Biological and Economic Effects of ENSO
Understanding and simulating ENSO is of primary importance in climate modeling because of the far-reaching biological and economic implications. One of the most important effects ENSO has on the planet is the noticeable ecological shifts that occur in response to either a warm or a cold phase. These changes in local ecosystems when coupled with human interaction can affect species survival in a particular region. For example, overfishing in a region alone can lead to species reduction and when combined with a change in fish population related to ENSO phase near extinction has been observed for several species (Barber and Chavez 1983). Near or total extinction of a species is a tragedy, and the economic impact can be the permanent dismantling of a regional industry. Agriculture is also often a victim when an extreme unanticipated ENSO event occurs. When crops fail, great social and economic strife can follow.
To understand how ENSO can affect the biological realm so profoundly requires looking at the basic physical process that leads to the change in ecosystem. For the coast of Peru, the resulting change in ecosystem alters the phytoplankton population which then causes proportional changes in fish, bird, and marine mammal populations (Barber and Chavez 1983).
The phytoplankton in the Eastern Equatorial Pacific live within the mixed layer of the ocean on a combination of sunlight (which decreases exponentially from the surface downward) and nutrient rich water (which reaches surface from below the thermocline via upwelling). During a warm event, sunlight and nutrients are less available to the phytoplankton. Near the coast of Peru, winds favorable for coastal upwelling are still present during the warm phase (and may actually be stronger). Nutrients are absent during this phase because the depth of the thermocline has increased to a level below the depth of entrainment. As a result the upwelling is not transporting as much nutrient-rich water to the surface. Second, less sunlight will reach the bottom of the mixed layer because the thickness of the mixed layer has also increased during the warm phase (Cane 1983) and sunlight decreases rapidly through seawater. Since the phytoplankton are roughly uniformly distributed in the mixed layer the amount of sunlight reaching any given phytoplankton is reduced and so the population is smaller. Less phytoplankton in this region leads to less biomass for higher organisms.
The reduction of phytoplankton is felt profoundly in higher tropic organisms as well. In the El Niño of 1982, arguably the strongest warm phase ever, the absence of fish, birds, and seals was extensively documented. In the anomalous currents some species of sea life, such as shrimp, simply drift from their normal habitat with no apparent change in population. For others, such as anchovy, the 20-fold reduction of their primary food source decreased their growth, survival rate, and reproductive fitness. The severity of the 1982 El Niño had such profound effects on anchovy that concentrated schools near the coast were found with numerous dead anchovies-an occurrence not observed in any of the recent past warm events. The jack mackerel were equally affected by the 1982 event. Due to the changing environment and diminished food source, the mackerel, usually seen offshore, moved into a tight 30-km band along shore. The increased density allowed predatory fish and fisherman to nearly eradicate the mackerel (Barber and Chavez 1983). The Galapagos fur seal was also gravely impacted. Due to the reduction of fish, adult seals were less successful at finding food. Observations show that adult females stayed at sea for about 5 days (3.5 days longer than normal) and returned with too little milk to feed their pups. The effects of the 1982 event were strong enough to kill not only the majority of pups, but also many adults (Pauly et al. 1989).
The reduction of multiple fish species during a warm phase is not isolated to the equatorial region. Near Christmas Island the disappearance of small fish and squid caused the blue-faced booby and great frigate bird to abandon nests on the island. In June 1982 the number of great frigate birds was 20,000; by November, the number was fewer than 100. Because of the relatively long lifespan of the birds, the parent s can abandon the baby birds and attempt to survive on the reduced food supply. Keeping the babies would jeopardize the survival of the flock by forcing competition for too little food. However, the magnitude of the 1982 event caused many of the adults to perish (Schreiber and Schreiber 1984).
The marked biological effects of ENSO also have important economic implications that compel us to continue ENSO prediction studies. The impact on the South American fishing industry is apparent from the discussion above; however, there are far-reaching global effects on agriculture and other sectors as well. From the 1940s to the 1980s guano from Peruvian seabirds was in great demand for use as fertilizer in Europe and the United States, creating a large industry in Peru (Glantz 2000). In this case, the reduction in seabird populations near Peru, due to El Niño, reduces the production of guano.
Today, agriculture is much less dependent on guano, but the effects of ENSO on the industry are still present. Changes in seasonal rainfall in the Southeastern United States related to ENSO are a key influence on agriculture in that region. For instance, in the Southeastern states, crops such as corn and tobacco, which are sensitive to precipitation, are significantly affected by ENSO (Hansen et al. 1998). Interestingly, the failure of the fishing season due to warm ENSO events may also impact crops planted in the Southeast United States. In years when demand for fish is high, the price of livestock feed, which contains fish protein, increases. In response to this increase in price and demand of livestock feed, soybean farmers tend to plant more soybeans, which can substitute for fish protein, 2 years after warm events (Hansen et al. 1998).
Accurate seasonal climate predictions can inform decision makers on when and where to plant crops. Being forewarned about the nature of upcoming ENSO events can not only protect farmers from crop failures but can also help farmers choose crops that may be in higher demand. During warm event winters, the prices of bell peppers and snap beans increase. The increase in hard-freezes for the South East United States reduces crop yields of winter-harvest vegetables. In Florida, diminished efficacy of fertilizers and damaged plant roots results from the excess rainfall during El Niño years. Because Florida is the primary producer of winter-harvest bell peppers and snap beans, farmers in regions outside Florida with mild winters can capitalize on this shortage (Hansen et al. 1999).