The cycle of oxygen

The air we breathe contains about one fifth oxygen. This gas is invisible, without smell and without taste – but it is vital for us. Because we need oxygen to gain energy in our metabolism. Without this gas, neither humans nor most animals can survive.

Almost all the oxygen in the air is produced by plants through photosynthesis. In this process, the plant forms important nutrients from carbon dioxide and water with the help of sunlight. Oxygen is also produced as a by-product of photosynthesis.

The oxygen that the plant does not need is released into its environment. A large beech tree, for example, produces about as much oxygen in one hour as 50 people need to breathe in the same time. Humans and animals breathe in this oxygen, consume it and exhale carbon dioxide. In photosynthesis, plants absorb this carbon dioxide while at the same time producing new oxygen. A cycle is created between plants, humans and animals.

In the course of the Earth’s history, much more oxygen was released than the living creatures consumed when breathing. Thus, more and more oxygen entered the atmosphere. From the growing proportion of oxygen, the ozone layer was able to form high up in the stratosphere, which protects us from dangerous UV radiation.

Since people have been burning more and more oil, natural gas and coal, this natural oxygen cycle has been severely disrupted: burning consumes oxygen and at the same time carbon dioxide is emitted. For this reason, the amount of carbon dioxide in the air has risen sharply over the last 250 years. The increase in this trace gas is the main cause of the man-made greenhouse effect and thus also of the warming of the atmosphere.

Breathtaking: Mount Everest conquered without oxygen equipment!

No sensible person would have thought it possible: Reinhold Messner and Peter Habeler have climbed the highest mountain on earth without oxygen equipment. Completely exhausted but happy, the two extreme mountaineers arrived at base camp yesterday.

Their summit attempt on Everest begins on 8 May, at half past five in the morning, after an icy night in the tent. They have been on their way up from base camp since 6 May. The warnings of many doctors do not scare them: they want to climb the roof of the world without artificial oxygen. One failed attempt is already behind them. They are now making another attempt from an altitude of almost 8,000 metres. The climb in the thin high-altitude air is an ordeal, every step is torture. But the two are in top shape, and they have experience.

At noon they reach an altitude of 8,800 metres. Their legs are heavy as lead, the fatigue hard to describe. But they overcome their pain and trudge on, as if in a trance. Finally they achieve the seemingly impossible: they stand on the summit of Everest. A world record! From exhaustion, they let themselves fall into the snow. After a long break, Messner takes his camera out of his backpack and films. Back in the tent, they radio the base camp: they have made it!

During the night Messner is tormented by terrible eye pain: he is snow-blind. Habeler is injured in the ankle. Nevertheless, on 10 May the two of them manage the descent to base camp. Only now do they realise their success, a sense of triumph fills them. The sensation is perfect: Peter Habeler and Reinhold Messner have proved that Mount Everest can be climbed without oxygen equipment.

In the death zone

Doctors had warned Reinhold Messner and Peter Habeler that moving around at 8,000 metres without artificial oxygen was extremely dangerous to one’s health. Brain cells could die and controlled thinking could cease, and there was also a risk of unconsciousness. “You will come back as idiots,” they said briefly and drastically.

In fact, altitude sickness is not to be trifled with. Starting at about 2,000 metres, the thinning air can make itself felt through shortness of breath, dizziness, headaches or vomiting. With increasing altitude, the lungs absorb less and less oxygen, and the body is undersupplied. Above 7,000 metres – in the death zone – most people become unconscious if they do not receive supplementary oxygen. In the worst case, the extreme altitude leads to death. This fact has already cost many mountaineers their lives. The fact that Habeler and Messner climbed the summit without breathing apparatus really borders on a miracle. It can only be explained with the most precise planning, fabulous physical fitness and an iron will.

Record CO2 emissions

Global emissions of carbon dioxide have never been as high as they are today. In 2010, it even rose more sharply than ever before. This has now been announced by the US Department of Energy. The figures exceed even the worst fears.

