The Earth’s Climate Is Warming Because Humans Release Too Much Carbon Into the Air

The greenhouse effect is a process that occurs when certain gases in the Earth’s atmosphere trap the energy and heat from the sun (Figure 1). These gases are called greenhouse gases. You can think of this layer of gases as a blanket around the Earth: it keeps the Earth warm. Without the greenhouse effect, most heat would escape back into space and the Earth would be too cold for us to live on.

However, due to human activities such as driving cars, deforestation, burning coal and gas to make electricity, and raising cows for meat and milk, we emit more greenhouse gases into the atmosphere than in the past. Carbon dioxide (CO₂) is one of the most important greenhouse gases we emit. CO₂ emissions have caused the greenhouse effect to become stronger, leading to increasing temperatures across the globe. This global warming is changing Earth’s ecosystems and weather and resulting in major climate changes. For many locations, this means hotter heatwaves, longer periods without rain (droughts), or too much rain at once, causing floods! You may have noticed such events occurring more often where you live, too.

To combat climate change, we need to think about ways to reduce our CO₂ emissions. We can eat less meat, use less electricity or gas to warm our houses, and ride our bikes instead of driving. To help us, nature has come up with a nice solution as well: trees!

Trees Draw Carbon From the Air

Trees store carbon in their trunks, roots, branches, and leaves. Trees need carbon to grow—they draw CO₂ from the air like vacuum cleaners. Air enters the leaves through tiny pores and, once inside, the carbon is captured from the air, while oxygen is released back into the environment.

Trees can store carbon in two more ways, which are perhaps less easy to see, but not less important (Figure 1). In addition to storing carbon in their bodies, trees also increase the amount of carbon stored in on the forest floor and in the soil (Figure 1). In autumn, most trees let go of their leaves, which form a carpet of dead leaves, called the litter layer, on the forest floor. Carbon makes up more than half of the mass of the litter layer. In autumn, earthworms, woodlice (pill bugs), and other tiny organisms living in the soil break down the litter layer into smaller parts. The dead plant material becomes part of the soil. In this way, forest soils can form a massive invisible carbon-storage room. More than half of the carbon stored in a forest is actually below our feet, in the soil [2].

Forest Edges Can Take up More Carbon Than Forest Interiors

Not every tree captures the same amount of carbon. We found that forest edges store more carbon than forest interiors! The forest edge is the outer area of the forest, the part that is close to surrounding grasslands, agricultural fields, or roads. The forest interior is the inner part of the forest, far away from the edges. Forest edges are champions of carbon storage. First, forest edges store more carbon in their wood and leaves than forest interiors do. Second, in the litter layer, it is a tight race between forest edges and interiors. In cold, northern Europe, the litter layer in forest edges has a higher carbon stock than the litter in the interior. In warm, southern Europe, the litter layer in the forest interiors is the winner. Whether edges or interiors store more carbon in the litter layer thus depends on the climate. Third, the forest edges and the interiors store about the same amounts of carbon in the forest soil—a shared first place! Overall, if we sum up the three ways in which forests store carbon, forest edges are the carbon champions (Figure 2) [3].

There are several reasons that forest edges have higher carbon stocks—light, temperature, and nutrients. Imagine a tree growing in the forest interior, surrounded by many large trees where it is cool, humid, and dark. In the forest edge, at least part of each tree is not surrounded by other trees, since forest edges border open spaces such as streets or agricultural fields. Edge trees thus receive more sunlight and warmth. Edges also receive more nutrients, like nitrogen, from outside the forest. Read more in this Young Minds article. More nitrogen enters the forest edge than the forest interior (Figure 2). Hence, in forest edges, conditions are very good for trees to grow quickly and produce lots of wood and leaves, in which they store carbon. Sometimes, when part of a forest is cut, a new edge is created. Trees that were previously growing in the shady forest interior end up at the forest edge. These trees start to take up more carbon because they receive more nutrients and sunlight than they did before.

Forest Edges Worldwide Are Increasing Due to Fragmentation

It is important to know how forest edges differ from the forest interior because the total area of forest edges is increasing worldwide. Forests are cut when humans need land for agriculture, roads, or towns. As a result, large forests are split up in multiple smaller forest patches, with other land uses in between. This process is called forest fragmentation. Due to forest fragmentation, the proportion of forest land that is close to a forest edge increases (Figure 3).

In the European Union (EU), 40% of all forests lie close to an edge [4]. Hence, there are 4 million km² of forest edge in the EU, which is equivalent to 580 million football (soccer) fields. The total length of forest edges in the EU equals 100 times the distance from the Earth to the moon. Impressive numbers, right? Yet, forest edges are currently ignored when the carbon storage of forests is estimated. The carbon storage of forests is thus underestimated, because forest edges can store more carbon than forest interiors. If we take the edge effects into account, the extra carbon stored in forest edges in the EU is the equivalent of a forest area of more than 1.4 million football (soccer) fields, or one third of the size of Belgium.

Does this mean that fragmentation is a good thing, because it creates more edges that capture a lot of carbon? No, definitely not! Our study does show that it is important to protect even very small forest patches. Small forest patches are often carelessly removed, because people think they do not make a big difference—but they do, thanks to their large proportion of edge areas. Therefore, we should both protect small forests and plant new forests, even very small ones. In the fight against climate change, small forest patches should not be neglected!

Glossary

Greenhouse Effect: ↑ A process that occurs when greenhouse gases in the Earth’s atmosphere trap the energy and heat from the sun, like a blanket around the Earth.

Greenhouse Gases: ↑ Gases in the atmosphere that can trap the energy and heat from the sun at the Earth’s surface. Carbon dioxide (CO₂) is an example of an important greenhouse gas.

Litter Layer: ↑ The layer of dead leaves on the forest floor.

Forest Edge: ↑ The outer area of the forest, close to surrounding grasslands, fields, or roads.

Forest Fragmentation: ↑ Splitting up large forests into multiple smaller forests. Fragmentation happens when humans cut forests to create space for agriculture, roads, or towns.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This study was supported by H2020 European Research Council, Grant/Award Number: 757833.