Background information fact; by edgardowelelo@yahoo.com, Master of the Game
The solar system was created between 4.5 and 5 billion years ago. The Sun and Planets were formed from a huge cloud of dust and gas produced in the Big Bang. Gravity caused the cloud to collapse towards its centre and begin to rotate. At the centre of this spinning disc, temperatures rose to a point where hydrogen fused to form helium and our Sun was born. Further out, dust particles were drawn together by electrostatic and other forces and gradually grew into larger and larger rocks. Gravity drew these rocks together, and slowly these bodies grew to form the planets. The four (4) planets nearest the Sun – Mercury, Venus, Earth and Mars – were made up largely of solid material with high melting points, rocky planets with metal cores. Further away from the Sun are the gas giants – Jupiter, Saturn, Uranus and Neptune. Early in its history, the Planet Earth began to sort itself into a number of different layers. The natural decay of radioactive material at its centre generated enormous quantities of heat, which melted most of the rock, forming a liquid ‘mantle’. Around this mantle – and no thicker, relatively, than skin on a peach – formed the cooler solid crust. Then, as still today, the molten rock (magma)/or liquid rock erupted through the thin surface layer. This volcanic activity was accompanied by the release of gases such as nitrogen and carbon dioxide that formed the basis of our planet’s atmosphere. With the gases came water vapour in such large quantities that when it condensed it formed the oceans, and it was in those oceans that the first simple life forms evolved 4 billion years ago. Volcanic activity and the release of gases and water vapour were also happening on Earth’s near neighbours, but as far as we know, no life has evolved on any of the other planets in our solar system. The main reason for this seems to be simple cosmic luck. Planet Earth happened to end up orbiting the Sun at just the right distance. Mercury is closest to the Sun at an average 57.9 million km (36 million miles) and has never developed an atmosphere. It experiences the largest daily temperature fluxes of any planet, with average daytime surface temperatures of 430°C (806°F) plummeting to a night – time average of -180°C (-292°F). It is hard to imagine any life form that could survive these extremes, let alone the daily variation. Venus is 108.2 million km (62.7 million miles) from the Sun and has a thick atmosphere rich in carbon dioxide. The strong greenhouse effect of this thick atmosphere ensures that Venus stays hot around the clock, with an average surface temperature of 480°C (896°F). If Venus is too hot for life, Mars is probably too cold. It is 227.8 million km (141.6 million miles) from the Sun and has a thin atmosphere with very little carbon dioxide. Average surface temperatures are always – 50°C (-58°F), and any water that might exist beneath the planet’s surface or at its poles remain permanently frozen. The Planet Earth, though – orbiting at 149.6 million km (93 million miles) from the Sun – seems to be at just the right distance for life. Our medium – thick atmosphere has contained just the right amount of carbon dioxide to help keep Planet Earth at a perfect average surface temperature of 17°C (63°F). We live on the “Goldilocks” planet because, just like the porridge in the fairy tale, Planet Earth is neither too hot nor too cold but “just right” for life.
LIFE FROM THE MOON
Again, cosmic luck was on our side. It is generally believed that the Moon was formed about 4.5 billion years ago, when a planet about the size of Mars collided with the early Planet Earth. The huge impact threw into space an enormous quantity of the Planet Earth’s crust, which orbited the planet before gradually coalescing to form the Moon. By chance the planet that hit Planet Earth also had a liquid – iron core. In the heat of the impact, this joined up with Earth’s existing liquid iron, and our Planet Earth ended up with a much larger iron core. It is this CORE that produces the Planet Earth’s magnetic field, which itself acts as a defensive shield from the particles that stream out of the Sun as solar wind. At both poles of our Planet Earth, the magnetic field is less strong, and solar particles break through into the atmosphere to create the spectacular coloured light shows of the aurora borealis and aurora australis. For the most part, though, the Planet Earth is protected from deadly ultraviolet radiation that otherwise would scorch the surface and destroy all life. The impact was to have other profound influences. So much of the Planet Earth’s surface was thrown into space to form the Moon that only about 30 per cent of the original Earth’s crust was left. What remained was so thin that the continental plates moved around more easily. This continental drift has played a key role in driving evolution. Over the millennia, the freely moving landmasses (continents) have continually shaped our Planet Earth’s surface. Their collisions have created great mountain ranges such as the Himalayas and ripped apart wide trenches such as Africa’s Rift Valley. In the process, new and different habitats have constantly been created, and a wide variety of life evolved to exploit these new and changing environments. Without the collision that created the Moon, the plates would be locked together as they are on Venus, and there would