Planet Earth’s nine lives

Our planet is like a cat with nine lives. People are harming some of these lives without realising. My guide shows you what’s going on… and what’s going wrong!

I’m making a cool quiz about what you can discover in this guide for you to try and see how well you can do. And there’s more: try my Nine Lives Crossword Puzzle. All the answers are in this guide…

Click to make bigger

People say that cats have nine lives. They fall out of trees or nearly get run over but usually live to see another day. But then, one day, a cat may take one risk too many… and that is the end for that particular puss.

In a sense, our planet Earth too has nine ‘lives’. Nature has looked after these planetary ‘lives’ very well for a very long time. Earth has been quite safe most of the time for most things that have lived on it. Penguins have lived on the planet for many millions of years as have myriads of other animals and plants. Modern humans have only been around for a few tens of thousands of years but, like other living things, didn’t threaten any of the planet’s ‘lives’. Until now, that is.

Today humans are pushing up against the limits of what they can take from the planet and what they can do to it. If they push too hard, some of our planet’s ‘lives’ will be damaged and that’s not good. Not good for you, your friends and family, cats, penguins and much of life on Earth.

Nobody quite knew this until some smart scientists from around the world got together to see if they could work out how many ‘lives’ the planet really had. They discovered that Earth had just nine ‘lives’ which they named ‘planetary boundaries’. Because they were tightly linked, hurting one also hurt others. Knowing this, they then needed to find out how much an Earth ‘life’ could be pushed by humans before it was in real danger.

Click to make bigger

Their idea was to give people a guide for how to live without risking any of the planet’s ‘lives’: a manual for helping planet Earth run smoothly without pushing the nine ‘life’ limits too far.

Because scientists understand a lot about the workings of our planet, they could say quite clearly what the limits were: how far a ‘life’ boundary could be pushed before it collapsed. It seems that Nature isn’t really in charge of planet Earth any more.

Click to make bigger

People are now in control and have the power to break the planet or fix it. It’s a choice you people can make, so you really need to know this and get good at taking care of the planet’s ‘lives’. This is responsible stewardship. But it hasn’t been the way people have treated their home planet at all up to now, mostly because hardly anyone understood or believed that the planet’s nine ‘life’ systems did have limits.

Click to make bigger

My guide is to show you in simple terms what’s wrong and what you people can do about it. That’s all one small penguin can do. But you can do everything that’s needed to fix the very big problems that are looming. Scientists can show the way but it has to be everyone everywhere acting together for the good of all.

But why, you might wonder, can’t the Earth — like a cat — lose some of its lives and still be just fine?

Click to make bigger

It turns out that the planet’s nine system boundaries (‘lives’, as I’ve called them) are interwoven like the different coloured threads in a tapestry.

Each set of threads depends on the others for the tapestry to hold together: they are interdependent. Take some threads away and the whole Earth ‘tapestry’ is damaged. That spells big trouble for life on our one and only home, our world, our planet: Earth.

Destroying the planet

Our planet can take care of itself. When people talk about ‘saving the planet’, they’re really referring to keeping the planet habitable for life, particularly human life

What, ALL life???

No, not all life. Some living things will survive almost anything. Things like cockroaches, rats and bacteria. But not – sob! – penguins

Big world? No, it’s a small world

Click to make bigger

To you and me, the world seems absolutely huge. Lots of people think this and that’s the reason why humans to this day use the land, sea and air to dump all the waste they make. But isn’t the amount of waste people make really tiny when you think of the vast size of the seas, lands and atmosphere? When there weren’t many people (and they lived in quite a simple way with no cars, planes, computers or electricity), that was true enough. But it’s all changed: there are now 7 billion humans on the planet making a whole lot of waste. This really is damaging the air, the sea and the land. By waste I mean pollution including, especially, greenhouse gases like carbon dioxide. Humans are also guzzling stuff which the planet only has in limited amounts. Stuff like oil, gas and coal (to power human industry), minerals, fresh water, fish, forests. All these are resources which will run out or are being destroyed faster than the planet’s natural systems can replace them.

Did you know…? Our planet weighs a whopping 6.6 sextillion tons – That’s 6,6 with twenty 0s. Land forms just less than one-third of our planet’s total area. The rest is ocean – great for fish and penguins.
Click to make bigger

The number of people on the planet grows at the rate of about seventy-five million every year. But the planet is not getting any bigger. The total number of humans is 7 billion, weighing in at around 4.5 million tons. Every person eats around 25 tons of food over their lifetime. All of this – apart from some seafood – has to come from farming. People are already using over one tenth of the land – 16 million square kilometres – for growing food. People also cover land with concrete and asphalt for cities and roads. Mining takes coal, oil, gas and minerals out of the Earth, which can’t be replaced. Taking out and using these minerals causes pollution everywhere, most of which you don’t see. People are cutting and burning forests and mostly not replacing them.

I know our planet seems big but it’s not so big as you might have thought. It is finite: it has its limits. And there’s only one planet for all of us. There’s no backup; no new continents to be discovered; no other habitable planets for people, penguins or any other wildlife to go!

