How Does an El Nino Start?
The answer lies in the interactions between the equatorial Pacific Ocean and the overlying atmosphere. A small change in the usual sea surface temperature pattern can produce a change in the winds along the equator. In turn, these wind changes affect the currents, which change the pattern of sea surface temperatures even more. In some years, for reasons still not completely clear, this process continues, with ocean temperatures affecting winds, which affect currents, and in turn ocean temperatures. The small changes thus become larger and larger. Eventually, in the biggest El Nino events, the difference in temperature between the western and eastern equatorial Pacific Ocean can disappear altogether. This is what happened in the 1982-83 event. As a result, the whole pattern of climate and atmospheric circulation across the Pacific and Indian Oceans and the surrounding continents was disrupted, with droughts in normally wet areas and heavy rains over normally arid regions.
The changes associated with El Nino continue to grow for about a year. Then they usually collapse quite quickly. Sometimes a mirror-image pattern of climate disturbances–with flooding in Australia, India, Indonesia, and northeastern Brazil and dry conditions on the Pacific coast of South America–occurs. This set of conditions is called La Nina. La Nina episodes also usually last about a year or so. The world was in a weak La Nina through much of 1995 and ’96.
The tendency for El Nino and La Nina episodes to last about 12 months means that, once we have determined that an episode is under way, we can often predict how the climate will develop in countries where this phenomenon is a major climatic influence. So we carefully monitor what is going on in the equatorial Pacific and the overlying atmosphere. Buoys are moored along the equator to collect information about ocean temperatures at the surface and below. These can tell us a lot about whether an El Nino (or La Nina) is developing. Recently, computer models have been developed to predict the behavior of El Nino a year or more in advance. These models of the ocean-atmosphere system in the tropical Pacific have been quite successful in predicting El Nino episodes of the past decade.
El Nino is not the only phenomenon causing climate variations. Some climate variations have occurred at much longer time scales. Ice ages and some other long-term disruptions of the climate system can be attributed to changes in solar radiation, due to long-term variations in Earth’s rotation on its axis and its orbit around the Sun. These variations appear to be responsible for much of the variation in global ice cover during the past 1.5 million years. Other mechanisms that affect climate include volcanic activity, the uplifting and wearing away of the land surface, and continental drift (which affects the size and relative positions of continents and oceans).
Not all the longer-term climate variations are understood, however. Nor have they all been gradual. There were quite rapid changes in climate during the last ice age, up to about 10,000 years ago. Changes of about 5 [degrees] C took place in only a few decades, at least in Greenland and the North Atlantic. These changes were linked to changes in the ocean circulation. Similar changes in the future could bring rapid climate changes to the world. Over the last 10,000 years climate variations have been smaller than during the last ice age.
A Global Warming?
One variation that has taken place recently is a gradual global warming. Although the data we have to check changes in global climate are by no means perfect, they do indicate that temperatures have risen about 0.5 [degrees] C since late last century. The rise in temperature is much the same whether we look at air temperatures measured routinely by weather services over the land or sea surface temperatures. Merchant and navy ships have been measuring sea surface temperatures routinely since the middle of the last century. In the early years temperatures were measured by scooping up water in buckets and measuring with a thermometer when the bucket reached the deck. Nowadays, engine intake temperatures are measured.
The different ways of measuring mean that care needs to be taken in comparing temperatures from the last century with those of today. The same applies for temperatures measured over land. The way thermometers were situated to measure temperatures in the last century was different from current practice, and again care must be taken to remove possible biases from the different methods of measurement. But when these factors are taken into account, it does appear that the world has been warming. At the same time, precipitation appears to have increased in the high latitudes of the Northern Hemisphere and decreased in the tropics, although these changes have not been as consistent or clear as the temperature increases. The relatively stable climate of the past 10,000 years means that this recent global warming appears quite unusual.
