Scientists find global warming intensified El Niño in the past; provides clues for the future

Scientists from France and Chile have found [*French] that the climatic phenomenon known as El Niño intensified significantly because of global warming in the past. Their results offer a new starting point to investigate whether the extreme El Niño events that occured at the end of the 20th century were due to (anthropogenic) climate change, and whether they are a first signal of ever more intense extreme weather events in the regions hit by the phenomenon. The findings are published in Geophysical Research Letters.

El Niño ('Child Jesus') got its name because it generally occurs at the time of Christmas along the Peruvian coasts. This mode of variability of the climate, also called ENSO (El Niño-Southern Oscillation), results from a series of interactions between the atmosphere and the tropical ocean. Its effects are drought in otherwise humid areas and, vice versa, precipitation and even floods in arid zones (schematic, click to enlarge).

Scientists qualify this phenomenon as 'quasi-cyclical' because its periodicity, which varies from 2 to 7 years, does not have a strict regularity. Drawing on research carried out for over 25 years by oceanographers, climatologists and meteorologists, the mechanisms driving El Niño events are becoming known better and better. However, it has been difficult to include and understand the influence of other modes of climate variability on the ENSO subsystem. More precisely, until now it was not known whether the intensity and the frequency of the phenomenon is undergoing changes because of planetary global warming.

Now a team made up of Chilean scientists and researchers from France's Institut de Recherche pour le Développement (IRD) sheds new light on the question. The researchers took sediment cores, going 80 metres deep, in the bay of Mejillones, located in the north of Chile, and analysed the geochemical data contained in the samples. In particular they zoomed in on the by-products of the decomposition of diatoms - unicellular planktonic algae - which allowed for a precise determination of the changes of ocean surface temperatures in this area between 1650 and 2000.

The researchers found a sea temperature drop of more than 2°C between 1820 and 1878. The same decrease was detected in two samples collected near the South American coast more than one thousand kilometers north and south of Mejillones. The findings prove the ocean temperatures observed since 1820 affected the entire coastline of western South America, from central Chile to the north of Peru. The entire stretch of ocean where the Humboldt current can be found was thus the theatre of a significant cooling during this period.

However, this conclusion is paradoxical because the beginning of the 19th century coincides with the end of the 'Small Ice Age' which was accompanied by a reheating of the planet. In order to supplement and double check the data, the scientists studied minerals contained in the sediment samples, which made it possible to confirm that these minerals were transported by the winds from the continent. From this they concluded that the dominant trade winds in the region were strengthened at the time, pushing back the layer of warm surface water towards the west, resulting in an upwelling of cold water found usually in the deep along the Pacific coasts of South America:
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The assumption was confirmed by the measurement of the organic carbon flow which is directly related to an increase in the concentration of nutrients in the surface water. The increase in this flow, consistent with the observation of a 'cold phase' between 1820 and 1878, proves that the rise in the concentration in nutrients is the consequence of an upwelling of cold water.

The researchers then put forth the assumption that, in a climatic context of warming like the one which followed the end of the Small Ice Age, the important variation in temperature between the continent's land mass and that of the ocean would be responsible for this strengthening of the trade winds.

Whereas the coastal desert of Atacama was heated quickly during this period, the temperature of sea water would have increased much more slowly. Because this difference persisted and even grew, dominant winds intensified. By pushing back surface waters towards the west, these winds would thus have allowed the cooling of coastal water, modifying the normal mode of El Niño, which is usually characterized by a heating of the water.

Between the end of the Small Ice Age and the onset of the warming effects caused by anthropogenic climate change, the ENSO changed its mode.

These historical climatology studies also explain the observations made by chroniclers at the time who describe floods and an abrupt change in El Niño's behavior, occuring around 1820, along the peaceful coasts of South America.

Since the beginning of the 19th century, from the end of the Small Ice Age onwards, the ENSO has been characterized by abnormal rains occuring at in central Chile during the southern winter and on the northern coast of Peru during the next southern summer.

The new results underline the complexity of the interactions between global changes in the climate, the particularities of the El Niño-Southern Oscillation and regional climatic changes.

Now it remains to be determined whether the very strong intensity of the two extreme El Niño events occuring at the end of the 20th century, in 1982-1983 and again in 1997-1998, are also related to the recent intensification of the warming of the land mass caused by global warming. If that were indeed the case, El Niño could then become increasingly intense and have destructive impacts on the coasts of South America, but also in other areas of the planet.

The research was undertaken by the Institut de Recherche pour le Développement in collaboration with the Universities of Chile and Concepción. They follow upon the doctoral thesis of Gabriel Vargas supported at the University of Bordeaux I and financed by the IRD.

(Note, Chilean universities recently launched a bioenergy program based on drought-tolerant energy crops to be planted in the arid zones along the coast. The project will have to take into account potential for more extreme El Niño events in the future.)


Translated for Biopact by Laurens Rademakers, 2007, CC.

References:
Vargas G., Pantoja S., Rutllant J. A., Lange C. B., Ortlieb L. "Enhancement of coastal upwelling and interdecadal ENSO-like variability in the Peru-Chile Current since late 19th century", Geophysical Research Letters, 2007, 34 (13), doi:10.1029/2006GL028812

AlphaGalileo: El Niño affecté par le réchauffement de la planète - December 10, 2007.

IRD: Les recherches sur le climat à l'IRD - dossier [*.pdf] - 2007.

Biopact: Chile and the U.S. to cooperate on biofuels development - July 14, 2007