Class 9 Science Chapter 13: Earth as a System: Energy, Matter, and Life

The Earth System and Solar Radiation

Rock, water, ice, air and life are not five separate subjects but five parts of one machine, and the Sun is what drives it. Measure what arrives — about 1.4 kilowatts on every square metre above the atmosphere — and the scale of the thing becomes clear.

What you will be able to do

• Name the five spheres of the Earth system and give examples of each.
• Explain how a disturbance in one sphere leads to changes in others.
• Describe the electromagnetic spectrum and which parts of it reach the Earth.
• Define insolation and the solar constant, and state the value of each.
• Calculate the solar energy received by a given area in a given time.

Key points

• Life on the Earth runs on a constant flow of energy and matter, and the Sun is the main source of that energy.

• The Sun is not the only driver: heat from deep inside the planet, and reactions going on among the gases, the waters and the rocks, set matter and energy moving as well.

• The Earth is treated in this chapter as one system made up of interacting spheres.

• The geosphere is the solid rock, the soil, the landforms such as the Deccan plateau and the Thar desert, together with the Earth's interior.

• The hydrosphere is liquid water, whether surface water such as oceans, rivers like the Ganga and Brahmaputra system and lakes, or groundwater.

• The cryosphere is water in its solid form, such as ice and snow, including the Himalayan glaciers, the snow of Ladakh and the polar ice caps.

• The atmosphere is the air surrounding the Earth that we breathe, cleaner in the mountains and forests.

• The biosphere takes in everything that lives, together with the places it lives in — mangrove and forest, farmland and reef, and the plankton adrift in the open sea.

• Three things tie the five together and hold them in a fine balance: the warming the Sun supplies, the travelling of air and of water, and the passing round of nutrients.

• Disturb one sphere and the others change too: less snowfall in winter may leave less water in a lake in summer, and so less water to grow the grass that feeds the sheep.

• At the largest scale, a warmer Arabian Sea puts more vapour into the air, the southwest monsoon becomes unsteady in consequence, and the rain falls unevenly — flood in one part of India, drought in another.

• Warmer air would take the glaciers and the polar ice faster; low ground would flood, and over a long enough period the sea would stand higher against coastal cities — while habitats went, and with them the living systems that depend on them.

• The Sun's radiation is the Earth's main source of energy, and it reaches us as electromagnetic waves that cross a vacuum at the speed of light.

• Unlike electromagnetic waves, sound waves are mechanical waves and need a medium to travel through.

• The speed of light in vacuum is three hundred million metres per second.

• These waves run over an enormous range of frequencies. At one extreme lie the short, rapid ones — gamma rays and X-rays; at the other the long, slow ones — infrared and the radio waves.

• The high frequency waves, gamma rays and X-rays, carry very high energy and can be harmful for life on the Earth.

• The entire range of electromagnetic radiation is called the electromagnetic spectrum.

• The solar radiation reaching the Earth is concentrated mainly in the ultraviolet, visible and infrared range, about 99 per cent of the Sun's energy falling within these wavelengths.

• Between them those three stretches of the range settle what the climate does and keep living things going.

• The short, energetic ones are for the most part stopped high overhead; the long, slow ones do get through but bring too little energy to make any difference to the warmth of the planet.

• Ultraviolet of the shortest wavelengths is largely stopped high up by the ozone; life below is shielded, and the stopping itself warms the air a little.

• What the eye can see comes all the way down, and it is this that green things use to build their food — on which nearly everything else feeds — while warming ground and water somewhat besides.

• The long-wave part heats the ground directly, and the ground then sends that warmth back upward into the air.

• Not all of it escapes: carbon dioxide, methane and the water vapour in the air catch and hold a share, and that is what keeps the planet warm enough for anything to live on it.

• Ultraviolet rays lie in the wavelength range of 100 to 400 nanometres, a nanometre being a thousand-millionth of a metre, and have a much higher energy than visible light.

• Take enough ultraviolet over enough time and eyes and skin suffer, with cancer more likely; anyone much exposed should wear glasses that block it and put on sunscreen.

• Ultraviolet rays are also useful for killing germs in water purifiers and for powering fluorescent lights.

• How much of the Sun's radiation actually gets down to the ground goes by the name of insolation, and it is what heats both the ground and the air above it.