Waste energy

How much stuff do you and your family throw away every day? How much fuel do you burn in your family car and house heating or cooling system? Probably far more than you’d think. Then multiply by 7 billion…!

That’s 148,940,000 square kilometres; 57,491,000 square miles if you really want to know!

‘Habitable’ means somewhere you can live.
Click to make bigger

Humans live in tribes where people may, for example, look similar or speak the same language. These tribes have got really big and occupy large pieces of land. These enormous groups of related peoples and the land they live in have a name: ‘states’ or ‘nations’.

Click to make bigger

The United States of America, the People’s Republic of China and Russia are three really big nations. The trouble with nations is that they act selfishly in what they think is their own best interests. Although all the nations of the world have formed an organisation called the United Nations, this has little power to act in everyone’s interests. So rich nations tend to grab much more than their fair share of everything and pour out much more than their share of pollution into everybody’s air. Selfish or what? Because everyone suffers, especially the poor. It’s very rare for nations to act together to fix global problems. But it has happened and has to happen much more.

And that’s the reason for the scientists getting together to try and show that the planet’s nine ‘lives’ and their limits are every nation’s problem but no nation can solve them alone. The problems are really big but they can be fixed if only all nations work together to do it.

The European Union

This is a big group of nations which have agreed to act in partnership – though they still squabble

Click to make bigger

If you’ve looked at some of my other guides, you’ll already know something about what humans are doing which affects our planet. Most of the nine planetary ‘lives’ are still okay but three of them are not. Bad news, you might think. But the good news is that you people can fix them if you have the will. You and your fellow humans, acting together, have so much power if you choose to use it wisely. It may take you some time but you can do it!

The scientists now know there are nine planetary boundaries (‘lives’). Think of the white line which marks out a tennis court

If you hit a ball which lands outside the boundary, you lose a point. Everyone can see where boundary lines are so the game works. But what if you didn’t know where the boundaries were? How could you know if the ball was ‘in’ or ‘out’? Humans have been ‘playing’ with our planet’s resources without even thinking that there might be boundaries. But now, thanks to science, the boundary lines are clear. No-one will now be able to claim that they didn’t know about these limits, though some people will be sure to ignore them and claim they don’t believe the evidence.

Here’s another way of looking at this. Imagine you’re driving a car about in the desert It’s dark and the headlights go out and you know there are cliffs which plunge down to a canyon somewhere nearby. Do you carry on driving and hope for the best? Or do you wait till daybreak so you can see clearly where those cliffs (boundaries) are? I think I’d wait till daybreak, wouldn’t you?

The nine planetary boundaries:

  1. Climate change: the planet is heating up and could soon do so much faster
  2. Acid oceans: the seas are becoming hostile to animals with shells
  3. Ozone depletion: the ozone layer which protects us is damaged
  4. Aerosol loading: soot and smoke from burning fuels and forests
  5. Nitrogen and phosphorus cycles: nature can’t cope with all the fertilisers used by farmers
  6. Fresh water: most fresh water on or under the land is now being used by humans. Many rivers don’t even reach the sea any more
  7. Land use: there’s a limit to how much land humans can farm
  8. Biodiversity loss: life itself is in danger as extinction looms for many animals and plants. Whole ecosystems and even biomesare threatened and yet people depend on them utterly
  9. Chemical pollution: artificial chemicals used around the world harm humans and wildlife

Sounds boring? Or too complicated? Big sciency words can be off-putting, I know. That’s why I’ve written this guide for you: to explain the scientists’ new understanding of how our planet works; a basic book of rules for running planet Earth safely. Because now, you people with your powerful technologies, are almost like gods. You have the power to change the planet for good or for ill. Use your power wisely… and soon!


It was Johan Rockström who thought up this metaphor. He was the lead scientist in this planetary boundaries work. You’ll see and hear him at the end of this guide in a video


This metaphor – which I’ve modified slightly – comes from Jon Foley, University of Minnesota, and one of the original planetary boundaries team. You can see him in a great, very short video at the end of this guide.

What in the world are aerosols?

Aerosols are tiny particles which are so light that they’re supported by the air and loft high into the atmosphere to spread around the planet

About Nitrogen

Nitrogen, chemical symbol N, is vital for plant growth. It makes up most of the air we breathe but is non-reactive unless combined with other elements. It is the reactive forms of N which are the problem

What is phosphorus?

Phosphorus, chemical symbol P, is also vital for plant growth


Natural underground water stores, which can be vast, are called aquifers

What are ecosystems?

An ecosystem is an interconnected web of living things. Damage one part of it and the whole system may collapse

What are biomes?

Biomes are much bigger than ecosystems and may contain many of them. Examples are the Amazon rainforest and the Great Barrier reef, Australia. Both of these have been badly damaged (2020) by human actions.

Climate change: the planet is heating up and could soon do so much faster

Click to make bigger

You probably know a good deal about climate change, especially if you’ve read my guide. Climate change is often called ‘global warming’ which gives the impression that everywhere on the planet is getting hotter. This isn’t quite true. Overall, our planet is heating up, but some parts – like the polar regions – are warming much faster than others.

