The carbon cycle and human impact

To get energy, humans break down carbon-based substances and exhale carbon dioxide (CO₂). Without carbon, we and everything else living on Earth simply wouldn't function.

Just like water, carbon circulates in nature. The two central parts of the carbon cycle are photosynthesis and combustion. The sun powers the carbon cycle and, via photosynthesis, carbon dioxide is bound as carbon atoms in plants. This is called carbon storage.

Photosynthesis works like this: There is carbon dioxide present in the air. Green plants absorb this carbon dioxide and, together with energy from the sun and water (absorbed via their roots), plants convert carbon dioxide into glucose, which they use for nourishment. Oxygen is also created during photosynthesis, which the plant then releases into the atmosphere.

Humans and animals breathe in this oxygen, which we need to burn food and release energy to the cells in the body. At the same time, carbon dioxide is formed, which is then exhaled. This is called cellular respiration. The same process occurs when wood is burned. Then, too, oxygen, carbon and hydrogen are consumed, and energy (heat) and carbon dioxide are released. This process is called combustion.

How long does the carbon cycle take?

The carbon does not have one cycle but several, of different lengths. One day. One year. A hundred years. Or millions of years.

Through photosynthesis and cellular respiration, carbon atoms circulate daily in a short cycle.

A slightly longer process is when carbon is stored in the body of a mammal via food that originates from the plant kingdom. When the animal dies, decay sets in, the body breaks down and carbon dioxide returns to the atmosphere.

A significantly slower cycle is when carbon atoms are absorbed in tree trunks and branches. Many trees can live to be several hundred years old, and the carbon is only released back into the atmosphere when the tree dies and decays. If the tree is planted to be harvested for fuel, the process occurs much faster.

Crude oil, natural gas, black coal and lignite are called fossil fuels. They have been formed from dead organisms (plants and animals) that did not break down completely when they died but were packed together over millions of years underground. When crude oil is pumped out of the ground, refined, and used as gasoline and diesel in engines, carbon dioxide that has been out of the carbon cycle for millions of years is released.

Carbon can also be found in the ocean

Even water and oceans are part of the carbon cycle. The surface water absorbs large amounts of carbon dioxide, which dissolves in the water. Carbonic acid (H₂CO₃), among other things, is formed. About half of all carbon dioxide (or dissolved carbon) is used by marine organisms that need the carbon to grow, or by clams and corals that use it to build shells. The shell mostly consists of a mineral called calcium carbonate (CaCO₃).

When sea creatures die, they sink to the bottom of the ocean and form sediment. That in turn can, after a very long time, be converted into limestone that binds carbon. For example, the Gotland limestone is estimated to be 400 million years old. Our oceans are the world's largest carbon sinks. However, when seawater gets warmer, less carbon dioxide is captured by the ocean.

How do humans affect the carbon cycle?

The natural cycle of carbon is disturbed by human activities. The use of fossil fuels for transport and energy emits large amounts of carbon dioxide into the air. Human influence on the carbon cycle results in an increased greenhouse effect, which in turn drives climate change.

By cutting down and burning large areas of rainforest, for example in the Amazon and Borneo, we humans have reduced nature's ability to store carbon dioxide from the atmosphere.

In Sweden, large-scale forestry affects the natural carbon cycle. The most common felling method is clear cutting, which means cutting down all the trees in a place and then planting new ones. Clearcuts emit carbon dioxide for 10–12 years, and it can take the new saplings up to 20 years to capture the same amount of greenhouse gases that a clearcut emits.

Cement production is another example of how the carbon cycle is disrupted. When cement is made, it releases carbon dioxide that has been bound in limestone for millions of years. Two-thirds of the emissions from cement manufacturing come from the burnt lime and the rest from the fossil fuels used to heat the limestone.

Almost all the food we humans and our domestic animals eat causes emissions of carbon compounds (greenhouse gases), either in Sweden or in the countries from which the food is imported. High demand for soy feed leads to large areas of rainforest being felled, causing important natural carbon sinks to disappear. Domesticated animals are mostly fed with crops grown on arable land. All kinds of cultivated crops store carbon dioxide through photosynthesis as they grow. Before sowing and during harvest, heavy machinery that runs on diesel and emits carbon dioxide is used, and when fields are ploughed, carbon is released from the ground. Cows and sheep release the gas methane (CH4) when they digest their food. Although methane is a stronger greenhouse gas than carbon dioxide, it breaks down faster in the atmosphere than carbon dioxide does.

People's private consumption of gadgets and clothing also affects the carbon cycle. The largest part of the emissions caused by Swedish consumption today takes place abroad, where the goods are produced. The transport from the manufacturing country to Sweden adds to the total emissions.

How to create better balance in the carbon cycle

By developing and switching to renewable energy sources, emissions from burning fossil fuels can be reduced. Methods of transport and travel also need to change so that they do not emit carbon dioxide.

When we shop, we can choose goods that last a long time and are made from renewable materials that can be recycled or reused, instead of being incinerated immediately after they have been discarded.

By eating less meat and choosing meat from animals that have grazed free-range on grass in forests and open fields, climate emissions from the production of animal feed can be reduced. Agriculture's carbon dioxide emissions can be reduced by ploughing land less often, and by increasing the cultivation of perennial crops.

Nature-based solutions

Nature can be helped to absorb more carbon dioxide by preserving old natural forests and allowing all forests to grow in a natural way. The trees then need to be allowed to grow and store carbon for a long time, and if they are cut down, it should be done without destroying the land. If the trees are used to build houses instead of fuel, the carbon atoms continue to be bound as long as the houses remain.

Nature can also store more carbon if more old wetlands are restored. During the 19th and 20th centuries, many natural wetlands were drained to create arable land or forest plantations. In the wetlands, carbon is stored in the form of peat, which consists of dead plants that have only partially broken down in the low-oxygen environment. Wetlands are a natural carbon sink that can keep large amounts of carbon dioxide out of the atmosphere. They can also prevent forest fires from spreading.

Creating more green areas such as parks and green roofs in cities and urban areas will increase the number of plants available to absorb carbon dioxide. Trees in cities are also important in providing shade and coolness for people and animals as the climate gets warmer.