When scientists first realised that increasing CO2 in the atmosphere was linked to climate change, some were puzzled. There was so little of it in the atmosphere— how could it change the climate of an entire planet? Then they discovered that CO2 acts as a trigger for that potent greenhouse gas, water vapour.
Carbon dioxide also lasts a long time in the atmosphere: around 56 per cent of all the CO2 that humans have liberated by burning fossil fuel in the past century is still aloft, which is the cause—directly and indirectly—of around 80 per cent of all global warming.
The fact that a known proportion of CO2 remains in the atmosphere allows us to calculate, in round numbers, a carbon budget for humanity. We can do this in gigatonnes— a gigatonne is a billion tonnes. The carbon budget tells us how much more carbon we can put into the atmosphere before we trigger dangerous changes, which are widely acknowledged to occur at between 450–550 parts per million of CO2.
Prior to 1800, the start of the Industrial Revolution, there were about 280 parts per million of CO2 in the atmosphere, which equates to around 586 gigatonnes of carbon. (To make comparisons easy, figures like this relate only to the carbon in the CO2 molecule. The actual weight of the CO2 would be 3.7 times greater.)
Today the figures are 380 parts per million or around 790 gigatonnes in total.
If we wish to stabilise CO2 emissions below that threshold of dangerous change, we will have to limit all future human emissions to around 600 gigatonnes. Just over half of this will stay in the atmosphere, raising levels to around 1100 gigatonnes, or 550 parts per million, by 2100.
This will be a tough budget for humanity to abide by. Over a century, it equates to around 6 gigatonnes per year. Compare this with the average of 13.3 gigatonnes that accumulated each year throughout the 1990s (half of this from burning fossil fuel). And remember that the human population is set to rise from 6 billion now to 9 billion in 2050. You can see the problem.
Even in the long view, this rise in CO2 is exceptional. Its concentration in the atmosphere in times past can be measured from bubbles of air preserved in ice. By drilling more than three kilometres into the Antarctic ice cap, scientists have drawn out an ice-core that spans almost a million years of Earth history.
The power of the ice core record to tell us about the climate and atmosphere in times past came home to me when I visited the University of Copenhagen’s ice core store in Denmark. I had arrived straight from an Australian summer and the store was –26°C. The hardy Dane showing me about seemed unaware of my shock. Concern for my freezing nose instantly vanished, however, when my guide held out a cylindrical piece of ice around one metre long and pointed to a layer of ice in it around five centimetres thick. That ice, he said, fell as snow over central Greenland in the year Jesus was born, and the tiny specks I could see in it were bubbles of air trapped in the ice. From those bubbles scientists could tell what the levels of CO2 and other atmospheric gases were that year, which reveals quite a lot about the state of the climate. The atmosphere mixes so quickly, he said, that it’s possible those bubbles contain a few molecules breathed out by the Holy Family during that first year.
The unique record of the ice cores demonstrates that during cold times CO2 levels have dropped to around 160 parts per million, and that until recently they never exceeded 280 parts per million. The Industrial Revolution, with its steam engines and smoky factories, changed that. By 1958, when Keeling began his measurements of CO2 atop Mauna Loa, it comprised 315 parts per million.
It is our servants—the billions of engines that we have built to run on fossil fuels such as coal, petrol and oil-based fuels, and gas—that play the leading role in manufacturing CO2. Most dangerous of all are the power plants that use coal to generate electricity. Black coal (anthracite) is composed of at least 92 per cent carbon, while dry brown coal is around 70 per cent carbon and 5 per cent hydrogen.
Some power plants burn through 500 tonnes of coal per hour. They are so inefficient that around two-thirds of the energy created is wasted. And to what purpose? Simply to boil water, which generates steam that moves the colossal turbines to create the electricity that powers our homes and factories.
Most of us have no idea that nineteenth-century technology makes our twenty-first-century gadgets work.
There are around thirty other greenhouse gases. Think of them as glass windows in a ceiling, each gas representing a different window. As the number of windows increases, more light is admitted into the room, where it is trapped as heat.
After CO2, methane is the next most important greenhouse gas. Methane is created by microbes that thrive in oxygenless environments such as stagnant pools and bowels, which is why it abounds in swamps, farts and belches. It comprises just 1.5 parts per million of the atmosphere, but its concentration has doubled over the last few hundred years.
Methane is sixty times more potent at capturing heat energy than CO2, but thankfully lasts fewer years in the atmosphere. It is estimated that methane will cause 15 to 17 per cent of all global warming experienced this century.
Nitrous oxide (laughing gas) is 270 times more efficient at trapping heat than CO2. It is far rarer than methane but it lasts 150 years in the atmosphere. Around a third of our global emissions of this gas come from burning fossil fuels. The rest comes from burning biomass (plant and animal material) and the use of nitrogen-containing fertilisers. While there are natural sources of nitrous oxide, human emissions now greatly exceed them in volume. Today there is 20 per cent more nitrous oxide in the atmosphere than there was at the beginning of the Industrial Revolution.