Sometimes changing weather systems can channel cold air away from the Arctic, giving other parts, like northern Europe or eastern North America, very cold winters. People get mixed up thinking that a cold winter means the climate is getting colder, not warmer. So you often hear people say, “I don’t believe in global warming. This winter is so cold. Global warming is a big hoax.” That sort of stuff. You can see why they say it, but it really just shows that they don’t understand (or don’t want to know about) what climate change means. There’s all kinds of scientific evidence which shows without doubt that the planet as a whole is getting hotter.

We know why the climate is changing. It’s because humans burn gigantic amounts of carbon-based fossil fuels: coal, oil and natural gas. Burning these produces energy for industry (like electric power stations) and transport (cars, trucks, planes, ships). Most of the waste gas from this burning are made up of carbon dioxide (CO2) and go straight into the air.

CO2 is the main greenhouse gas. Without it the planet would freeze, because it acts like a blanket, trapping some of the sun’s heat in our atmosphere. So too much CO2 is bad because it’s like piling on too many blankets. And there is already too much. CO2 levels have shot up by one third in just 200 years. Today it’s rising faster than ever, as more people use more and more energy.

There are many other greenhouse gases which human industry and farming produce, often by accident. The best known of these is methane, which is around 25 times more powerful than CO2 in its warming effects.

Scientists know that natural systems like climate tend to change quite suddenly from one stable state to another. We’ve been very lucky since the end of the last ice age (about 11,500 years ago) because the climate has been quite stable: just right for humans (and penguins too). But the extra burden of greenhouse gases that people are unintentionally pumping into the atmosphere is ‘forcing’ the climate towards another, hotter, stable state.

Climate change: the planet is heating up and could soon do so much faster

Click to make bigger

This is happening more rapidly than expected because of something called feedback. One important example of feedback is happening right now in the Arctic. Until recently, the Arctic Ocean has been mostly covered by a blanket of floating ice. This ice, being white, reflects almost all the sun’s heat back into space. When it melts away, we are left with dark ocean waters which, because they are so dark, absorb most of the sun's heat so the water gets warmer still. And because the warmer water is exposed to the air, some of it evaporates as water vapour, which is itself a powerful greenhouse gas. So warming increases yet more. This is another feedback.

Tackling climate change: Right now, climate change is the most important planetary boundary because we’re already over the limit for CO2. The limit the scientists suggest is 350 parts per million of CO2 in the air. It’s 416 ppm now (May 2020) and continues upwards faster than ever . There are two ways people can fix this. The best way would be to quit using fossil fuels almost totally. This is unlikely to happen fast enough even though world nations are coming together regularly to try to find ways to do it. But, like it or not (and the ‘greens’ definitely don’t!), there are alternative ways to bring down greenhouse gas levels in the air:

Click to make bigger
  • Generate electric power using a mix of renewables like wind, solar and nuclear power. More on this
  • Geoengineering is a big topic [more] and there are many reasons why some schemes might neither work nor be a good idea. Environmentalists (‘greens’) don’t like it, partly because they (probably rightly) think that once people start using it, the pressure to solve the real problem – burning too much fossil fuel – will be forgotten. Geoengineering could seem an easy option to avoid having to stop using fossil fuels. Though really, it’s a sort of emergency toolbox for people to use if things get desperate. So it makes good sense that scientists are already doing lots of experiments to find out what could work and how safe and easy it is to do.
Click to make bigger

Humans are the cleverest creature to have ever lived on planet Earth. You can fix climate change, the biggest problem of all. You just have to agree to all pull together and do what needs to be done. And there’s a bonus: if climate change really is sorted, several of the other nine planetary boundaries will also be largely sorted too. But single countries can’t do this on their own. The whole world needs to unite and agree how to fix it and then do it! This happened successfully with the ozone treaty. And in December 2015 at a huge meeting of the leaders of the world’s nations in Paris (France), everyone agreed on a fair deal to tackle climate change. Unfortunately, the USA — one of the world’s biggest polluters — is now pulling out of the deal. But they are on their own since the rest of the world is determined to stick with the agreement. More on this…

What are the polar regions?

You knew this, didn’t you!? The Arctic and Antarctic, the icy parts at the top and bottom of the planet are the polar regions. The Arctic is warming particularly fast

Carbon capture and storage

There are plans to capture and store some of these waste gases but it’s only happening on a very small scale. It’s going to be very expensive plus there are other problems. Best not to burn carbon fuels in the first place!

That is from the start of the Industrial Revolution, which is when people started burning coal big time
This period of time is known as the Holocene. Some scientists suggest we are now entering a new period called the Anthropocene: a period in which the climate is now becoming unstable due to human influence. ‘Anthropo’ means ‘people’ so anthropology means the study of people

The albedo effect

Have you noticed that if you leave something black in the sun, it gets much hotter than something white. It’s the same with dark ocean waters: they absorb far more heat from the sun than they would if covered in white reflective ice. This is the albedo effect.

Measuring greenhouse gases

350 parts per million (shortened to ‘ppm’) sounds very small. It is just 0.035 percent but in the case of CO2 is critically important

United Nations’ climate conferences

These conferences, called COPs (‘Conferences of the Parties’) are arranged by the United Nations Framework Convention on Climate Change or UNFCCC. They try to get countries to agree to big cuts in greenhouse gases. This is proving very difficult!