• Reckon the energy arriving each second on each square metre held square to the rays, taken at the very top of the air, and that average figure is what is termed the solar constant.

• The figure comes to roughly 1.4 kilowatts on every square metre — which is to say some 1400 joules arriving each second on each square metre.

• It stands for what is on offer before the air has taken anything: nothing yet absorbed, nothing scattered, nothing turned back.

• The figure is worth having because the whole accounting of the planet's heat — and with it the study of climate and of weather — has to start somewhere, and it starts there.

• On the way down a part of it is taken up and a part flung aside by the gases, the cloud and the dust hanging in the air.

• What is left at the ground therefore falls short of the figure above the air: on a clear day the most that arrives is about a kilowatt on each square metre.

• Sitting where it does, between the tropics and a little beyond, the country is well supplied with sunshine in every month of the year.

• This makes solar insolation highly significant for India, since it drives the southwest monsoon, which influences the country's climate and agriculture.

• It puts within reach, besides, an enormous quantity of power that renews itself and can be drawn on indefinitely.

• Anna Mani, who broke ground in this country's study of the air, set about measuring how much sun each part of India receives, and did so through the 1950s.

• The volume she brought out with S. Rangarajan in 1982, Solar Radiation Over India, was the first atlas of its kind the country had.

• What she measured showed how much sun the country had to work with, and that is now being turned to account as solar installations go up across India.

• The energy received by a surface is found by multiplying the intensity of the radiation by the area and by the time.

• For an insolation of 1 kilowatt per square metre, one square metre receives 3.6 million joules in one hour.

• That amount of energy is roughly what is needed to melt 5 kilograms of ice and heat the water so obtained to 100 degrees Celsius, and it also equals one unit of electricity as shown on a household bill.

• A useful sum to attempt: what area of ground, panelled over, would meet the whole of India's present electricity demand?

• Assume a figure for insolation and a figure for how much of it a panel turns into electricity, and the arithmetic says that panelling over some part of the Thar — not even the whole of it — would do.

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• Spheres and Quantities
• True or False: Sunlight and the Earth
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• Unscramble: Spheres and Radiation

Albedo, Latitude and the Role of the Atmosphere

The same sunlight falls on snow and on black soil and produces opposite results; it falls square on the equator and slanting on the poles and produces opposite climates. Over all of it sits a layered envelope of air that lets some in and holds some back.

What you will be able to do

• Define albedo and relate it to how a surface heats up.
• Explain the urban heat island effect and what causes it.
• Explain why equatorial regions are warmer than polar regions.
• Describe the troposphere and the stratosphere and how temperature varies in each.
• State the two protective roles of the atmosphere and explain the ozone hole.

Key points

• Different materials absorb and heat up differently under sunlight, and land heats up faster than water.

• There may be some variation depending on the material the land is made of and the soil's colour.

• A dark face takes in a greater share of what falls on it; a pale one throws more of it back and stays the cooler for doing so.

• It is why a blacktopped road grows so hot underfoot while a pale pavement beside it does not, and why a dark shirt is unbearable in the heat where a white one is not.

• What fraction of the arriving radiation a surface throws back is termed its albedo, from an old Latin word for whiteness.

• Where that fraction is large the surface keeps cool, having turned most of the light away; where it is small the surface warms fast, having kept most of it.

• Snow has an albedo of about 0.80 to 0.90 and ice about 0.50 to 0.70, while crushed rock is around 0.25 to 0.30.

• Snow and ice reflect a large proportion of the incoming solar radiation, which is what makes the polar regions very cold.

• Surfaces such as black soil and ocean water have lower albedo, absorb more solar radiation, and so stay relatively warmer.

• All objects radiate heat, so a concrete house feels hot inside at night, especially in a hot summer, because of the heat the concrete re-radiates after dark.

• Traditional houses built with thick mud and wooden walls stay cool even in hot summers, because they re-radiate less.

• A city runs hotter than the countryside round it, the gap widest in summer and after dark; the name for this is the urban heat island effect.

• Compared with rural areas, cities have more built-up area: buildings of steel, concrete and brick, and roads of concrete and asphalt.

• All these materials absorb solar radiation and retain heat, and the heat they re-radiate warms cities more than the countryside around them.