The rarest of all greenhouse gases are members of the hydrofluorocarbon (HFC) and chlorofluorocarbon (CFC) families of chemicals. These products of human ingenuity did not exist before industrial chemists began to manufacture them. Some, such as the tongue-twisting dichlorotrifluoro-ethane, which was once used in refrigeration, are ten thousand times more potent at capturing heat energy than CO2, and they can last in the atmosphere for many centuries. We shall meet this class of chemicals again later, when we come to the story of the ozone hole.
For the moment, though, we need to know more about the carbon in CO2. Both diamonds and soot are pure forms of carbon; the only difference is how the atoms are arranged. Carbon is everywhere on the surface of planet Earth. It is constantly shifting in and out of our bodies as well as from rocks to sea or soils, and from there to the atmosphere and back again.
Were it not for plants and algae, we would soon suffocate in CO2 and run out of oxygen. Through photosynthesis (the process whereby plants create sugars using sunlight and water) plants take our waste CO2 and use it to make their own energy, creating a waste stream of oxygen along the way. It’s a neat and self-sustaining cycle that forms the basis of life on Earth.
The volume of carbon circulating around our planet is enormous. Around a trillion tonnes of it is tied up in living things, while the amount buried underground is far, far greater. And for every molecule of CO2 in the atmosphere, there are fifty in the oceans.
The places where the carbon goes when it leaves the atmosphere are known as carbon sinks. You and I and all living things are carbon sinks, as are the oceans and the soil and some of the rocks under our feet.
Over eons, much CO2 has been stored in the Earth’s crust. This occurs as dead plants are buried and carried underground, where they become fossil fuels. On a shorter time scale, a lot of carbon can be stored in soils, where it forms the black mould that gardeners love.
Even the belching of volcanoes (which contains much CO2) can disturb the climate. And meteorites that collide with Earth can also disrupt the carbon cycle by upsetting the oceans, atmosphere and the crust.
Scientists know where the CO2 that we produce goes. This is because the gas derived from fossil fuels has a distinct chemical signature and can be tracked as it circulates around the planet. In very round figures, 2 gigatonnes is absorbed by the oceans and a further 1.5 gigatonnes is absorbed by life on land each year.
The contribution made by the land results partly from an accident of history—America’s frontier phase of development. Mature plants, trees and forests don’t take in much CO2 for they are in balance, releasing CO2 as old vegetation rots, then absorbing it as the new grows. The world’s largest forests—the coniferous forests of Siberia and Canada—and the tropical rainforests don’t absorb as much carbon as new forests.
During the nineteenth and early twentieth centuries, America’s pioneers cut and burned the great eastern forests, and burned and grazed the western plains and deserts. Then shifts in land use allowed the vegetation to grow back. As a result, most of America’s forests are fewer than sixty years old and are regrowing vigorously, absorbing around half a billion tonnes of CO2 annually from the atmosphere. And remember, trees are built of air, not the ground they sprout from: timber, leaves and bark were once, not that long ago, CO2 in the atmosphere.
Newly planted forests in China and Europe may be absorbing an equal amount. For a few crucial decades these young forests have helped cool our planet by absorbing excess CO2.
But as the Northern Hemisphere’s forests and shrub-lands recover from their damage at the hands of the pioneers, they will extract less and less CO2, just when humans are pumping more of it into the atmosphere.
If we take a long term view, there really is only one major carbon sink on our planet—the oceans. They have absorbed 48 per cent of all the carbon emitted by humans between 1800 and 1994.
The world’s oceans vary in their ability to absorb carbon. One ocean basin alone, the North Atlantic—which comprises 15 per cent of the ocean surface—contains almost a quarter of all the carbon emitted by humans since 1800. Shallow seas are a carbon kidney and have removed 20 per cent of all carbon dioxide emitted by humans.
Scientists are worried that changes in ocean circulation brought about by global warming might degrade the effectiveness of this ‘carbon kidney’. There are many ways that this could happen, one of which you can see in a warm can of soft drink. That fizz on opening the can fades—indicating that the liquid has quickly released the CO2 that gives it its sparkle. Cold drinks hold their fizz longer. Cold sea water can hold more carbon than warm sea water, so as the ocean warms it becomes less able to absorb the gas.
Sea water also contains carbonate. It reaches the oceans from rivers that have flowed over limestone or other lime-containing rocks, and it reacts with the CO2 absorbed into the oceans. At present there is a balance between carbonate concentration and the CO2 absorbed. As the CO2 concentration increases in the oceans, however, the carbonate is being used up.
The oceans are becoming more acid, and the more acid an ocean is the less CO2 it can absorb.
Before the end of this century the oceans are predicted to be taking in 10 per cent less CO2 than they do today. In the meantime we continue to pour more and more CO2 into the atmosphere.