Earth: A Unique Life Sustaining Planet

Step outside on an ordinary morning and nothing feels remarkable. There is air to breathe, water in the tap, and a temperature your body can cope with. That ordinary feeling hides something extraordinary. Of all the worlds studied so far, Earth is the only one known to carry life, and it manages this because several separate conditions happen to be satisfied here at the same time. Take away any one of them and the whole arrangement fails.

The right distance from the Sun. Earth travels around the Sun at an average distance of about 15 crore kilometres, which is roughly 150 million kilometres. Sunlight takes a little over eight minutes to cross that gap. At this distance the surface receives enough heat to keep most of its water in the liquid state, and not so much that the oceans boil away. A planet placed much closer would lose its water as vapour, and one placed much farther out would keep it locked up as ice. The band of distances around a star in which liquid water can survive on the surface of a planet is called the habitable zone, and Earth sits comfortably inside it.

Enough mass to hold an atmosphere. Gases escape from a small world because its gravity is too weak to keep them. Earth is massive enough for its gravitational pull to hold a deep envelope of air pressed against the ground. The Moon is far smaller, its pull is far weaker, and whatever gases it may once have had drifted away into space long ago. That is why the Moon has no air, no wind and no weather, and why its surface goes from scorching in direct sunlight to bitterly cold in shadow, with nothing in between to even out the difference.

Liquid water, and plenty of it. Close to 71 percent of the surface of our planet is covered by water. Every living thing we know of is mostly water inside, and the chemical changes that keep a body alive take place in watery solution. Water also carries heat around the globe through ocean currents and through the water cycle, so no region becomes impossibly hot or impossibly cold.

A magnetic shield. Deep below the crust, the outer core of Earth is molten metal rich in iron. Its slow churning makes the whole planet behave like a giant magnet. The magnetic field that results reaches far out into space and turns aside most of the stream of charged particles that the Sun throws out in every direction.

A steady spin and a gentle tilt. Earth turns once in about 24 hours, so no side faces the Sun long enough to roast or stays in darkness long enough to freeze solid. The axis is tilted by about 23.5 degrees, and that tilt is what gives us seasons rather than one unchanging climate.

Now compare our neighbours. Venus is almost the same size as Earth but is wrapped in a thick blanket of carbon dioxide, and its surface stays near 460 degrees Celsius, hot enough to melt lead. Mars is about half as wide as Earth, its air is thin and mostly carbon dioxide, and between the cold and the very low pressure of that thin air, liquid water cannot last on the open surface. Same solar system, same Sun, very different outcomes. That contrast is the best argument there is for looking after the one planet that works.

The atmosphere is the envelope of gases held around Earth by gravity. It has no sharp upper edge; it simply grows thinner and thinner with height until it fades into empty space. Most of its mass is squeezed into the lowest few kilometres, which is why air feels perfectly ordinary at sea level and noticeably thin on a high mountain.

What air is made of. Dry air is about 78 percent nitrogen and about 21 percent oxygen. The remaining one percent is mostly argon, with carbon dioxide at roughly 0.04 percent and traces of neon, helium, methane and other gases. On top of this dry mixture sits water vapour, whose share is not fixed at all: it is close to nothing over a desert and can be three or four percent over a warm sea. Dust, pollen, smoke and salt particles float in the mixture as well, and they matter more than they sound, because water droplets need such particles to condense on when a cloud forms.

The troposphere is the layer we live in. It reaches from the ground to about 8 kilometres over the poles and about 16 kilometres over the equator, roughly 12 kilometres on average. It holds about three quarters of the mass of the atmosphere and nearly all of its water vapour, so clouds, rain, snow, thunderstorms and cyclones all belong here. Temperature falls steadily as you climb, which is why tall mountains stay capped with snow even in a hot country.

The stratosphere sits above it and reaches up to about 50 kilometres. It contains the ozone layer, and because ozone soaks up ultraviolet radiation from the Sun, temperature in this layer rises with height instead of falling. The air here is dry and steady with very little up and down movement, so long distance aircraft prefer to cruise in its lower part, above the bumpy weather.

The mesosphere extends from about 50 to about 85 kilometres and is the coldest part of the atmosphere. It is also the layer that shields us from small pieces of space rock. A meteoroid entering at very high speed slams into the air here, squeezing the air ahead of it so hard that both the air and the rock become white hot, and the rock usually burns away completely. The bright streak we call a shooting star is that burning, not a star at all.

The thermosphere lies above about 85 kilometres. The gas there is unbelievably thin, yet the few particles present take in so much energy from the Sun that their temperature is extremely high. It would still not feel hot to a person, because there are far too few particles there to pass that energy on. A part of this region is electrically charged and is known as the ionosphere. It bounces certain radio waves back towards the ground, which is how a radio signal can reach far beyond the horizon. Auroras, the coloured glows seen near the poles, also happen at these heights, and space stations orbit here.