I’ll be looking at this later in the guide
Click to make bigger

Acid oceans: the seas are becoming hostile to animals with shells

You know what acid is, don’t you? Vinegar is a weak acid but it burns your throat if you take a swig of it! Maybe you’ve heard that the oceans are getting more acid. Most people probably haven’t and so don’t know why it’s happening or why it’s important.

Carbon dioxide from the air dissolves in seawater and makes it more acid With ever more CO2 in the air, more dissolves in the oceans so the more acid they become.

What does it matter if the oceans get a bit more acidic? There are 3 reasons

  • shellfish
  • corals
  • certain plankton

On the importance of phytoplanktonThe ocean is home to myriads of tiny life forms called phytoplankton. These remove CO2 from the ocean by doing what all green plants do: photosynthesis. Scientists have found that these simple tiny ocean plants remove about half of CO2 produced by humans use of fossil fuels. This is more than is taken out of the air by all the world’s rainforests and other land plants. So they may be small but they are very important and most of them make their skeletons out of calcium carbonate. This can’t happen if the oceans become too acid

Shellfish and corals are animals which build their shells or skeletons out of calcium carbonate. The animals make this by using calcium and CO2 both dissolved in the seawater. If the seas get too acid the sea creatures can’t build their shells and skeletons and they will die. So these zillions of tiny creatures are providing us with a free means of getting carbon out of both the atmosphere and the sea and locking it away safely into what will become limestone rock (mostly calcium carbonate). The shell-building plankton whose tiny bodies sink to the bottom of the sea when they die also become another type of limestone rock.

The acidity of surface ocean waters – which is where all the plankton and corals live – has increased by one third since humans started burning fossil fuels on a grand scale. Scientists reckon things will be critical in some oceans by mid-century. We are getting close to the boundary but we’re not there yet. There’s still time to stop the damage from happening. How? By quitting burning fossil fuels and getting the energy you need from other sources like renewables and nuclear power.

Now for some simple chemistry!

Fizzy water is made by squirting carbon dioxide into it under pressure. The CO2 dissolves in the water and when you take the top off the bottle, there’s a fizz and lots of bubbles of CO2 form in the water. But this carbonated (=fizzy) water is a weak acid. So if you drop a sea shell from the beach into it, it will begin to dissolve in the acid. [Sea shells are made of the same stuff as limestone: a mineral called calcium carbonate. So are coral reefs and the shells of all shellfish.] The same happens with rain which absorbs CO2 from the air. And the same happens with the oceans.

On the importance of phytoplankton

The ocean is home to myriads of tiny life forms called phytoplankton. These remove CO2 from the ocean by doing what all green plants do: photosynthesis. Scientists have found that these simple tiny ocean plants remove about half of CO2 produced by humans use of fossil fuels. This is more than is taken out of the air by all the world’s rainforests and other land plants. So they may be small but they are very important and most of them make their skeletons out of calcium carbonate. This can’t happen if the oceans become too acid

What is calcium carbonate?

This is a very common mineral which exists in two forms: calcite and aragonite, chemical formula CaCO3. Limestones are all made of it and it’s useful because it ‘locks away’ carbon. Sea creatures which use it for their skeletons are helping to remove carbon from the air and sea

Why plankton are so important

Plankton are very tiny but very numerous; so numerous that you can see them from space when they form ‘blooms’. They are bottom of the food chain which means that all larger sea creatures depend on them, directly or indirectly, for their food. Without plankton, there’d be no fish and no corals.

Click to make bigger

This is something of a good news story for a change. You probably know something about it already. Scientists discovered there was a problem with ozone and all the world’s countries agreed to fix the problem. And the treaty they all agreed to seems to be working quite well.

But what is ozone? It’s a gas molecule made of three oxygen atoms (O3) and is not like the oxygen we all need to live. The oxygen we breathe is a molecule formed by just two atoms (O2) and makes up about 21 percent of the atmosphere. Almost all the rest of the air is made of nitrogen gas. The ozone form of oxygen makes up only a tiny amount There are two odd things about ozone:

  • most life – including people – couldn’t exist without it
  • it’s poisonous! So there’s ‘good’ and ‘bad’ ozone. I’ll explain because this sounds silly but it is both true and very important.

Almost all ozone exists high in the stratosphere It is formed continually as powerful ultraviolet (UV) radiation from the sun splits some ordinary oxygen (O2) molecules into oxygen atoms (O) which react with each other to make ozone. It’s a very good thing that this happens because the ozone acts as a special shield from dangerous UV radiation from the sun. Without the ozone shield to absorb most of the UV radiation, there would be nothing to protect us living things from it. The sun would be like a giant death ray gun which never stopped. So that’s why life can’t survive without ozone. The small amount of UV radiation that does make it to the Earth’s surface can cause nasty diseases in people, like skin cancer, so sunbathing at the beach is not a good idea. The bottom line is we can’t survive without ozone, or we would all be burned to a crisp by the sun’s UV rays.