be far fewer habitats on Planet Earth today. The collision had one other dramatic effect. It knocked our Planet Earth so that it was no longer spinning on a straight axis with respect to the Sun. The angle of tilt it created was roughly 23 degrees off the perpendicular, and that tilt remains today. Without this, life on Planet Earth would be very different. Day length all over the world would be the same all year round. The Sun’s warming influence would also remain constant throughout the year and there would be no seasonal change. Without the warmth of summer, the poles would on average be far colder, and their frozen influence would extend further towards the equator. There would be no cycle of wet and dry seasons in the subtropical regions, and the world’s deserts would be far more extensive. There would be no need for animals to migrate, and life would probably be far less diverse. Ours is the largest moon relative to its mother planet, and this gives it a powerful gravitational influence. It is the pull of the Moon’s gravity that plays the dominant role in creating the oceans’ tidal cycle. Less well known but equally important is the dampening and stabilizing effect the Moon’s gravity has on Earth’s angle of tilt. Without such a large moon so close by, the Planet Earth would be at the mercy of the gravitational influence of the Sun and Jupiter. The power of this pull would vary as Jupiter circled closer or further away. Without the Moon acting as a stabilizing gyro, the Planet Earth’s angle of tilt would vary chaotically and at times reach as far as 90 degrees to the perpendicular. This would leave the North Pole pointing directly towards the Sun, causing the whole ice cap to melt and widespread flooding of our Planet Earth. The Moon then is a vital climate regulator on Planet Earth, providing the stability for life to evolve.
SUN PLUS WATER EQUALS LIFE
All life on our Planet Earth ultimately depends on two vital ingredients – energy from the Sun and liquid water. This at least was the accepted view until 1977, when deep – sea explorers discovered a completely new ecosystem of animas around hot volcanic vents on the floor of the ocean abyss. It is totally dark at around 3000m (10,000 feet), and for a while nobody could work out how this community, as productive as the richest coral reef, was getting its energy. Eventually they discovered that specialized bacteria were fixing energy from the sulphides pouring out of the vents. But even the animals in this food chain are not living totally independently of the Sun’s energy. They all use oxygen to burn the compounds supplied by the fixing bacteria or obtained by eating the bacteria themselves. This oxygen is created in shallow, sunlit waters by plants that are themselves dependent on the Sun for their energy. At first sight the vent communities seem to live independently of the Sun, but ultimately they could survive only on a sunlit planet.
THE SOLAR PUMP
The enormous variety among the different habitats on our Planet Earth, from the rich tropical jungles to the barren polar wastes, is shaped principally by the differing availability of the vital ingredients of sunlight and water. The amount of the Sun’s energy that reaches the Planet Earth is not evenly spread. More of it is available around the equator because there the Sun’s rays have to travel through less of the Earth’s atmosphere than they do at the poles. At higher latitudes, the lower angle of the Sun also means the energy is spread over a wider area than in the tropics. The amount of water available to life on land has a more complex distribution, but even that is largely influenced by the Sun. Ninety per cent (90%) of the world’s fresh water is created by evaporation off the ocean, and most of that occurs near the equator in warm tropical seas. The other 10 per cent comes from the surface of lakes and rivers or is released by plants. The water vapour is carried high into the atmosphere on rising warm air, and in the process it cools and forms clouds, which are blown round the world by winds. Exactly where these clouds release rain depends on many factors, but mountains play a key role. They (mountains) cause the clouds to rise and cool, and their water vapour condenses as rain. Because most of the Suns’ energy falls around the equator, it is here that you find most of the rising hot air that carries moisture into the atmosphere. As it rises and cools, it produces torrential downpours. Having lost its moisture, this air is deflected by the spin of the Planet Earth and flows north and south away from the equator. As it reaches cooler higher latitudes, it sinks, and this dry air creates the bands of desert lands found along both tropics. In the oceans, the variety and quantity of life is also determined mainly by the availability of energy from the Sun. The top 100m (340 feet) or so of sunlit, shallow waters contains 90 per cent of life in oceans. Here it is the availability not of water but of vital nutrients, especially phosphorus and nitrogen, that has shaped marine communities. Though tropical seas receive the largest amounts of sunlight, they (tropical seas) are, with the exception of the coral reefs and seagrass beds, largely deserts. This is because these waters are calm, allowing most of the nutrients to sink to the depths. The ocean’s greatest riches tend to be found in rough temperate seas or where upwelling currents provide a good supply of nutrients.