• This raises the energy demand for air conditioning, which further stresses urban ecosystems.

• Out in the country and under the trees there is far more growing, and growth cools twice over: it shades the ground, and the plants themselves give off water.

• The whole business is a plain demonstration that what people choose to do with a piece of ground will alter the climate of that ground.

• The amount of radiation a region receives also depends on its latitude, and this uneven heating determines the atmospheric conditions of a place.

• The planet being a ball, the light comes down nearly square on in some belts and at an increasing slant in others.

• Near the equator a given beam lands on a small patch of ground; near the poles the same beam is smeared across a much bigger one.

• So the middle belt stays warm the year round and the two ends stay bitterly cold, and the standing gap in temperature between them follows directly from that unequal warming.

• The Earth's spherical shape and the tilt of its axis of rotation also give rise to the seasons and to the changing length of daytime through one revolution around the Sun.

• Solar radiation is therefore not evenly distributed across the globe, and this uneven heating of the surface drives global winds and ocean currents.

• The air surrounding the Earth is called the atmosphere, and it is held in place by the force of the Earth's gravity.

• Nitrogen makes up 78 parts in a hundred of it and oxygen 21, the remainder being argon, carbon dioxide, water vapour and a few others in small quantity.

• The atmosphere has a layered structure, and that structure helps explain key weather patterns.

• Nearly all weather phenomena take place in the troposphere, which has an average height of about 12 kilometres and is heated from the Earth's surface.

• In the troposphere the temperature decreases with height, by about 6.5 degrees Celsius per kilometre, and as warm air rises it drives winds and storms.

• The height of the troposphere is greatest above the equator and least above the polar regions.

• In the stratosphere, from about 12 to 50 kilometres, lies the ozone layer, where ozone absorbs ultraviolet rays and heats the atmosphere, so that temperature rises with height.

• This increase of temperature with height calms the stratosphere, because there is no vertical mixing of air, which keeps weather confined to the troposphere.

• Above the stratosphere lie the mesosphere, the thermosphere and the exosphere, which play only a minor role in regulating climate at the surface.

• At about 100 kilometres above the Earth begins the region we call outer space.

• The first crucial role of the atmosphere is that it takes up part of the incoming solar radiation: the ozone layer blocks harmful ultraviolet rays, and clouds and other gases absorb some sunlight before it reaches the surface.

• The second is that it traps outgoing heat, since the surface absorbs sunlight and re-radiates it in the infrared region, and greenhouse gases such as carbon dioxide, methane and water vapour absorb that re-radiated heat and prevent it escaping into space.

• Strip the envelope of air away and the planet would be far too cold for anything to live on.

• The trouble is the surplus we ourselves put out, which strengthens that trapping and warms the planet; left to run on, it could end by making the place unfit to live on at all.

• Of the two innermost planets it is the farther that is hotter, and the reason is the thick envelope of gas about it, in which the trapping of heat has run away with itself.

• In 1934 the Indian scientist of the air K.R. Ramanathan went up to eighteen thousand feet in the Himalayas to take readings of ozone, and found less of it there than had been supposed.

• His work laid the foundation for understanding how ultraviolet absorption varies with altitude and with pollution, and he later led early monsoon forecasting efforts.

• The atmosphere is continuously modified by natural processes, which influence weather, climate and energy balance.

• Radiation from the Sun and rays from beyond it, meeting the air, bear on both weather and climate and interfere besides with the signals we send about.

• The ozone layer is vital to life because it acts as a protective shield, absorbing harmful ultraviolet radiation from the Sun.

• Should those molecules be broken up faster than nature can make them again, the shield grows thin and answers less well.

• Towards the close of the last century a family of manufactured chemicals — the chlorofluorocarbons, put to use in cooling machines and in spray cans — tore heavily into the ozone above Antarctica, and the gap that opened came to be spoken of as the ozone hole.

• Increased ultraviolet radiation at the Earth can harm living organisms and ecosystems.

• By an agreement among nations, the Montreal Protocol, the use of those chemicals was cut back, and the shield is mending, if slowly — which is worth noting as what scientific cooperation across borders can achieve.

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• Surfaces, Layers and Shields
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Winds and Ocean Currents

Heat a surface unevenly and the air above it will not stay still. The same rule works on a hillside before breakfast and across a hemisphere over a year — and once the winds are moving, they drag the ocean along with them.