Click to make bigger

So ozone is good and yet it’s poisonous?
Fortunately, 90 percent of all the ozone floats high up in the stratosphere where it’s bitterly cold and the air is so thin nobody could survive anyway. Even the world’s highest mountains don’t reach anywhere near that high. The tiny amounts of ozone which you and I breathe in at the surface of our planet do us no harm because they are really tiny amounts. It can become a problem in the polluted air – smog – which hangs over many cities. This is because strong sunlight reacts with pollutants like car exhaust and industrial chemicals to make ozone in large enough amounts to be harmful to health, especially for people with breathing difficulties like asthma.

The ozone ‘hole’
Scientists began studying ozone levels in the stratosphere about 50 years ago. In the 1970s, they suddenly realised that amounts of ozone above the Antarctic – my home! – were falling rapidly, especially in the spring. A giant hole formed each spring and this let in much more UV radiation than normal. The hole became so large that people in Australia and New Zealand began to have trouble with skin cancers. Similar holes also began to form over the Arctic. The scientists discovered that the reason the ozone was disappearing was long-lived and man-made chemicals called CFC It turned out that these CFCs were big time ozone-eaters and people were using them in large amounts for things like refrigerators. When the CFCs escaped into the air, they floated up into the stratosphere. UV radiation from the sun knocks chlorine atoms out of the CFC molecules. The chlorine, which is deadly poisonous, quickly destroys the ozone molecules in complicated reactions. This all happens in high and very cold polar clouds during the winter. The chlorine then returns to destroy more ozone and so the telltale ozone hole forms in spring.

Click to make bigger

And the good news? Thanks to careful science, people quickly realised what was happening and took steps to stop it. There is now an agreement between all countries to quit using CFCs and other ozone-eating chemicals. This is called the Montreal Protocol. Thanks to this agreement, the worst danger seems to be over and it looks as if we won’t be going over this planetary boundary. It’s double good news because it also shows that humans can work together to stop bad things happening to our planet.

Ozone in the air

Ozone makes around 0.000004% overall up of the atmosphere and up to 8 parts per million molecules of all gases in the atmosphere at between an altitude of 30 and 35 kilometres. This is the action zone where the radiation-absorbing reactions take place

What is the stratosphere?

The stratosphere is part of Earth’s atmosphere. It begins around 10 kilometres (6 miles) above us and the air is very thin. Almost no living creature can survive – unless in the pressurized cabin of an aircraft! The part of the atmosphere we live in is the densest part. It’s called the troposphere and is where almost all weather happens (rain, hurricanes, snow and so on)

Oxygen and ozone

O+O2=O3 Simple maths!

What are CFCs?

Their chemical name is chlorofluorocarbons. When they drift up into the stratosphere, the CFC molecule breaks down in sunlight, freeing a chlorine atom. This is where the trouble begins. The chlorine released from CFCs catalyzes the break down of ozone (O3) to oxygen (O2). The chlorine atoms continue to do this many times – catalysts aren’t destroyed in the reaction they catalyze – which is why even minute quantities are so damaging to the ozone layer

What about vitamin D?
Human skin is like a sort of factory for making vitamin D, but it only works when the sun shines on it. Vitamin D is very important because it makes strong bones among other things. So a little exposure to the sun is good.
Click to make bigger

Aerosol loading – smoke, dust and soot

An aerosol is a spray can that you use, right? Yes but it’s also smoke and dust that affects people’s health and pollutes the air. Not nice, huh? Humans make a big mess wherever they go, churning out masses of smoke or dust aerosols from

  • car, truck, ship and aircraft engines,
  • fires from cooking, heating, burning forests
  • industry – burning fuels like coal
Click to make bigger

This smoke and dust is made up of very tiny particles, which are so light that they get wafted up into the atmosphere where they stay for weeks or months. They are called aerosols and they affect the climate and people’s health. Soot particles – a major part of these aerosols – cause the climate to warm up because they are black and absorb heat from the sun. Sulphate particles (from burning coal mostly, and also from large volcanoes occasionally erupting) can cause local cooling. A brown haze of smoke sits permanently over India and is called the Asian brown cloud. It seems that this not only causes warming but also partly blocks the life-giving monsoon rains which Indian farmers depend on for growing food.

Nobody knows what limits should be set for the dusty haze of smoke, soot and sulphate particles humans are making. It looks like it should be a lot less than now.

Click to make bigger

Nitrogen gas on its own is inert It makes up most of the air we breathe and is vital for life. But there’s a big problem because humans are using enormous amounts of artificial nitrogen fertilisers for growing crops. People can do this because clever chemistry means that nitrogen gas can be 'fixed' from the air and made into a form which plants can use to grow big and fast. A good thing, you might think, because it means more people can be fed.

But all this extra nitrogen is beginning to cause major pollution problems, damaging the natural nitrogen (N) cycle by massive overload. Nature has always ‘fixed’ N too but the natural nitrogen cycle is now overwhelmed by all the artificial N fertilisers farmers use.

Scientists reckon that people can safely make around 35 million tons of nitrogen fertilisers each year and not overload the system. At present, people are making four times that amount and nature can’t cope with it. Farmers use so much N fertiliser, often unnecessarily, that it makes the soil acid and kills wildlife. Because it’s soluble, it dissolves in rain and runs off into rivers, lakes and the sea where it causes havoc by making 'dead zones' where all the oxygen in the water gets used up and nothing can live. Fish and coral reefs die. The best known ‘dead zone’ is in the Gulf of Mexico though there are many others around the planet caused by this over-fertilisation.