The exosphere is the outermost region, where the last stray particles slowly leak away into space and the atmosphere quietly ends.

Put together, the atmosphere supplies the oxygen animals breathe and the carbon dioxide plants use, filters dangerous radiation, burns up incoming rock, carries water from ocean to land, lets sound travel, and works like a blanket that stops the surface from freezing at night. Even the colour of the sky is its doing, because air particles scatter sunlight and make the daytime sky blue rather than black.

Water is so familiar that it is easy to miss how unusual it is. It stays liquid across a wide range of everyday temperatures, it dissolves an enormous variety of substances, and it takes in and gives out a great deal of heat without changing temperature quickly. Those three properties together are the reason living things are built around water.

Water inside living things. Roughly two thirds of the human body is water. Blood is mostly water and carries digested food and wastes in solution, along with oxygen held by the pigment inside its red cells; sap does a similar carrying job inside a plant; the fluid within every cell is water based. Digestion, breathing and every other life process depends on substances being dissolved and carried from place to place, and that needs a liquid. No liquid water, no life of the kind we know.

The water cycle. The Sun drives a constant circulation of water between sea, sky and land. Heat makes water evaporate from oceans, rivers, lakes and damp soil, and plants add more through transpiration from their leaves. The vapour rises, cools, and condenses on tiny floating particles to form clouds. When the droplets grow heavy enough they fall as rain, snow or hail. Some of the fallen water runs off into streams and returns to the sea, some soaks into the ground and becomes groundwater, and some evaporates straight back into the air. The total quantity of water on Earth hardly changes at all; it simply keeps moving from one store to another. In our country this cycle arrives in a very concentrated form as the monsoon, so a weak monsoon is felt in wells, canals and crops for many months afterwards.

How little of it we can actually use. About 97 parts out of every 100 of the water on Earth is salty sea water, which cannot be drunk or used on fields without expensive treatment. Of the small fresh remainder, most is frozen in ice sheets and glaciers or lies deep underground. That leaves under one part in a hundred of all the water on the planet in the rivers, lakes, ponds and shallow groundwater that people, animals and farms actually draw from. A water planet, in other words, with a very thin margin of usable water.

Water as a thermostat. Land heats quickly during the day and cools quickly at night. Water does neither. Oceans absorb a huge quantity of heat in the hot months and give it back slowly in the cold ones, so places near the coast have milder days and nights than places far inland at the same latitude. On the scale of the whole planet, the oceans smooth out temperature swings that would otherwise be violent enough to make life very difficult.

Because usable water is scarce, the way we treat it matters more than the total figure suggests. Pumping groundwater faster than rain can refill it makes borewells go deeper and eventually run dry, and it can also concentrate the salts and chemicals left behind. Letting rain soak into the ground, instead of allowing it to rush off concrete into a drain, is one of the simplest repairs available to us, and it is something a school or a housing colony can do without waiting for anyone else.

Two invisible shields stand between the surface of Earth and radiation from space. Neither can be seen or felt, and life on land would be impossible without both.

The magnetic shield. The core of our planet has two parts: a solid inner core, and around it an outer core of molten metal that is largely iron. This liquid metal is in constant slow motion, and moving metal carries electric current, which produces magnetism. The result is that Earth behaves like an enormous bar magnet whose poles lie near, but not exactly at, the geographic poles. A compass needle lines up with this field, which is how sailors found their way across open oceans for centuries before any electronic instrument existed.

The field stretches far beyond the atmosphere into a huge region of surrounding space. The Sun continuously throws out a stream of fast moving, electrically charged particles, often called the solar wind. If those particles arrived unhindered they would strip away the upper atmosphere over long ages and damage living tissue at the surface. Most of them are instead deflected around the planet by the magnetic field. Some are guided along the field lines towards the polar regions, where they crash into gas particles high in the atmosphere and make them glow. Those shifting curtains of green and red light are the auroras. Mars has no global magnetic field of this kind today, and its atmosphere is very thin, which is a strong hint about how useful such a shield is.

The ozone shield. Ordinary oxygen exists as molecules made of two oxygen atoms joined together. Ozone is a different form of oxygen in which three atoms are joined, written as O3. It forms high in the stratosphere, where energetic ultraviolet radiation splits oxygen molecules apart and the freed atoms attach themselves to other oxygen molecules. Ozone is never plentiful; even inside the ozone layer it is only a trace gas among far more abundant ones. Yet that trace absorbs the greater part of the harmful ultraviolet radiation before it can reach the ground.