What you will be able to do

• Explain how uneven heating produces differences of pressure and so wind.
• Describe the valley breeze and the mountain breeze and say when each blows.
• Trace the pressure belts from the equator to the poles and the circulation between them.
• Explain why planetary winds follow curved paths.
• Describe what drives ocean currents and what they do for climate and for life.

Key points

• Air on the move is what we call wind, and it runs from wherever the pressure stands high towards wherever it stands low.

• Those differences of pressure arise chiefly because the Sun does not warm the surface of the planet evenly.

• The unequal warming of ground and water is what gives the breezes that blow off the land at one hour and off the sea at another.

• The identical principle accounts for winds and for ocean currents, however great the difference in size.

• The same unequal warming raises winds on a small scale too, among them the two that come and go daily in the hills.

• Up in hill country a hillside and the ground at its foot neither take up warmth nor give it off at the same pace.

• By day the faces of the hills that catch the Sun grow warm sooner than the ground lying below them.

• The air over those slopes becomes warm and rises, creating a low pressure region.

• Cooler air from the valley then moves up the slopes to replace the rising warm air, and this flow of air is called a valley breeze.

• After sunset the situation reverses, the mountain slopes losing heat faster and becoming cooler while the valley floor remains relatively warmer.

• The air on the slope becomes the colder and heavier and drains down into the valley; that is the mountain breeze.

• Anyone living at Shimla or Dehradun, or anywhere in the Himalayan valleys, knows this daily turning of the wind.

• Such winds bear on the weather of the hills, on what can be grown there, and on the ordinary course of life in those places.

• They help regulate temperature and moisture conditions, and support soil and crop health.

• Unequal warming of the ground is thus what starts the wind, and it is a large part of how the whole atmosphere circulates.

• Uneven heating of the Earth from equator to poles creates belts of low and of high pressure.

• A difference of pressure on that scale starts the air travelling across great stretches of the globe, and winds of that kind are termed planetary.

• About the middle of the world the Sun beats down hard, the warmed air goes up, and the pressure it leaves behind stands low — the equatorial low.

• As this air rises it moves poleward at higher altitudes, and on cooling it becomes denser and sinks around 30 degrees North and South, forming the sub-tropical high pressure belts.

• Out of those high-pressure regions the air creeps back towards the middle along the ground, and with that the round is complete.

• Not every part of what comes down there goes back the way it came, however.

• The rest travels poleward along the ground and lifts again near 60 degrees on either side, where cold air out of the polar regions meets it; that is where the sub-polar lows sit.

• At the poles themselves, near 90 degrees, the cold is such that heavy air settles, and those are the polar highs.

• Air from these polar regions flows towards the sub-polar belts, completing another circulation cycle.

• The turning of the planet pushes these winds off any straight line they might have taken.

• They therefore travel in curves, and never make straight from the high pressure to the low as one might expect them to.

• In the northern half of the world they are pushed rightward and in the southern half leftward, and the cause in either case is the planet turning on its axis.

• When great bodies of sea water travel steadily and without cease, what we have is an ocean current.

• As with winds, planetary pressure differences also drive the ocean currents.

• The great winds, rubbing against the top of the sea as they pass, drag that water along with them and so start the surface currents going.

• Wind is not the whole of it: how warm the water is and how salt, the planet's spin, and where the continents happen to lie all have a hand in where the sea goes.

• The waters of the equatorial and the polar regions have different temperatures.

• Warm equatorial waters travel over the surface towards the poles, while colder and denser waters flow slowly back towards the equator through deeper ocean levels.

• Ocean water has different salinities depending on the location.

• Where there is less salt the water is lighter for its bulk and keeps to the upper part; where there is more it is heavier, goes down, and travels at depth.

• Pushed aside by the turning of the planet, these travelling bodies of water fall into great wheeling circles, which are called gyres.

• North of the middle they turn with the hands of a clock; south of it, against them.

• Continents further modify these paths by blocking and redirecting currents.

• These flows count for a great deal, both in holding the climate of the planet steady and in keeping life going.

• Carrying warmth out of the hot middle towards the cold ends, they narrow the gap in temperature between one part of the world and another.