For years, experts have been warning about the speed of global warming. Apparently without success: for the proportion of the climate-damaging gas carbon dioxide in the air is rising rapidly. Especially in the industrialised countries, it is constantly pouring out of chimneys and exhaust pipes. The new figures are frightening: in 2010, the world emitted a total of over 33,500 million tonnes of carbon dioxide. That is 1,900 million tonnes more than in the previous year, an increase of six percent!

According to the US study, China and India are primarily responsible for the horror increase. Both countries are growing economically. They get their energy mainly from coal-fired power plants – and thus produce a lot of CO2. Overall, China is the record holder for greenhouse gas emissions, followed by the USA, Russia and India.

Policies on global climate protection have failed completely so far. China and the USA refuse to curb their CO2 emissions. Russia, Canada and Japan also refuse to comply with directives when the main polluters balk at meeting international limits. Bad for the climate, as the new study clearly confirms on the basis of the figures.

The Keeling Curve

The world’s first CO2 measuring station was opened far away from car exhausts and factories: In 1958, the US climatologist David Keeling began to regularly measure the carbon dioxide content of the air on the volcano Mauna Loa on the island of Hawaii. This location was chosen quite deliberately. Because neither chimneys nor forests influenced the result, an average value of the trace gas in the air could be measured here. A second station in Antarctica also fulfilled these conditions. After two years, Keeling presented his results to the world: The level of carbon dioxide in the air was rising! In the following years, Keeling continued to fight for regular CO2 measurements of the atmosphere. With success: the result is the so-called Keeling curve, a collection of data that records the carbon dioxide content of the air to this day and documents the significant increase in CO2.

A shell of gas

Seen from space, it appears like a fine bluish veil that wraps around the Earth: the atmosphere. It is the envelope of air that surrounds our planet. Compared to the diameter of the Earth, this envelope is quite thin: if the Earth were the size of an apple, the atmosphere would be about the thickness of its skin.

Without the atmosphere, there would be no life on this planet, because plants, animals and humans need air to breathe. It protects us from the cold and from harmful radiation from space. It also allows meteorites to burn up before they can hit the earth’s surface. This air envelope is vital for us – but what is it actually made of?

The atmosphere is a mix of different gases. A large part of this gas mixture is nitrogen: at 78 per cent, this is almost four-fifths of the entire atmosphere. Only 21 percent consists of oxygen, which we need to breathe. The remaining one percent is made up of various trace gases – gases that only occur in the atmosphere in traces. These trace gases include methane, nitrogen oxides and, above all, carbon dioxide, or CO2 for short. Although the proportion of CO2 is quite small, this trace gas has a huge influence on our earth’s climate. This can be seen in the greenhouse effect, which heats up our planet.

The fact that the Earth has an atmosphere at all is due to gravity. It holds the gas molecules on the earth and prevents them from simply flying out into space. In fact, the air becomes thinner and thinner with increasing altitude and thus decreasing gravity. At altitudes as low as 2000 metres above sea level, this can become unpleasantly noticeable for people: They suffer from altitude sickness with shortness of breath, headaches and nausea. Extreme mountaineers who want to climb high peaks like the 8,000-metre peaks of the Himalayas therefore usually take artificial oxygen with them on their tour.

How did our air come to breathe?

What do humans and animals need to live? Food and water, of course, but above all oxygen! We get it from the air we breathe. But it wasn’t always like this: the primordial atmosphere consisted of gases like carbon dioxide and foul-smelling hydrogen sulphide in addition to water vapour. We would immediately suffocate on this “air”. But what has changed since then? Why is there oxygen in the atmosphere today? And since when?

If you look back in the history of the Earth, you will find traces of living beings that must have needed oxygen more than two billion years ago. So there must have been oxygen in the air back then.

Even older are fossilised traces of microscopically small bacteria called blue-green algae. These organisms were the first to be able to use the energy of sunlight for their metabolism. They absorbed water and carbon dioxide from their environment and, with the help of solar energy, converted them into sugar, which served them as an energy store. In addition, this chemical reaction produced oxygen – as a waste product, so to speak. However, the bacteria could not do anything with the oxygen and simply released it into the environment.