Why that matters is easy to state. Strong ultraviolet radiation burns skin and raises the risk of skin cancer, damages the eyes, weakens the defence system of the body, reduces crop yields, and kills the tiny floating organisms that sit at the base of ocean food chains.

Damage and repair. During the twentieth century a group of manufactured chemicals called chlorofluorocarbons, or CFCs, came into wide use in refrigerators, air conditioners, aerosol sprays and foam packaging. CFCs are safe to breathe and do not catch fire, which is why nobody worried about them at first. But they drift slowly up to the stratosphere, break apart there, and destroy ozone. In the 1980s scientists reported a severe seasonal thinning of the ozone layer over Antarctica, and the term ozone hole entered everyday language. In 1987 countries agreed to phase out the production of CFCs, and their use has fallen sharply since then. The ozone layer is expected to recover slowly over the coming decades. It stands as one of the clearest cases of a worldwide environmental problem being tackled by co-operation between nations.

One caution about ozone. It protects us only when it is high above our heads. Ozone formed near the ground, when sunlight acts on vehicle exhaust, is itself a pollutant that irritates the lungs and damages plants.

A greenhouse is a small glass or plastic house used to grow plants in cold places. Sunlight passes in through the transparent walls, warms the soil and plants inside, and the warmed air cannot easily escape, so the inside stays warmer than the outside. Earth does something similar, but with gases in place of glass.

How the natural greenhouse effect works. Energy arrives from the Sun mainly as visible light, and light passes through the atmosphere without much difficulty. It is absorbed by land and sea, which warm up. Every warm object gives out energy, so the surface radiates energy back outwards, but it sends it out as infrared radiation, which is invisible heat radiation of a different kind. Certain gases in the air absorb infrared radiation strongly. They take in this outgoing heat and send a part of it back down towards the ground. Heat that would have escaped straight to space is therefore held in the lower atmosphere for a while, and the surface settles at a higher temperature than it otherwise would.

The gases responsible are called greenhouse gases. Water vapour is the most abundant of them; then come carbon dioxide, methane and nitrous oxide, along with small amounts of ozone and manufactured gases such as CFCs. Notice that nitrogen and oxygen, which together make up about 99 percent of dry air, play almost no part in this. A gas can be rare and still be powerful.

It is natural, and we need it. Estimates suggest that with no greenhouse effect at all, the average surface temperature of Earth would be roughly 33 degrees Celsius lower than it is, somewhere near 18 degrees below zero instead of the roughly 15 degrees above zero we actually have. The oceans would be frozen and the planet would be a white desert. So the natural greenhouse effect is not the problem. The problem is that we have been making it stronger.

The enhanced greenhouse effect. Burning coal, petrol, diesel, kerosene and natural gas releases carbon dioxide. Cutting down forests makes matters worse twice over, since growing trees take carbon dioxide out of the air and burnt trees put it back. Flooded paddy fields, cattle, landfill sites and leaking gas pipelines add methane, which traps heat far more effectively than carbon dioxide does for the same mass. Heavy use of nitrogen fertiliser adds nitrous oxide. Measurements show that carbon dioxide in the air has climbed from roughly 280 parts per million before large scale industry to more than 400 parts per million today.

What follows from that. The average surface temperature of Earth has risen by a little over one degree Celsius since the late 1800s. One degree sounds trivial until you see what moves with it: glaciers and ice sheets melting, sea level creeping upward and threatening low lying coasts and islands, longer and fiercer heat waves, rainfall arriving in shorter and heavier bursts with dry spells in between, stronger cyclones, coral reefs bleaching, and harvests becoming harder to predict. Farming in our country leans heavily on the timing of the monsoon, so it is directly exposed to changes of this kind.

The remedies all point one way: burn less fossil fuel. Electricity from sunlight and wind, buses and trains in place of single occupant cars, efficient appliances and better built houses, less wasted food, and protecting and replanting forests each cut the amount of greenhouse gas we add to the air.

A pollutant is a substance that harms air, water or soil when it is present in the wrong place or in too large an amount, and pollution is what results. It is worth remembering that many pollutants are perfectly ordinary substances behaving badly. Carbon dioxide is harmless in the breath you just let out and a serious problem in bulk. Cow dung is a fertiliser in a field and a contaminant in a well.

Air pollution: what, and from where. Vehicles release carbon monoxide, oxides of nitrogen, unburnt fuel vapour and fine soot. Thermal power stations and factories release sulphur dioxide, oxides of nitrogen, ash and smoke. Burning of crop residue and household garbage, brick kilns, construction dust, and cooking on wood or dung in a poorly ventilated kitchen all add their share. Some of these particles are small enough to be pulled deep into the lungs, and daily air quality readings published in our cities report exactly those.