• The North Atlantic Drift is the Gulf Stream continued — that powerful current taking warm water off the lower east coast of North America right across the ocean.

• The Drift runs on to north-western Europe and keeps harbours open through the winter at latitudes where they would otherwise freeze.

• A climate so evened out serves people as well, since harbours that stay open keep goods moving.

• Ocean currents also support a massive ecosystem by transporting nutrients.

• At Pune, in the Indian Institute of Tropical Meteorology, elaborate computer models are run which knit together the flows of energy through air, sea, land and ice, in order to reproduce the workings of the Indian monsoon.

• They are fed from satellites, from buoys moored in the Indian Ocean, and from stations as far off as Antarctica, the aims being better forecasts from season to season and some grasp of what a warming world will do to where the monsoon rain falls in India.

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• Breezes, Belts and Currents
• True or False: Air and Water on the Move
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• Unscramble: Circulation

Biogeochemical Cycles: Water, Carbon, Nitrogen and Oxygen

The Earth gains almost no new matter. Every atom of carbon in a leaf, every atom of nitrogen in a muscle, has been round before and will go round again — and where one of those rounds is interrupted, everything downstream of it feels the interruption.

What you will be able to do

• Define a biogeochemical cycle and say why such cycles matter.
• Describe the water cycle and how climate change is altering it.
• Distinguish the fast and slow carbon cycles and explain the ocean's part.
• Describe the five steps of the nitrogen cycle and the bacteria that carry them out.
• Describe the oxygen cycle as a balance of consumption and production.

Key points

• Whatever lives is always giving something to, and taking something from, the air about it, the water near it, and the ground and rock beneath it.

• That traffic between what lives and what does not is how matter and energy get from one sphere of the planet to another.

• What this keeps up is the supply: the substances life cannot do without — carbon, nitrogen, oxygen — come round again and again and are never used up for good.

• Such a round, carried on between what lives and what does not, is spoken of as a biogeochemical cycle.

• No system of living things stands quite apart from the rest, and it is that connection which lets a damaged one mend and keeps the whole in some sort of balance.

• In the water cycle, water evaporates from water bodies such as rivers, lakes and oceans, and condenses to form clouds.

• Down it comes again as rain, as hail or as snow, and finds its way at last back to the sea.

• A portion never takes that route, soaking instead through the earth and stone below and lodging there as the water underground.

• Water dissolves minerals from soil and rocks, supports all terrestrial organisms, and transports those nutrients to the oceans, where they support marine life.

• A shifting climate is unsettling this round. Warmer air carries more moisture, so the rain comes down harder in some districts and scarcely at all in others.

• Ice wasting in the high mountains swells the rivers now and lifts the level of the sea in time, which puts coastal cities — Mumbai and Chennai among them — in hazard.

• Rain arriving all at once means more runs off and takes the soil with it, while less soaks in, so the groundwater is not replenished and farming through the dry months becomes very hard.

• The water cycle thus links the cryosphere through glaciers, the hydrosphere through rivers and oceans, the atmosphere through moisture, the geosphere through soil erosion and reduced infiltration, and the biosphere through crops and fisheries.

• Carbon forms the backbone of life, since every protein, carbohydrate, fat and DNA molecule contains it.

• It keeps moving between four places: the air, where it is carbon dioxide; living things, plant and animal; the rock, as carbonates and as the coal and oil laid down long ago; and the sea, dissolved in the water or built into shells.

• The several parts of this round do not run at anything like the same pace.

• The quick route runs in days or in years: with sunlight, a plant takes carbon dioxide out of the air and makes glucose of it.

• Carbon dioxide is released back into the atmosphere through respiration, and animals eat plants or other animals, so that when they die carbon dioxide returns to the air through decomposition.

• The long one takes ages to complete: what has died is laid away beneath and turned by slow degrees into coal, into oil and into gas.

• Heating, cooking, transport and industry all burn these fuels, and burning hands their carbon straight back to the air as carbon dioxide. What the ground took millions of years to store is returned in moments.

• The atmosphere and ocean water continuously exchange carbon dioxide.

• Ocean water absorbs atmospheric carbon dioxide to form carbonate and bicarbonate ions, which phytoplankton use for photosynthesis, and some marine organisms use to form shells.