Phosphorus is another element which plants need to grow and once again, humans are overdoing it and this adds to the problems of dead zones. People are using twice as much as they can safely use, say the scientists.


The nitrogen gas we all breathe is inert. It doesn’t react with anything and just stays as ordinary nitrogen gas. Did you know that 78 per cent of the air is made of nitrogen and only 21 per cent is oxygen? If there was much more oxygen, there would be many more serious fires. About 300 million years ago during a period geologists call the Carboniferous, oxygen levels soared to as much as 35 per cent. As well as wildfires, the extra oxygen meant that enormous insects and amphibians could evolve.

‘Fixing’ nitrogen: why it’s so important

Nitrogen gas is ‘fixed’ by combining it with hydrogen gas to make ammonia which is then turned into solid nitrogen compounds. These will dissolve in water and can be used by plants. They can also be used to make explosives. So the N is no longer inert like the gas in our atmosphere. It is reactive. ‘Fixing’ N artificially is called the Haber process and it uses a lot of energy from fossil fuel (natural gas)

Fixing nitrogen

Certain types of plants have the ability to ‘fix’ nitrogen from the air. They have formed a special relationship with a type of bacteria which actually does the ‘fixing’. In return for nitrogen compounds it needs to grow, the plant gives the bacteria both a home in its roots and the sugars the bacteria need to live. This cooperation is an example of symbiosis and is quite common in the natural world

Too much food

This is called eutrophication and is caused by algae which have so much fertiliser ‘food’ that they use more oxygen than they produce. This means that all oxygen-breathing creatures – like fish – die

Over 400 at the last count in 2008

Greedy people

Isn’t it funny that humans always use more than their fair share, comments Amy, my junior editor. So long as they get lots of what they want, they don’t seem to realise that this is affecting the planet… and penguins too!

Click to make bigger

Fresh Water: something we take for granted

Without fresh water, no people could live. Happily, there’s plenty of it about… but only in some places. In deserts, there’s no water. Humans waste alot of water without even realising it. In hotter countries, where climate change is starting to take effect, there is not much water. What there is may be dirty and can carry diseases. But people are taking so much water that they’re creating more deserts. You might wonder how that can be.

All fresh water comes from rain. The rain falls on the land and makes big rivers and lakes. Much more of it sinks into the ground and gets stored there. These natural underground stores of water are called aquifers or, simply, groundwater.

Click to make bigger

As you know, people need fresh clean water to drink and wash. But that’s only a small amount of the vast amount of water that humans take for things like farming and industry. A lot more goes on watering lawns and golf courses in dry regions.

One famous example of people seriously damaging a whole region by taking too much water is the Aral Sea in central Asia (look left at the image). So much water gets used for growing water-hungry cotton that the once-large inland sea has almost dried up. Fishing boats rust in the middle of what is now desert. This has even changed the climate in the region, making it hotter in summer and colder in winter.

It’s a fact: people are already taking so much water from rivers that around a quarter of them no longer reach the sea for at least some of the year.

Scientists reckon that people can safely take about one third of the water from rivers (but not from those in remote areas). That limit hasn’t been reached yet and, with care, it won’t be.

Click to make bigger

Land: not enough for everyone

Most of our planet is covered by the seas and oceans. Only about one third is land, and much of that is either desert or covered in ice. Think of Greenland and almost the entire continent of Antarctica. That doesn’t leave a lot for all the land-living animals – including humans.

Before people started farming about 10,000 years ago, most of the land was either forest or grassland. Today, it’s very different. Most of the forests have been cut down or burned to make space for growing crops or raising animals. Roads and cities take up yet more land. The animals that used to live there are now either extinct or in big trouble, because people need ever more land for food as numbers continue to increase.

There’s obviously a limit and the scientists reckon that humans can use at most about 15 per cent of land which isn't covered in ice. Right now, people are farming around 12 per cent and this is still going up. But people are clever and are getting better at growing more food on the same amount of land. This is because farmers and scientists have a better understanding of things like how to breed better food plants and animals (genetics), soil fertility, water needs and so on.

There are now 7 billion humans and the figure increases by about 80 million every year. More humans want more resources and poor people, not surprisingly, want decent living standards like people already have in richer countries

Icy continents

Large areas of land are still covered by thick ice sheets. I’m sure you know the two biggest? Antartica, a huge continent and Greenland which is a very large island. If all the ice on Greenland melted, sea levels around the world would rise by a whopping 8 metres; over 26 feet


More tropical rainforests are still being cut down or burned to make way for cattle ranching, biofuel production and farming for food.


Breeding can be conventional in which plant breeders select the best crop plant or animal – like evolution speeded up. Or it can employ genetic engineering

Biodiversity: this is the big one!

The world is full of animals and plants. There are millions of them. You see them all the time in wonderful wildlife programmes on the TV so how can there possibly be a problem?