At that time, there was plenty of sunlight and carbon dioxide and the oceans were comparatively warm. These were the best conditions for the blue-green algae, and so they were able to proliferate and spread. In the process, they produced more and more oxygen, which accumulated over millions of years, first in the oceans and later in the atmosphere.

Thus, the waste product of these bacteria created the conditions for higher life forms in the water and on land. The bacteria later gave rise to the chloroplasts that capture solar energy in every plant to this day. The principle of so-called photosynthesis has also remained the same: With the help of sunlight, water and carbon dioxide are converted into sugar and oxygen. The sugar serves as a nutrient for the plant, the oxygen is released into the air and inhaled by humans and animals.

What pollutes the air?

A thick haze hangs thickly over the ground. Such a grey veil of mist can often be seen, especially in large cities and conurbations. Here, the air quality suffers because there are lots of dust particles floating around. Because they are too small to see with the naked eye, these suspended particles are also called fine dust. In addition to particulate matter, toxic gases such as carbon monoxide or sulphur dioxide float in the lower atmosphere and pollute the air.

A large part of these exhaust gases is produced by burning petroleum, coal and other substances. Cars, power plants, waste incineration and residential heating systems blow a lot of dirt into the air. In addition, there is dust kicked up – from roads, but also from factory farming, for example. The “exhaust fumes” of farm animals also contribute to the fact that the air is getting worse and worse. But it is not always humans who pollute the air: Volcanic eruptions can also contribute to higher particulate matter levels in the atmosphere.

The more pollutants there are in the air, the worse it is for our health: the respiratory tract can become ill, and the circulatory system and brain are damaged. Not only humans and animals suffer from the polluted air, plants are also damaged: If too much carbon dioxide and sulphur oxide are suspended in the air, acid (carbonic and sulphuric acid) forms in combination with water. What results is so-called “acid rain”, which causes the soil to become acidic. Plants growing in such soil become dry and die. This is called “forest dieback”. This can also happen far away from where the exhaust fumes enter the air, because the wind carries the acid rain clouds away for hundreds of kilometres.

Air pollution is particularly bad in cities with millions of inhabitants in India, Pakistan and Iran, or as in Mexico City. In Germany, there are regulations on how much air pollution is allowed. But even here, the values are not always adhered to and car traffic continues to increase.

In order to keep pollutants in the air low, it is therefore particularly important that enough forests and parks clean the air. Trees, like all green plants, absorb carbon dioxide from the air and produce oxygen, which is essential for life. “Green lungs” in big cities, i.e. green spaces and forests close to the city, are therefore particularly important for our health. And if you get on your bike more often instead of driving, you also help to keep the air clean.

The greenhouse effect

In a greenhouse, vegetables or flowers can thrive even when it is cold outside. This is because greenhouses are made of glass. The glass – or even a transparent film – allows the short-wave rays of the sun to reach the inside unhindered: The air warms up. For the long-wave heat radiation, on the other hand, the glass is impermeable, so the heat can no longer escape. That’s why it’s cosily warm in a greenhouse.

Something similar is happening on a large scale on Earth. The greenhouse gases carbon dioxide (CO2) and water vapour are naturally present in the atmosphere. Water vapour enters the air through evaporation, carbon dioxide through us breathing out. Volcanic eruptions also contribute to the natural carbon dioxide content of the air. Both gases have the same effect as the glass of a greenhouse: they allow the short-wave rays of the sun to reach the earth. At the same time, like an invisible barrier, they obstruct the long-wave heat radiation on its way back into space. The heat accumulates and the atmosphere heats up.

Without this natural greenhouse effect, hardly any life would be possible on Earth, because it would be far too cold for most living things. Instead of the current average temperature of plus 15 degrees, there would be an icy minus 18 degrees Celsius. The earth’s surface would be frozen!