• Dying, they go down to the bottom, and what they were made of stays locked there as carbon for a very long while.

• Human activities such as burning fossil fuels and deforestation have raised atmospheric carbon dioxide by about 35 per cent since 1960, from 315 parts per million to 420, an unprecedented rise in the history of human civilisation.

• The record of that rise is called the Keeling curve, and its sawtooth pattern shows seasonal dips as yearly plant growth in the Northern Hemisphere absorbs carbon dioxide.

• A certain amount of carbon dioxide is wanted, since without it the planet would be too cold for anything; but the amount is what matters.

• Excessive carbon dioxide intensifies the greenhouse effect, leading to global warming, the melting of glaciers and Arctic sea ice, rising sea level and more extreme weather conditions.

• In India this may lead to more intense monsoons, since warmer air holds more moisture, and to threats to agriculture from changing rainfall patterns.

• A great deal of India's electricity still comes from coal and oil, but renewable capacity is being added fast, and that will cut what goes into the air.

• Carbon makes up about 49 per cent of the dry weight of living organisms.

• Seventy-one parts in a hundred of all the carbon there is lies in the sea, and it is that reservoir which governs how much carbon dioxide the air holds.

• The atmosphere holds only a very small fraction, about 1 per cent, of the total global carbon.

• Without nitrogen nothing that lives could build its proteins or the strands that carry its inherited instructions.

• Most of it is in the air, and there is the difficulty: N₂ will hardly react with anything, and neither plant nor animal can take it as it stands.

• It must first be converted into soluble compounds that living beings can absorb.

• Taken all together, the passage of nitrogen among air, soil, water and living things is termed the nitrogen cycle, and it falls into five stages: fixation, assimilation, ammonification, nitrification and denitrification.

• Certain bacteria do the first work — Rhizobium, housed in the swellings on the roots of pulse crops, and Azotobacter, living loose in the soil — turning the gas of the air into ammonia.

• Nitrifying bacteria like Nitrosomonas convert ammonia into nitrite, while Nitrobacter convert nitrite into nitrate, and this process is known as nitrification.

• What the soil then holds is drawn up by the roots of plants; the creatures get theirs at second hand, by eating plants or by eating one another.

• Dead bodies and waste alike are taken apart by bacteria and fungi, which put nitrogen compounds — ammonia among them — back into the soil; that step is ammonification.

• A last group, Pseudomonas among them, sends part of that nitrate back to the air as free gas — denitrification, which shuts the round and holds the quantity in any place steady.

• A stroke of lightning does a little of the same work, making a trifling quantity of nitrogen oxides out of the air it passes through.

• Today most nitrogen is fixed artificially by the Haber-Bosch process, devised in the early 1900s, which makes ammonia from atmospheric nitrogen and produces most of the fertilisers used today.

• Bread out of the air, as it was called, turned farming upside down: it carried the Green Revolution in this country and feeds billions, and above half the nitrogen atoms in a human body have come through it.

• The cost is in power: the process takes something like one or two per cent of all the energy used on Earth, and too much fertiliser has spoiled soil and water alike.

• Oxygen is one of the Earth's most abundant elements, and about 21 per cent of the atmosphere consists of free oxygen gas.

• It is an essential component of most biological molecules, such as carbohydrates, proteins, nucleic acids and fats.

• It is not always found free. Down in the crust it is locked up with metals and in the stuff of minerals, and overhead it is locked up in carbon dioxide.

• In discussing the oxygen cycle we focus on the processes that regulate the oxygen level in the atmosphere.

• Organisms such as plants and animals use oxygen for respiration and release carbon dioxide, and the combustion of fuels likewise uses oxygen and releases carbon dioxide.

• Plants restore oxygen through photosynthesis, using sunlight, water and carbon dioxide to form glucose and release oxygen.

• Breathing and burning take it; photosynthesis puts it back; and held in that balance oxygen keeps passing between the air, the ground, the sea and living things, so that everything alive in every sphere goes on.

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Human Impact on Earth's Processes

Every one of the cycles has a point where human activity now enters it: carbon at the chimney, nitrogen at the fertiliser bag, water and soil at the felled forest. The damage is real and traceable — and so, in the one case where nations acted together, is the repair.