What air pollution does. It irritates the eyes, nose and throat, sets off coughing and asthma attacks, and over years reduces how well the lungs work, with children and elderly people suffering most. Carbon monoxide is especially dangerous because it attaches itself to the blood pigment that normally carries oxygen, so the body is starved of oxygen even when the air seems fine. Smoke and fog together make smog, which cuts visibility and traps pollutants near the ground. Sulphur dioxide and oxides of nitrogen dissolve in rain water and turn it acidic. This acid rain eats into marble and limestone buildings, strips nutrients from soil, damages leaves, and makes lake water unfit for fish. Marble monuments in our country have visibly suffered from a mixture of acidic gases and settling dust.

Water pollution: what, and from where. The largest single source is sewage that reaches rivers and lakes without treatment. Industries add effluents carrying dyes, acids and heavy metals such as lead and mercury. Fertilisers and pesticides wash off farmland whenever it rains. Detergents, discarded plastic, waste from festivals and offerings, oil leaks, and hot water released by power stations all add to the load.

What water pollution does. Sewage carries the germs of cholera, typhoid, jaundice and diarrhoea, and these still cause a great deal of illness wherever drinking water and sewage are allowed to mix. Fertiliser washed into a lake feeds a sudden heavy growth of algae; when that algae dies and rots, the rotting uses up the oxygen dissolved in the water and the fish suffocate. Heavy metals collect in the bodies of fish and pass on to whoever eats them. Plastic breaks into fragments that animals swallow and cannot digest. Groundwater in some regions carries harmful levels of fluoride or arsenic, and pumping too hard can make such problems worse.

Making water safe. Water fit to drink is called potable water. Clear water is not automatically potable, because germs and dissolved chemicals do not always change how water looks. At a treatment plant, water is first allowed to stand so that solids settle, then filtered through beds of sand, then disinfected with chlorine or with ultraviolet light. At home, boiling for a few minutes kills germs, a candle filter removes suspended dirt, and chlorine tablets are useful after a flood. The real cure, though, lies at the other end of the pipe: treating sewage and industrial waste before releasing it, rather than trying to clean it out of drinking water afterwards.

Natural resources are the useful things we take from the environment: air, water, soil, minerals, fuels, forests and wildlife. They fall into two groups. Renewable resources are replaced by nature within a human lifetime, such as sunlight, wind, flowing water and forests, provided we treat them with some care. Soil sits awkwardly in between, because it does form naturally but so slowly that soil washed away by erosion is not replaced within a human lifetime. Non renewable resources exist in a fixed stock that cannot be refilled at any useful speed, such as coal, petroleum, natural gas and metal ores. Coal and petroleum formed from the remains of organisms buried for millions of years. What we burn in a decade took an unimaginably long time to make, and burning it also loads the air with carbon dioxide and other pollutants.

Conservation does not mean refusing to use resources. It means using them carefully, without waste, so that they remain available later and so that using them does the least possible damage on the way.

The three Rs, in that order. Reduce comes first, because the cleanest unit of electricity is the one you never needed and the cheapest litre of water is the one you did not spill. Reuse comes second: refill bottles, carry a cloth bag, hand on clothes and books, and repair rather than replace. Recycle comes last, not because it is unimportant but because recycling itself consumes energy and water. Sorting waste at home into wet and dry is what makes recycling possible at all. Wet kitchen waste can be composted into manure for pots and gardens, and old electronic items should go to an e-waste collection point rather than into a bin, since they hold both valuable metals and poisonous ones.

Water. Close the tap while brushing, mend a dripping tap promptly, use a bucket rather than a running shower, water plants in the cool of the evening so that less evaporates, and reuse the water in which vegetables were washed. On a larger scale, drip irrigation delivers water to the root of each plant instead of flooding a whole field, and rainwater harvesting catches rain from a roof or a courtyard and either stores it in a tank or sends it into a recharge pit so that it refills the groundwater below.

Energy. Switch off lights and fans when leaving a room, use LED lamps, choose appliances with a good star rating, keep the refrigerator door shut, dry clothes in the sun, and walk, cycle, share a ride or take a bus for short journeys. Rooftop solar panels and solar water heaters turn sunlight straight into useful energy with no fuel bill and no smoke.

Forests and wildlife. Forests hold soil in place, help refill groundwater, absorb carbon dioxide and shelter wildlife. Using less paper, planting and actually caring for trees, protecting existing forests and supporting national parks and wildlife sanctuaries all count. Community action can be decisive: in the Chipko movement, villagers in the Himalayan foothills clung to trees to stop contractors from felling them, and their protest changed forest policy.

None of these steps looks dramatic on its own. Their power comes from arithmetic. A country of well over a hundred crore people multiplies every small daily habit into something enormous, in either direction. Earth is the only home we have found that will have us, and looking after it is not a favour we do the planet, which will carry on regardless. It is simply looking after ourselves.