heavy El Niño rainfall. As indicated in the adjacent figure, the study site was approximately 80 kilometers (50 miles) from Lima at a depth of 184 meters (604 feet).

Northern Peru sees the heaviest El Niño rainfall, generating floods capable of dispersing large amounts of fine-grained river sediments. Smaller amounts of rainfall in central and southern Peru, which are not caused by El Niño, don’t result in flooding with the same dispersal capability.

The study authors classified the flood event signal as very strong when the concentration of stone fragments, known as lithics, was more than two standard deviations above the centennial mean. The frequency of these very strong events over the last 12,000 years is illustrated in the next figure; the black and gray bars show the frequency as the number of 500- and 1,000-year floods, respectively. Radiocarbon dating of the sediment cores was used to establish the timeline.

It can be seen that the number of very strong Peruvian flood events peaked around 9,500 years ago and again about 2,500 years ago, since when the number has been decreasing. No extreme floods occurred at all from about 5,500 to 7,500 years in the past.

A more detailed record is presented in the following figure, showing the variation over 20,000 years of the sea surface temperature off Peru (top), the lithic concentration (bottom) and a proxy for lithic concentration (middle). Sea surface temperatures were derived from chemical analysis of the marine sediment cores.

As indicated in this figure, the lithic concentration and therefore El Niño strength were high around 2,000 and 10,000 years ago – approximately the same periods when the most devastating floods occurred. The figure also reveals the absence of strong El Niño activity from 5,500 to 7,500 years ago, a dry interval without any major Peruvian floods.

But it’s seen that El Niños were strong in other eras too. During this 20,000-year span, El Niños first became prominent between 17,000 and 16,000 years before now, at the same time that sea surface temperatures jumped several degrees. The initial rise in both El Niños and ocean temperature was followed by roughly centennial fluctuations, alternating between weaker and stronger El Niño activity. After the gap from 7,500 to 5,500 years ago, El Niños surged again, as did sea surface temperatures.

On a finer scale, El Niños during the last two millennia were distinctly stronger than their modern counterparts between 2,000 and 1,300 years ago, then relatively weak during the MWP (Medieval Warm Period) from about 800 (1,300 years ago) to 1300. During the LIA (Little Ice Age) from about 1500 to 1850, El Niños strengthened once more before falling back to their present-day levels.

It may seem counterintuitive that El Niños, which release vast amounts of heat from the Pacific Ocean into the atmosphere and often raise the global temperature by several tenths of a degree for a year or so, are associated historically with prolonged periods of cooling such as the LIA. But ecologist Jim Steele has explained this phenomenon as arising from the absence of La Niña conditions during an El Niño. La Niña is the cool phase of the so-called ENSO (El Niño–Southern Oscillation), El Niño being the warm phase.

In a La Niña event, east to west trade winds cause warm water heated by the sun to pile up in the western tropical Pacific. This removes solar‑heated water from the eastern Pacific, resulting in upwelling of cooler subsurface waters there that replace the surface waters transported to the west and cause a temporary decline in global temperatures. But at the same time, the ocean gains heat at greater depths.

With the absence of this recharging of ocean heat during an El Niño, global cooling sets in for an extended period. Such cooling is usually associated with lower heat output from the sun, characterized by a falloff in the average monthly number of sunspots. Conversely, La Niñas usually accompany periods of higher solar output and result in extended global warming, as occurred during the MWP.

El Niño and La Niña have been major influences on our climate for many millennia and will continue to be. Until they are better understood, we can’t be sure they play less of a role in global warming than greenhouse gases.

Next: Sudden Changes in Ocean Currents Warmed Arctic, Cooled Antarctic in Past