What you will be able to do

• Explain how excess carbon dioxide disrupts the carbon cycle and the oceans.
• Describe eutrophication and what causes it.
• List the effects of deforestation across air, water, land and living things.
• Distinguish stratospheric ozone from ground level ozone.
• Describe what individuals, nations and international agreements can do.

Key points

• Rising carbon dioxide levels from the use of fossil fuels lead to extreme weather and to loss of biodiversity.

• Human actions disrupt the biogeochemical cycles right across the Earth's spheres.

• Excess atmospheric carbon dioxide increases the amount the ocean absorbs, which makes sea water more acidic.

• That acidity could threaten tiny plankton and coral reefs, and so disrupt marine ecosystems.

• However, warmer ocean water reduces the ocean's capacity to absorb carbon dioxide as an effective carbon sink.

• The burning of fossil fuels and deforestation together saturate natural carbon sinks such as forests and oceans.

• The natural workings of the planet have altered markedly in the course of the last half century.

• A good part of the electricity used in this country is still got by burning such fuels, with all the damaging output that entails; the swift spread of power drawn from the sun is the encouraging side of the account.

• The excess carbon dioxide intensifies greenhouse warming, and so disrupts the carbon cycle.

• The overuse of fertilisers in agriculture adds excessive nitrogen, in the form of nitrates, to rivers and lakes.

• That nitrogen causes the widespread growth of algae, known as algal blooms, which deplete oxygen and kill fish.

• This process is called eutrophication, and it threatens water bodies and coastal fisheries.

• Deforestation creates multiple effects across air, water, land and living organisms.

• Take the trees off and there is less photosynthesis and less water given off to the air, which can leave the district drier than it was.

• It also alters the surface albedo of the cleared land.

• With nothing left below ground to bind the earth in place, it is the more easily washed or blown away.

• In time the places where creatures lived are gone altogether, and as their homes disappear the variety of living things in that country falls away.

• Vehicular emissions react with sunlight to form ground level smog.

• The same working produces, low down where we breathe, a gas that does the lungs no good at all.

• Ozone in the stratosphere, by contrast, blocks ultraviolet radiation and is protective of life, so where the gas sits decides whether it helps or harms.

• These pollutants make city air unhealthy.

• We have reached every sphere of the planet, but what is done locally, together with agreement between nations, can set the natural systems right again.

• Under the Montreal Protocol, nations acting together set the shield overhead on the road to mending.

• Kyoto and Paris, by which countries undertook to cut their carbon dioxide, have not worked nearly so well.

• Using less power and husbanding the sources of it; turning to the sun and the wind, which renew themselves; setting out trees; going carefully with water; and farming in ways the land can bear — all of these help right the balance.

• This country has set out trees by the billion, greatly enlarged what it draws from the sun and from other renewing sources, and pressed for farming of the sustainable kind.

• Individuals can also contribute by saving resources such as water, food and energy.

• This can be done by reducing waste, and by reusing and recycling materials.

• Taken together, such efforts are what will hold the account of the natural world steady.

• Mission LiFE — Lifestyle for Environment — is India's proposal to the world, put before the UN climate conference of 2021.

• It encourages people to adopt mindful, eco-friendly lifestyles.

• Old custom and old writing in this country understood long ago that the planet works as a single connected whole.

• The planet runs on energy passing through and matter going round between its spheres; consume beyond what can be sustained and that arrangement is upset.

• By urging plain habits — using less power, going carefully with what is scarce — it makes the case that what households and neighbourhoods do adds up to a future that can be kept going.

• What arrives from the Sun as radiation is the chief supply of energy this planet has.

• Most weather processes, such as evaporation, condensation and precipitation, occur in the troposphere.

• How much sun a place gets depends on the planet's shape, on how far north or south it lies, and on the tilt of the axis — which is why the surface is heated so unevenly.

• That unequal warming of the surface is what raises the winds and sets the ocean currents going.

• Matter and energy are continuously cycled between the living and the non-living systems.

• Atmospheric oxygen is used in combustion, in respiration and in the formation of oxides, and is restored mainly through photosynthesis.

• Water, carbon, nitrogen and oxygen are never at rest here, passing again and again between air, sea, ground and living things.

• These rounds of matter keep nourishment within reach of what lives, hold life going, steady the climate and keep systems of living things in balance.

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