Some scientists have suggested that changes in sunspot numbers (which may reflect changes in solar radiation) may be causing the recent warming, but the general consensus is that at least part of the warming is due to the enhanced greenhouse effect, due largely to the burning of fossil fuels. The greenhouse effect occurs because some atmospheric gases (especially water vapor, carbon dioxide, and methane) affect the radiation balance of the atmosphere. As described earlier, Earth absorbs radiation from the Sun, mainly at the surface. This energy is then redistributed by the atmospheric and oceanic circulation and radiated back to space at longer (“infrared”) wavelengths. Anything that alters the radiation received from the Sun or lost to space–or that alters the redistribution of energy within the atmosphere, and between the atmosphere, land, and ocean–can affect the climate. The greenhouse gases cause outgoing infrared radiation from the surface to be absorbed and reemitted by the atmosphere. This acts to warm the lower atmosphere and Earth’s surface. Therefore, an increase in greenhouse gases (and the atmospheric content of carbon dioxide has increased about 30 percent over the past couple of centuries) should lead to warming. There is considerable uncertainty about exactly how much warming should result from increased greenhouse gases, but it seems likely that continuing to increase the carbon dioxide in the atmosphere will lead to continued warming over the next few decades.
Is the Climate Becoming More Extreme?
If this enhanced greenhouse effect leads to a global climate warmer than the present by a couple degrees, the major effect on society and the economy would likely be felt through the impacts of the more extreme weather and climate events. For instance, changes in the number or intensity of tropical cyclones, or changes in the frequency of droughts, could have major consequences. In recent years, meteorologists have started to examine how such extreme events might change, if we continue to increase the amount of greenhouse gases in the atmosphere. Of course, this is a difficult task. The climate models we use to investigate such questions do only a rather rudimentary job in reproducing how extreme weather events work, at present. For example, they are unable to reproduce tropical cyclones with the intensity of observed cyclones, so it is difficult to extrapolate to a future climate. Tropical cyclone activity appears to have increased in the northwestern Pacific in recent decades but has decreased in the Atlantic. It is not clear if these changes are due to human interference in the climate system, or whether they will continue in the future.
Some of the climate models used to investigate possible changes due to an enhanced greenhouse effect have suggested that intense rainfall events may increase in frequency. There is some observational evidence that such an increase may already be happening, at least in Australia and the United States, but there is no certainty that this observed increase is the result of the enhanced greenhouse effect. One change in extreme events that could be confidently expected to accompany a general warming would be a drop in the number of cold nights (including frosts). This does appear to have taken place in several parts of the world in recent decades.
Other human actions also appear to have the potential to affect global climate. Aerosols (small particles) in the atmosphere are increasing, mainly as a result of the emission of sulfur dioxide from fossil fuel burning and also from biomass burning. These can absorb and reflect solar radiation. In addition, changes in aerosol concentrations can alter cloud amount and cloud reflectivity. These processes tend to produce cooling, which can, in some areas, offset warming due to an enhanced greenhouse effect. The lifetime of these aerosols in the atmosphere is much shorter (days to weeks) than most greenhouse gases (decades to centuries), so their concentrations (and thus their climatic impact) respond much faster to changes in emissions.
The description above indicates some of the many processes that can affect global climate. Evaluating just how much each process is contributing to recent climate trends, such as the recent warming, is difficult. The Intergovernmental Panel on Climate Change, after a thorough examination of the evidence, determined that the balance of evidence suggested that there has been a discernible human influence on global climate, and that the climate was expected to continue to change in the future, due to human influences on the atmosphere. The interaction of the atmosphere with the ocean is one factor complicating the detection and prediction of climate change. The oceans store large amounts of heat and carbon dioxide, which delays any warming that would be caused by an enhanced greenhouse effect. Understanding this delay is crucial for good predictions of future climate change.
Will El Nino Change?
One aspect of the climate system for which we need to be able to predict its response to global climate change is El Nino. If the future climate change leads to a change in the frequency of El Nino episodes of the intensity of those of 1877-78 or 1982-83, would this lead to changes in the frequency of famines around the world? Unfortunately, the models we currently use cannot predict how El Nino might react to an enhanced greenhouse effect, but better models are being developed. These models need to reproduce the way the oceans and the atmosphere interact, so they can reproduce the current behavior of El Nino. Certainly meteorologists and oceanographers have recognized the importance of this question. As well as using models to look to the future, they are examining evidence of ancient El Nino episodes to gauge how liable they are to disruption in a changing climate. This work, still in its infancy, should, with the new models of the ocean-atmosphere system under development, lead to clearer answers over the next few years.