EQUATORIALRICHES AND POLAR POVERTY
With sunlight and fresh water abundant and available all year round, it is hardly surprising that the most productive habitat on our Planet Earth is in the tropical regions around the equator. The tropical rainforest is life at its most abundant, and plants grow here in greater profusion than in any other habitat. Up to 200 different tree species grow in a single hectare (2.5 acres), compared to 10-20 tree species in temperate woodland. Though tropical rainforests now cover just 3 per cent of our Planet Earth’s land surface, they (tropical rainforests) are thought to contain more than 50 per cent of the animal and plant species yet described. Growing conditions for plants are perfect, with warm year – round temperatures between 20°C and 28°C (68-83°F) and continual reliable rainfall that averages 2500mm (100 inches) a year. As plants are the primary producers and almost all animals are ultimately dependent on them for their energy, the amount of carbon fixed by the plants per year through photosynthesis is a good measure of the total productivity of a habitat. Tropical rainforests are the record – breakers on land by this measure, producing 1000-3500g (35-125 ounces) of carbon per square metre (11 square feet) each year. They also win out in terms of another good measure of a habitat’s productivity – the total weight of animals and plants, the biomass, which reaches 4500g (160 ounces) in every square metre. The coral reefs that are restricted to the same tropical regions of our Planet Earth are often called the rainforests of the seas, and in terms of productivity they deserve this accolade. They produce between 1500 and 3700g (53-130 ounces) of carbon per square metre per year. This figure is particularly impressive considering the fact that the shallow tropical seas in which they grow are so low in the vital nutrients of nitrogen and phosphorus. But coral reefs and the other animals that share this habitat have reached these heights of productivity through very efficient recycling of their nutrient needs.
THE DRIER TROPICS
As you travel north and south away from the equator and towards the tropics, the climate changes. Though there is plenty of sunshine and it is warm all year round, rainfall is less reliable. Each year there a distinct wet and dry season, and the plants that grow there have to be able to cope with these changing conditions. The monsoon forest of northern India and Southeast Asia is a good example. The evergreen trees typical of tropical rainforest are replaced by deciduous species such as teak and ebony, which save water by dropping their leaves each year. These trees are shorter, more widely spaced and have deeper roots than those in the tropical forests. This is the land of the langur monkey and tiger, which each year live through a dry season from November to April before being deluged by the monsoon rains. In drier regions such as northern Australia, trees like the eucalyptus are even better equipped to deal with drought conditions between ‘the big wets’. In the driest subtropical regions, trees are replaced by grassland – the vast open landscapes of the savannahs. This is a habitat that supports unusually high numbers of large animals. It is warm all year, but the lives of the plants and animals here are dominated by the annual cycle of wet and dry. For much of the year, the East African Savannah is parched golden brown, but come the rains, it flushes with new green growth. The annual migration of the wildebeest in the Serengeti follows this new grass. The savannahs are only half as productive as the tropical rainforests, fixing 200 – 2000g (8-80 ounces) of carbon per square metre per year, but they achieve that with just a tenth of the biomass of plants in the tropical rainforest. Savannahs are one of the most efficient ecosystems on the Planet Earth.
THE THIRSTY LANDS (THE TROPICS OF CANCER & CAPRICORN)
As you approach the tropics of Cancer and Capricorn, you reach the thirsty parts of our Planet Earth. These are the high – pressure regions, where warm, dry air is sinking and there is little rainfall – on average, less than 100mm (4 inches) per year, and in true desert regions, less than 50mm (2 inches). There is no shortage of sunlight, though, and because humidity levels are so low and there is very little cloud cover, 90 per cent of the Sun’s radiation reaches the ground. Day temperatures frequently exceed 38°C (100°F), and at night, when heat is rapidly lost back to the atmosphere, they can drop more than 44°C (79°F) in just a few hours. Though most deserts are hot all year round, some high altitude ones, such as the Gobi in Mongolia, can be extremely cold, with winter temperatures of – 21°C (-5.8°F). Cold deserts such as the Gobi are usually far inland and in the shadow of mountain ranges, where moist air from the oceans rarely penetrates. Life for animals and plants in the Planet Earth’s desert regions is about surviving extremes of temperature and very limited supplies of fresh water. Vegetation is sparse and annual productivity rates are as low as 300g (11 ounces) of carbon per square metre. These low levels are similar to those found at far higher latitudes and way out to sea in the open ocean. The vast blue desert of the tropical oceans receives ample supplies of sunshine, but once again, lack of nutrients limits phytoplankton growth – the algae at the base of most marine food chains. Tropical oceans are the famously calm seas of the doldrums, and there is little surface mixing to bring nutrients up from deeper water. Neither do these waters benefit from the regular supply of nutrients that rivers bring to shallower waters on the continental shelf.