The problem starts when we further increase the amount of greenhouse gases in the atmosphere. This happens primarily through the burning of oil, natural gas and coal. Heating our homes, driving cars, burning rubbish: Carbon dioxide is emitted during all these processes. This CO2 has the largest share in the man-made greenhouse effect. But the cultivation of rice or cattle farming also intensify the effect: large amounts of methane (CH4) – also a greenhouse gas – are produced in the stomachs of ruminants and in the flooded soils of rice fields. In addition, nitrous oxide, ozone and fluorocarbon are also greenhouse gases. Because all these gases slow down the heat radiation of the earth, the temperatures on our globe continue to rise.

The cycle of water

The water on earth is always on the move. Huge amounts of it are constantly moving – between sea, air and land – in an eternal cycle in which not a drop is lost.

The motor of the water cycle is the sun: it heats the water of the oceans, lakes and rivers so much that it evaporates. Plants also release water vapour into the atmosphere through tiny openings. The moist air rises, tiny water droplets gather at high altitudes and form clouds. The water falls back into the sea or onto the earth as rain, hail or snow. If it falls to earth, it seeps into the ground, feeds plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has existed almost as long as the earth has. It ensures that living beings on our planet are supplied with fresh water. And not only that: without the water cycle, the weather as we know it would not exist.

What water can do

No matter whether we drink tap water, jump into a lake or are surprised by a downpour – we constantly come into contact with water. And not only that: we ourselves are made of water, in fact about two-thirds of it. Without question, water is part of our everyday life. But what seems quite normal to us has all kinds of peculiarities. And water owes these above all to its structure.

Everything that exists on this earth is made up of tiny building blocks, the atoms. This is also the case with pure water: it is a compound of two hydrogen atoms and one oxygen atom. These combine to form a water molecule, or H2O for short. The individual water molecules are only loosely connected to each other.

This loose cohesion ensures that the bond between the molecules breaks down at high temperatures: the water evaporates. If, on the other hand, it cools down strongly, the molecules arrange themselves into a solid, regular lattice, the ice. The special thing about it: In solid form, water has a larger volume than in liquid form.

The arrangement of the water molecules provides yet another property: the surface tension of the water. Because of this tension, water spiders and water striders can walk effortlessly on a pond. But water can do even more: it is able to dissolve substances. Small grains of salt or sugar dissolve completely in water. Sea water, for example, contains large amounts of salt that we can taste but not see.

The fact that lemons ripen on the island of Mainau on Lake Constance is thanks to another ability of water: it can store heat. Lakes or seas heat up in summer and retain the heat for a long time. That is why temperatures fluctuate less on the coast than inland. Far from the coast, the temperature differences between day and night and between summer and winter are much greater than near the sea.

The blue planet

Seen from space, the earth’s sphere appears a strong blue. This is because almost three quarters of the earth is covered with water. Although water is transparent in small quantities, from a certain depth it takes on an increasingly strong blue shimmer. Because we see the mighty oceans as blue, the Earth is also called “the blue planet”. The term is especially true south of the equator. This is because the southern hemisphere is almost completely covered by sea, because a large part of the continents have migrated northwards due to plate movement.

The vast oceans contain almost all the water on earth. A lot of salt is dissolved in seawater, which is why it is not suitable as drinking water. What little fresh water there is on earth is mainly frozen in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, lakes and rivers or in the air.

But the view from the outside is deceptive: the Earth’s surface is largely covered by water, but measured against the diameter of the Earth, the oceans are only a wafer-thin layer. Therefore, the water makes up only a fraction of the Earth’s mass. By comparison, if the Earth were the size of a basketball, all the Earth’s water would fit into a ping-pong ball. And the drinking water would be proportionally even smaller than a single popcorn.

Water as a quick-change artist

Water is known to be liquid. However, this is not always true. In nature, water occurs in three states: As liquid water, as gaseous water vapour or as solid ice. Depending on external conditions, it changes from one state to the other.