In fact there are two problems:

Problem 1: animals and plants are going extinct very much faster than normal. I’ll explain why shortly. You will probably have heard of many creatures which are on the edge of becoming extinct. The well known ones are usually pretty, cuddly or large with big teeth. These are the ‘charismatic’ species and get lots of publicity; the celebs of the natural world..

Click to make bigger

The other uninteresting or tiny ones, which are just as important, get ignored or forgotten or people simply don’t yet know they exist.

Problem 2: all species tend to be part of webs of life, called ecosystems. If part of the web is damaged, then the whole thing can come tumbling down. The importance of these ecosystems is that they provide life-supporting services for people. There are many examples and humans are only just becoming aware of them and their importance – just as many are being damaged or destroyed.

Click to make bigger

One of the easiest ecosystem services to understand is pollination, mostly done by bees. Wild bees pollinate most of the world’s food crops. No bees, much less food for people – and bees are in serious trouble.

“The annual monetary value of pollination services in global agriculture could be as high as $200 billion” FAO

This is why I wrote a guide called Life in Danger – because much of Life IS in danger. Species are becoming extinct so fast that it’s like a sixth version of the five well-known mass extinctions. I’m sure you’ll know about the last big extinction: it killed off the dinosaurs about 65 million years ago, probably because of an asteroid hitting the planet. But this sixth extinction is not natural: it’s being caused by humans and, unlike an asteroid impact, humans should be able to stop it.

Nobody is doing it on purpose – it’s just that people are only just beginning to realise how powerful they are and what terrible things they do to other life without thinking about it. So why’s it happening?

  • People are taking over more and more land for farming, roads and cities
  • People are cutting down natural forests, destroying not just the trees but all the myriads of living things which live there (think orang-utans, gorillas and lemurs to name just three ‘cuddly charismatics’)
  • People have brought in alien species (think rats – everywhere, even on remote islands, cane toads and rabbits in Australia and kudzu vine in the USA) which kill or swamp (outcompete) native creatures
  • People are poisoning species on land and in water (think coral reefs, sometimes called the rainforests of the oceans) with pollution and chemicals used in farming
  • People are hunting all kinds of creatures for food or sport (this especially applies to fishing)
  • People are changing the climate so that many species of plants and animals can no longer live where they used to. There’s nowhere for them to go so they die

Species are always dying out and becoming extinct. This has been true since life began. Scientists reckon that the normal rate of extinction is around 10 extinctions every year for every million species.

Click to make bigger

Since humans have started their damaging practices, the extinction rate has shot up to 100 times that rate and seems to be increasing even more. The scientists suggest that people need to bring down this rate to something like between 10 to 100 extinctions for every million species each year. This would be a planetary boundary.

That’s still a lot, you might think, but better than what’s happening now. People can do something about this sixth extinction by reversing as many of the bad things as possible on that list I’ve just given you (above). Not easy, I agree, but it can be done by you clever humans, working together for the good of yourselves as well as all life.

What is biodiversity?

Short for ‘biological diversity’ it means the immense numbers of different living things on our planet

Extinction is forever

Extinct doesn’t just mean dead – it means gone for ever

What are species?

A ‘species’ is what scientists call one particular type of any sort of life, whether it’s you (human), me (penguin), or any type of life be it animal, plant, fungus or microbe.

The biggest mass extinction

This was around 250 million years ago at the end of the Permian geological period. Over 90 percent of life died

Dead zones caused by excess nitrogen are an example which I’ve mentioned in my ‘Nitrogen overload’ page
Did you know that there are almost 100,000 chemicals made by humans? Some of these are known to be highly poisonous (toxic). Many of these chemicals are deliberately poisonous because people use them for killing things – and have in times of war used some to kill each other. Some toxic chemicals bioaccumulate so top predators like bears, dolphins and people can be badly affected.

Click to make bigger

Pollution is now such a big thing that it affects people and the animals, plants and complex web of life that humans depend on over the whole planet. For example, when people burn coal, mercury forms a small part of the waste gases which pour out of smoke stacks into the air. It then drifts around the planet, blown by the wind and eventually comes back to earth in rainfall, so it pollutes lakes and even the oceans. Like POPs, mercury bioaccumulates. Amounts of mercury in large predator fish can be over a million times that of the water they swim in. Not good for the fish or for people who may eat them. [More on mercury.]

Radioactive pollution is another concern, as some radioactive elements produced by nuclear power remain radioactive for thousands of years, though there are possible ways of dealing with these.

Click to make bigger

Pollution also affects other planetary boundaries… for the worse. The aerosol boundary is tightly linked to pollution, both because aerosols are themselves pollutants and because they carry pollution like mercury far and wide. And as some creatures have already been driven close to extinction because people have destroyed their homes, they will already be weakened. More pollution and more extreme climate events could tip them over the edge. And climate change means that more crop pests and diseases will migrate so farmers will end up using more pesticides… more pollution.

Good news on lead

The legacy of lead remains in the form of pollution of the soils near roads. But airborne lead is dead!
Not many years ago, most fuel for cars contained lead (tetraethyl lead). This was to make them run better. But the lead, a highly-toxic heavy metal, spewed out of tailpipes as an aerosol along with the other exhaust gases. So people breathed it in – with serious effects, especially in children. Eventually, governments realised that lead in fuel was causing serious health damage to people, particularly those living near busy roads. So now, fuel in almost all countries around the world is lead-free.