TEMPERANCE
Halfway between the tropics and the polar circles are the temperate regions of our Planet Earth. Here you find the grasslands and deciduous woodlands of northern Europe and the prairie and coniferous woodlands of North America. There is less of the Sun’s energy here than in the tropics, but summers are still warm and moist – rainfall averages 500 – 1500mm (20-53 inches) per year. This is also a very seasonal part of the world, and the animals and plants that live here have to adapt to distinct winters, springs, summers and autumns. Winters, particularly in northern temperate climes, can be hard, and much of the fresh water is locked up as snow and ice. In Europe, the oak, beech, ash and chestnut trees of the broadleaf woodland shelter wild boar, deer and squirrels. The structure here is simpler than in tropical forests, with just two canopy layers. Shafts of sunlight easily penetrate to the forest floor, where bushes, flowers and mosses flourish. This is a very productive habitat supporting an average biomass of 3000g (106 ounces) per square metre. In North America coniferous trees are also common. The most spectacular are the giant redwood forests of the Pacific coast, where a mild and very moist coastal climate nurtures the tallest and largest trees on our Planet Earth. As you journey further north, coniferous trees become increasingly dominant. All across northern North America and Eurasia is a continuous belt of coniferous woodland called the BOREAL FOREST, or TAIGA. Every third tree on our Planet Earth is found in this forest, which is home to wolves and lynx, moose and reindeer. Winters here can be long and harsh, and for nine months of the year, most of the available water is frozen as snow or ice. When the spring melt does finally arrive, the ground quickly becomes waterlogged, because the permafrost layer keeps moisture at the surface. Vegetation is slow to decay, and the acidic soils trap nutrients needed for plant growth. The short growing seasons and demanding conditions make it impossible for deciduous trees to grow, but conifers do well because their conical shape and needle – like, waxy leaves help them survive the cold and drought of winter. Average rainfall is just 400-600mm (16-23.5 inches) per year, and in some regions as low as 150mm (6 inches). This is no more than many semi – desert areas receive, but in these colder conditions less moisture is lost through evaporation. But productivity in these northern boreal forests is only 200 – 1500g (7-53 ounces) per square metre – nearly half that of the deciduous woodlands to the warmer south.
THE GREAT GRASSLANDS
In the interiors of the vast landmasses of North America and Eurasia, you experience continental climatic conditions. Without the softening influence of the oceans and the moisture they provide, the winters here are cold and the summers hot and dry. Trees do not do well in these parched conditions and give way to the wide – open temperate grasslands. In the northern hemisphere, there is the North American prairie with its pronghorn antelopes, prairie dogs and remnants of the bison herds that were once millions strong. In Europe there is the Russian steppe, where until very recently large herds of saiga antelope used to roam. In the southern hemisphere exactly the same habitat is found in the South American pampas and the South African veldt. On either side of the equator, north and south, there always seems to be this mirror of similar habitats created by the same conditions of sunlight and rainfall. Productivity in the temperate grasslands is similar to that found in the savannahs nearer the equator – both depending on that great survivor, grass. Remarkably tolerant of both changing temperatures and lack of water, grass has become the most abundant plant on Planet Earth. Today it covers a quarter of the Planet Earth’s land surface and supports more large animals than any other habitat.
THE FROZEN POLES
At 65 degrees north, you cross the tree line and enter the bleak, barren world of the tundra. Here there are long, dark winters, with temperatures as low as – 30°C (-22°F). This is a high – pressure region, and the chilled air delivers little rain. Winters are too cold for trees to grow, and productivity here is as low as southern deserts, with only 100 – 400g (3.5-14 ounces) of carbon per square metre. In the winter only a few hardy residents are left, such as the musk ox, and the Arctic fox. In the short summer, though, visitors migrate from the south. Snow geese and caribou take advantage of the long days and short flush of new growth. In both the north and the south, 66.5 degrees marks the polar circle – the line beyond which, for at least one day of the year, the Sun never sets. As you approach the poles, the number of days increases until, at the North and South poles, the Sun rises and sets only once a year. To make things worse, the snow and ice reflect back 85 per cent of the little sunlight. The altitude and isolation of Antarctica make it colder still. Average annual temperatures are – 55° to – 60°C (-67° to -76°F), and only about 50mm (2 inches) of snow falls each year. Nothing grows except lichens on a few mountaintops piercing through the ice sheet that engulfs the continent. Productivity is as close to zero as you can find anywhere.