The state of water depends on its pressure and temperature. If liquid water exceeds the boiling point, it evaporates and floats in the air as gaseous water vapour. Water also changes into a gaseous state when it evaporates at room temperature. However, this happens more slowly than during evaporation. If, on the other hand, the temperature drops below 0° Celsius, the water freezes into ice. As soon as water changes its state between liquid, gaseous or frozen, it changes its properties.

The special thing about water is that it has its greatest density at 4° Celsius and takes up very little space. When it freezes into solid ice, it expands and increases its volume. At the same time, its density decreases. That is why ice is lighter than water with the same volume. This is why icebergs can float in the sea. For the same reason, a lake freezes over from above and not from below in winter. This is a good thing, because otherwise we would not be able to skate until the lake was completely frozen from the bottom to the surface.

So water expands when it freezes. If you prevent it from doing so, it exerts enormous pressure. Anyone who has ever left a bottle of water outside in the freezing cold knows the consequences: After some time, the bottle bursts and the ice spills out. Ice can also crack stone in this way. This happens when water flows into cracks in the rock, freezes there and pushes outwards due to the expansion. When this force causes pieces of the stone to break off, it is called frost blasting. Anyone who has ever driven into a pothole knows the consequences. Here, the constant alternation of wetness and frost has taken its toll on the asphalt.

How do clouds form?

How clouds are formed can be observed particularly well on cold winter days: When you exhale, steam comes out of your mouth – a whitish haze hangs in the air. It forms when the moist, warm air we breathe meets colder air. This is because warm air can store a lot of moisture – significantly more than cold air. When the warm air cools down, it can no longer absorb as much water. Then the excess water collects to form small water droplets that float in the air and become visible as a white veil. It is quite similar with the “real” clouds.

The power of the sun heats the land and the water on the surface. The heat turns some of the liquid water into gaseous water: it evaporates. Because warm air is lighter than cold air, it rises. If the moist, warm air cools down more and more as it rises, the excess water collects as droplets around tiny particles of dust or soot. This is also called water condensation. The droplets are still so small and light that they float in the air. A cloud has formed.

Clouds therefore always form when warm air cools down. This can happen when the ground and the air above it warms up and rises. Also, when the wind drives the air up a mountain range, warmer air is forced upwards. At altitude, it cools down and clouds form. The same happens when a zone of warm air meets a zone of cold air. The cold air causes the lighter warm air to rise and clouds form again!

But it does not rain immediately from every cloud. Only when the water droplets combine into larger drops due to air movement and are heavy enough, do they fall back to earth as rain. If the temperature is below 0° Celsius, the drops freeze into ice crystals. The precipitation then falls as snow, or in the case of thunderclouds as small graupel or large hailstones.

There are also clouds that form directly above the earth’s surface. This often happens in autumn when the air cools down more and more. The whole landscape then appears a whitish blur. If you can see less than a kilometre through this white haze, it is called fog.

Precipitation

No matter whether it rains, hails or snows – clouds are “to blame”. Because without clouds there would be no precipitation. However, it depends mainly on the temperature whether there is a downpour or wild snow.

Most precipitation on earth falls as rain. When small water droplets collide in a cloud, they join together to form larger and heavier drops. If they are too heavy to continue floating; if the temperature is above 0° Celsius, they fall to earth as rain.

At very low air temperatures, precipitation no longer falls as rain but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds due to water droplets. If the ice formations are large and heavy enough, they dance down from the sky as snowflakes.

If, on the other hand, strong updrafts move through a cloud that is piled high, hail can occur. Small drops from the lower part of the cloud are whirled upwards, where it is colder than below. There they freeze into small ice balls, about the size of pinheads. These ice pellets are called sleet. In a very high thundercloud, when the wind is strong and the pellets are tossed up and down in the cloud several times, more and more raindrops freeze to the pellets. The more the ice beads float around in the cloud, the larger and harder they become. From half a centimetre in diameter, these ice balls are called hail. Hailstones can become larger than tennis balls and have often caused a lot of damage.

