Earth Patrol

Background

The El Niño Southern Oscillation (ENSO)

El Niño is the name given to the oceanic and atmospheric phenomenon in the Pacific Ocean whereby unusually warm ocean conditions appear along the western coasts of Ecuador and Peru. These conditions may cause climatic disturbances of varying severity. El Niños occur about every two to seven years, and can affect climates around the world for months or years, after which they rapidly switch back to normal. The name El Niño, Spanish for 'the child', refers to the infant Jesus Christ; the phenomenon is probably so called because it usually begins during the Christmas season. Because El Niño is accompanied by a fluctuation in air pressure and wind patterns in the southern Pacific, the phenomenon is known as the El Niño Southern Oscillation, or ENSO.

The climatic disturbances caused by El Niño occur when the ocean currents are sufficiently warm and persistent to cause a reversal in the normal weather conditions of the eastern and western Pacific. Normally, the waters of the western tropical Pacific are warm. The air pressure is low over the warmer waters. Moist air rises in the region, causing the clouds and heavy rainfall characteristic of south-east Asia and northern Australia. In the eastern Pacific, the water is cold and air pressure is high, creating the arid conditions typical of coastal South America. The trade winds blow from east to west, pushing Sun-warmed surface waters westwards and exposing cold water to the surface in the east.

During El Niño, however, the easterly trade winds collapse and may reverse. As the slight weakening of the winds causes a change in sea surface temperatures, the change in wind and the accompanying pressure increases. The warm water of the western Pacific flows back eastwards, and sea surface temperatures increase significantly off the western coast of South America. The wet weather conditions normally present in the western Pacific now move to the east, and the arid conditions common in the east appear in the west. This brings heavy rains to South America and can cause droughts in south-east Asia, India, and southern Africa. It can also bring unusual weather to large parts of North America. The El Niño event of 1982-83 was the most severe this century: the temperature change was 14°C in some places.

The ozone hole

The ozone layer is situated from 19 to 48 kilometres above the Earth's surface. Ozone is formed there by the action of sunlight on atmospheric oxygen. Ozone is a form of oxygen which has three oxygen atoms in each molecule, rather than the normal two. Atmospheric ozone absorbs up to 90% of the sun's potentially harmful ultraviolet (UV) radiation. Solar UV radiation can damage crops, marine life and human health. It is therefore vitally important that the levels of ozone in the atmosphere are maintained.

There has been concern since the 1970s regarding the effect of chlorofluorocarbons (CFCs) on the ozone layer. CFCs have been used for many years as refrigerants, in foam plastics and in aerosol cans. When released into the atmosphere, CFCs are broken down by sunlight, and the chlorine reacts with and destroys ozone molecules. CFCs have now been banned in many countries. However, the ozone layer is also threatened by other chemicals such as bromine halocarbons and nitrous oxide.

Researchers in Antarctica in 1982 were the first to detect a periodic loss of ozone above the southern polar region. This so-called 'ozone hole' develops at the start of the Antarctic spring (in early October) and then closes up again, but never disappears completely. By 1995 the hole covered an area more then twice the size of the USA. Subsequent research showed that thinning of the ozone layer also occurs over Arctic regions, although not to the same extent as over the Antarctic. While there is a general decline in the amount of ozone throughout the atmosphere, the Antarctic is particularly vulnerable due to the additional effect of ice particles in the air and the effects of the wind, especially the south polar vortex.

The international response to this depletion has been twofold: to support and encourage research; and to reduce the amounts of ozone-depleting chemicals released. The Montreal Protocol of 1985 was signed by 49 countries and aims to have CFCs phased out by the year 2000. Research projects to monitor atmospheric ozone levels have included a satellite launched by NASA in 1991 to research the upper atmosphere.

Scanning the Earth

A series of satellites which monitor the Earth's resources have been developed since the 1960s, to orbit around our planet scanning its surface continuously using high-quality cameras and sensors. They have allowed us to see the Earth from a scale and perspective that was not previously possible. There are basically two types of satellite orbiting the Earth. One type follows an orbit passing over both poles, scanning the atmosphere and whatever is directly below it. These satellites typically orbit at 1500 kilometres above the ground; during each successive orbit of about 100 minutes the Earth has rotated by about 25°. The satellite therefore scans a different pole-to-pole strip each time it passes. The other type of satellite orbits above the equator at an altitude of approximately 35,900 kilometers, orbiting at exactly the same speed as the Earth rotates, so that it remains over the same position. These are known as geostationary satellites.

The cameras and sensors aboard the satellites can view Earth in light of various frequencies, including infrared as well as visible. The images have then to be interpreted by scientists on Earth in order for them to be useful. The images received are digitised and then analysed and manipulated on computers.

'False colours' can be used to highlight gradations in the images. For example, false colour is used to monitor plant growth and to discriminate healthy plants from those affected by disease and drought. These differences may be detected because plants absorb large quantities of red and infrared light when they are photosynthesising normally. False-colour images help to make maps, monitor the growth and health of forests and crops, find areas of flooding or drought, and locate forest fires. They can also show pollution in the atmosphere or oceans, plot the movement of icebergs, and locate precious minerals.

Weather forecasts have greatly improved with the use of weather satellites, some of which have geostationary orbits, so they can photograph almost a complete hemisphere of the Earth at one time. They are able to measure wind speed at sea level and higher in the atmosphere, measure the heights of and distances between ocean waves, photograph the constantly changing cloud patterns, watch the weather on a global scale, and track devastating tropical storms.

The idea of geostationary orbits was first suggested by Arthur C Clark in 1945, and has been used extensively for communications satellites. Other satellites, such as the Russian Molniya series, have very eccentric orbits; they travel along a long, narrow oval path, which swings low on one side of the Earth but high on the other. Communications satellites can relay telephone, telex and television signals across the world at the speed of light. Some signals beamed to the satellite from aerials about 30 meters across on the ground require special instruments to amplify them before they are sent back down to a receiving dish elsewhere on the Earth. These satellites are often networked, so they can relay information to each other as well as to the receiving dish; this allows them to cover larger areas of the Earth's surface.

In 1997 there were 150 communication satellites in orbit around the Earth. This number does not include the meteorological and scientific satellites. The number of satellites deployed secretly by governments and the military is unknown.

Satellites can be used to spy on other countries. They can 'listen in' to radio messages from thousands of sources across the world, observe the movements of troops, track ships' paths across the oceans, sense the positions of submarines beneath the ocean surface, and take photographs of military bases and missile launching sites. Such information allows the military to plan manoeuvres more accurately.