So how do the scientists decide how much pollution is too much? It’s such a complicated subject that there is no simple way of setting up a planetary boundary. It may not be easy to tackle climate change but at least people know what the limits are for greenhouse gases. With chemical pollution, all that scientists can do is monitor what's happening to living things.

You can do your bit to help cut pollution. Have you read my Pollution Guide? There’s some ideas for action at the end of it.

You can do your bit to help cut pollution. Have you read my Pollution Guide? There’s some ideas for action at the end of it.

What does ‘bioaccumulate’ mean?

It means that the toxic chemicals get concentrated in the bodies of animals which get eaten by bigger ones. Eaten, that is, with the toxic chemicals included. So the toxins increase in the big predators as they eat more contaminated prey. This can make them sick, or even kill them. It can also affect their reproduction so, even though they may appear healthy, they can’t have babies

Why mercury is dangerous

It is a very poisonous heavy metal – as is lead – and it bioaccumulates in animals’ bodies

Mostly the greenhouse gas CO2

POPs: What are they?

Persistent Organic Pollutants: DDT, PCBs and dioxins are examples

Dealing with radioactive waste

With new types of reactors, it will be possible to ‘burn’ the dangerous long-lived waste (actinides) and end up with a much smaller amount of waste. This will still be radioactive but the activity will die away in just a few hundred years

Aerosols can make you sick

Smoky aerosols like car and truck exhausts can cause asthma

Unwanted migrants

Just as people migrate to find new and better places to live, so too do pests and diseases. Global warming makes it possible for these pesky organisms to survive and prosper where, until now, it had been too cold


Some chemical are particularly troublesome. These are the POPs: persistent organic pollutants such as DDT and polychlorinated biphenyls (PCBs) are the best known but there are many more. These, along with heavy metals like mercury, tend to become more concentrated higher up the food chain. Top predators like eagles, polar bears and humans can end up with so much of these toxins that their young can be damaged: eagles’ egg shells become brittle and break; breast milk from humans and polar bears can be toxic to their babies

What next? If my guide has done its job, you will want to know more about planetary boundaries. You will also want to know what you can do to help change things. I think the most important thing you can do to get to understand as much as you can about what I called Earth’s nine lives. So what I’ve done on this final page of my guide is to give you some resources, mostly videos and web links. Note for younger kids And a reminder: could you please help me?

Try my crossword puzzle All the answers are in this guide…

The planet’s nine lives: the planetary boundaries in one image.

This wonderful image comes from Johan Rockström and his team at the Stockholm Resilience Centre, Stockholm, Sweden. They first published the idea of planetary boundaries in 2009. In this image, the green area in the centre is the safe area. Any human activities which lie inside are fine though of course they must stay there. The red areas flag up danger; the more red, the more danger. You can immediately spot the three problem systems because they’re way over the limit. So biodiversity is in really serious trouble. Nitrogen overload isn’t much better and neither is climate change. For now, the other systems are inside the safe green zone.

In this video (2019), Johan Rockström explains all about planetary boundaries.

Johan Rockström: Safe Future for Humanity on Earth  (2019) updates what we now know about all our planet’s 9 lives and how humans are the first species to have caused a mass extinction; the sixth.

Here’s another important video featuring award-winning environmental writer Mark Lynas. He explains about planetary boundaries and, importantly, what people can do to make things better. He says that humans now have such power over the planet and its systems that they can and must use this power to change the planet for the better. This will mean, among other things, using new nuclear technologies, geoengineering and genetic engineering. Mark Lynas’ latest book Our Final Warning: Six Degrees of Climate Emergency (2020) brings us up to date on the latest science. It is scary but the evidence is so powerful now that, scary or not, you humans have good to start making huge changes right away. You have been warned.

Jon Foley, University of Minnesota, and one of the original team headed by Johan Rockström, introduces planetary boundaries in 3 minutes.

Johan Rockström and photographer Mattias Klum have launched a beautifully illustrated e-book. This video gives you the basics of the book.

Climate change is causing terrible damage to the natural world. This video explains…

How to save the world’s oceans, a series of 18 videos from The Economist
Biodiversity and Climate Change from CBD on Vimeo

“Corporations are the ones gambling our planet away and our governments are running the casino,” claimed lawyer Polly Higgins in her book ‘Eradicating Ecocide’. Sadly, this brave and determined Scottish lawyer died in 2019. You can see and hear her in this video: Why earth destruction is a crime

And some links:

Tipping towards the unknown The original source of the critical planetary boundaries idea. Crossing the boundaries could be catastrophic, say the researchers. But there is hope.

The nine planetary boundaries Stockholm Resilience Centre, Stockholm University, Sweden, and where it all began

The End

Not just for adults

These videos and links have been made by adults for adults. Some of the videos are quite long but there’s no reason why you shouldn’t have a look. Having read my guide, you’ll probably recognise some of the stuff. If you’re doing science at school, you’ll probably follow most of the ideas quite easily. If in doubt, try them out!