In contrast to precipitation that falls from clouds, there is also precipitation that occurs close to the earth’s surface. When the temperature on the ground drops overnight, the air can absorb less moisture. Then the excess water settles on the ground, on plants or on objects: The moisture precipitates visibly as dew. If the temperature falls below 0° Celsius at night, the water freezes on the objects and forms a whitish layer. This is no longer referred to as dew, but as frost.

How is groundwater formed?

Like water in a sponge, groundwater collects in small and large cavities under the earth. It is formed when rainwater or melt water seeps into the ground or when water from streams, rivers or lakes flows through fissures into the subsoil.

Depending on whether the soil consists of loose sand or dense earth, the water travels downwards faster or slower. And it is only when the downward flowing water hits an impermeable layer of rock such as clay that the seepage is stopped. Then, above the impermeable layer, the groundwater collects in the cavities of the soil and is stored there. If the layer of “waterproof” rock tilts, then the groundwater itself flows downslope towards nearby streams and rivers. The places where the groundwater emerges again at the surface are called springs.

When it rains very heavily or a large amount of snow melts, more water accumulates on the earth’s surface in a short time than can seep away. The water then accumulates. If it cannot drain off quickly enough, flooding occurs.

As the groundwater flows through the different layers of the earth, it is filtered and purified. This is why drinking water can be obtained particularly well from groundwater.

Groundwater is part of the water cycle. It can stay inside the earth for less than a year, but also for several million years. Under the Sahara, for example, researchers have discovered groundwater that has been under the desert sand for many thousands of years.

From trickle to stream – flowing waters

Bubbling groundwater emerges from a spring and flows down the slope as a thin trickle or a small stream: a watercourse has been created. All flowing waters start out small. On their way towards their mouths, they merge with other watercourses and continue to grow until they have become a river or even a broad stream. At its lower end, the watercourse flows into another river, a lake or the sea.

Streams, rivers or creeks – terms that come fluidly to our lips are precisely distinguished from one another by scientists (geographers). They can be classified by their water volume, their length or their width: If the watercourse is less than half a metre wide, it is called a trickle; if it is more than 2 metres wide, it is called a stream. If the watercourse swells to a width of 10 metres, it is a river. And if it gets even wider, the river can be called a stream. The Amazon or the Nile, for example, are called rivers, but the Rhine and the Danube are also streams.

The amount of water in the flowing water increases from the source to the mouth. Nevertheless, it flows slower and slower downhill. This is because the slope down which it flows is steeper at the top than at the bottom. And because the water flows faster at the top and slower downstream, it can carry more sand and debris along the upper course than the lower. Thus, more sand and debris is removed from the upper course of a river, and more is deposited in the lower course.

Delta – watercourse between river and sea

Mighty and sluggish, the Nile pours into the Mediterranean. Like all large rivers flowing through a plain, the African stream slows down towards its mouth. From the slow flow, the cargo of eroded debris and sand sinks to the bottom and is deposited. With these deposits, the river builds its own obstacle that it has to flow around. The result is a finely branched branching of sandbanks, scree slopes and river arms that becomes wider and wider towards the mouth. From the air, this widely branched network looks like a triangle. Because of its shape, it is called a delta – after the Greek letter of the same name.

Over time, the river accumulates layer upon layer of sediment. The course of the river buries its own mouth and the delta protrudes further and further into the sea: the river lengthens. This process is particularly visible on the Nile. Its delta begins near Cairo and is now 160 kilometres long and 240 kilometres wide at the coast. And the Nile delta is getting bigger and bigger: shaped like a fan, it is constantly growing further into the Mediterranean.

For a delta to develop, other conditions must be met. The coast must be flat, the tides and the ocean current must be low, because only then will the sediments not be immediately transported away again by the moving sea water. The right conditions prevail, for example, in the lower reaches of the rivers Po or Danube. Both rivers flow into the shallow sea in a delta.