SEASONAL SHIFTS
The chance collision that created the Moon and left the Planet Earth spinning on a tilted axis has shaped the lives of wildlife more than any other factor. As the Planet Earth orbits the Sun, different parts are tilted towards it at different times. In the northern hemisphere, the North Pole is tilted away from the Sun in December, producing the dark, cold winter. As the Planet Earth continues its annual orbit, the North Pole is gradually turned towards the Sun, which with every passing day rises higher in the sky, and day lengths increase. In March, at the spring equinox, the Sun is directly over the equator and the night and day length is exactly equal. The Sun’s influence continues to increase in the northern hemisphere until, on 21 June, the summer solstice, it is directly over the Tropic of cancer. This is the height of the northern summer, when areas north of the Arctic Circle experience 24 hour – daylight. From then on, the North Pole begins to tilt away from the Sun, and day length starts to decrease. Summer turns to autumn, and by 21 September – the autumnal equinox – the Sun is again directly over the equator. Gradually, the southern hemisphere tilts closer to the Sun until, at the winter solstice on 21 December, the Sun is directly over the Tropic of CAPRICORN. This marks both the height of the northern winter and, of course, the southern summer. The transition from winter to summer at the poles is sudden and dramatic. In Antarctica, for instance, the continent effectively doubles in size in just a few months as the surrounding ocean freezes. This is the most drastic seasonal change on our Planet Earth, and with the exception of one bird and one seal, the wildlife has no option but to head north with the departing Sun. The temperate regions, halfway between the polar circles and the tropics, are where the seasonal rhythm of our Planet Earth is most obvious. Here are four (4) distinct seasons, but none extreme, and so there are many permanent residents, which adapt with the seasons. The temperate ocean also experiences four (4) seasons. In shallow waters, the lengthening days of spring fuel the phytoplankton bloom, which drives the marine food chain. Even in the total darkness of the deep – sea floor, a seasonal cycle exists. Through time-lapse photography, scientists have shown that the quantity of the marine “snow” (detritus from the waters above) follows a seasonal cycle that closely tracks the level of activity in sunlit waters above. In the regions around the tropics, where day length hardly varies, there are only two seasons – wet and dry. But even these are produced by the Earth’s orbit around the Sun. When the Sun is directly overhead, more water evaporates off the oceans and there is more rising hot air to carry it up into the atmosphere. This produces more clouds, storms and rain. In June, when the Sun is directly over the Tropic of Cancer, the northern tropical regions experience their rainy season – while in December, when the Sun is over Capricorn, the southern hemisphere enjoys the rains. Only in equational regions, where the Sun rises and sets at exactly the same time each day of the year, is there no seasonal cycle. For billions of animals on our Planet Earth, seasonal change in their habitats means they are constantly on the move, following the warming influence of the Sun or the changing supplies of precious fresh water. Every autumn, some populations of red admiral butterflies leave northern Europe and fly nearly 3,200km (2,000 miles) to North Africa to escape the cold. Every summer, 3 million caribou undertake the longest migration by any land mammal, up to 3000km (1,865 miles),in search of fresh pasture. European swifts and permanently on the wing as they migrate 18,000km (11,000 miles), following the Sun and their insect prey, first across the Sahara to West Africa and then, when food becomes scarce, on to East Africa and finally back again to Europe to breed. In the oceans, most of the baleen whales travel huge distances, from their breeding nurseries in warm tropical waters to their summer feeding grounds in the high latitudes. In fact, all the animals and plants on Planet Earth have lives dominated by a chance cosmic event – a collision that shifted our lucky Planet Earth by 23.5 degrees and, in the process, changed the whole history of life on Planet Earth.
NOTE AND REMEMBER (ROUGH, RICH SEAS)
Though life on land is richest in the tropics, the reverse is true in the seas. Distribution of life tends to be patchy, influenced by local factors such as currents, but generally, the seas in temperate latitudes are the most productive. Here, air travelling up from the equator meets polar air travelling down, creating unstable weather. In the North Atlantic, it takes the form of weather fronts that rush towards Europe, bringing dramatic storms to western coastlines. Particularly in shallow waters on the continental shelf, these storms mix up the sea as deep as 200m (650 feet). Phosphorus and nitrogen are brought up from the depths and, every spring, fuel a massive bloom of phytoplankton. By the end of the summer, the nutrients have been used up, but mixing occurs again in winter, recharging the ocean’s batteries. For example, the world’s second largest fish, the basking shark, it is found not in clear tropical seas but in nutrient – rich temperate waters, where it feasts on the plankton that multiply here.
