The comfortable temperature on earth is due to the unique combination of the distance to the sun and the presence of our atmosphere. As a result, the earth absorbs just the right amount of solar energy we need for life as we know it. And the atmosphere acts as an insulating blanket, keeping the temperature at a comfortable level. The blanket here is a collection of gases called ‘greenhouse gases’. Carbon dioxide (CO₂), water vapour, methane, nitrous oxide and ozone are the most important ones. Without the atmosphere containing greenhouse gases, the temperature on earth would only be -18 °C on average, because most of the heat generated by the sun would be reflected back from earth into space.

How do greenhouse gases retain heat?
Let’s zoom in on this way of warming, which is an interplay between radiation and molecules. Part of the (visible) sunlight radiated out of space reaches the earth’s surface, where the (short-wave) sunlight is absorbed by land, oceans and plants and converted into heat. The earth emits this heat in the form of (long-wave) infrared radiation. For the hardcore physicists among us: the earth is behaving here like a black body, which at a temperature of ‘around ambient temperature’ has an optimum in emission around 10 μm, in the infrared region. Without an atmosphere that contains gases that can absorb this infrared radiation, this heat would go straight into space. However, the greenhouse gases in the atmosphere – such as CO₂ – have the ability to retain heat. This is because these gases consist of molecules that can absorb radiation (energy) at wavelengths exactly within the infrared range. After absorbing infrared radiation, CO₂ molecules vibrate and emit the same infrared radiation in any direction to make a neighbouring molecule vibrate. This goes from molecule to molecule, with part of the heat returning to the earth’s surface and thus causing ‘global warming’. Another part of the heat radiation eventually disappears into space. The atmosphere with greenhouse gases in it thus acts as an insulating blanket that keeps part of the heat on the earth. Moreover: the more greenhouse gases in the atmosphere, the more heat remains ‘stuck’ on earth.

Together, greenhouse gases make up only about half a percent by volume of the atmosphere. Of all greenhouse gases, water vapour (on average 0.48 % by volume) is by far the largest contributor to the greenhouse effect. Carbon dioxide (0.041 % by volume) is a good runner-up. Because the atmosphere retains heat and does not radiate it directly into space, the average temperature on the earth’s surface is 33 °C warmer than if there were no greenhouse gases. A small amount of greenhouse gases therefore has a major influence on the temperature.

CO₂ and the carbon cycle
Carbon, the chemical element represented by the letter C, is one of the most important building blocks for life on earth. It is found in animals and plants, but also in algae in the ocean. However, the largest amount of carbon, over 99 %, is stored in the soil as sedimentary rock such as calcium carbonate (limestone, marble). The carbon cycle describes all processes on earth involving carbon: the exchange of carbon between the atmosphere, land and ocean. CO₂ plays an important role in the carbon cycle.
Until just before industrial development, some 200 years ago, the carbon cycle was well balanced. At that time, the atmosphere contains 590 gigatonnes (Gt, 10¹² kg) of carbon in the form of CO₂, which corresponds to a CO₂ concentration of 280 ppm (parts per million). The ocean contains tens of thousands of gigatonnes of C, and the plants and soil (without sedimentary rock) together contain several thousands of gigatonnes of carbon.
During the growing season, plants absorb several tens of gigatonnes of carbon in the form of CO₂ every year, which they convert into carbohydrates (photosynthesis) with the help of sunlight. Most of this is released when the plant dies or ends up in the stomach of humans or animals. The rest ends up in the soil.
The atmosphere also exchanges carbon dioxide with the ocean, because carbon dioxide can dissolve in water. A small part of this is converted into carbonic acid (H₂CO₃), which makes the water slightly acidic. Phytoplankton absorbs carbon through photosynthesis. CO₂ slowly exchanges between the ocean surface layer and the deeper ocean layers.

From industrial development onwards
Since industrial development began some 200 years ago, the concentration of CO₂ in the atmosphere has increased by 45 %: from 280 ppm then to 410 ppm (0.041%) today. The cause? In those 200 years, we have been rapidly releasing carbon from the ground (back) into the atmosphere in the form of CO₂. It took nature millions of years to convert CO₂ from the air into oil, natural gas and coal. The reverse process – combustion into CO₂ and water – happens roughly a million times faster. Fossil fuels such as coal, oil and natural gas are relatively easy to extract from the earth’s crust for direct use. They can be used to power machines and vehicles, to generate electricity, or as raw materials for products. Most of the 45 % increase is due to the burning of these fossil fuels, but also to large-scale deforestation, which allows less CO₂ to be absorbed by photosynthesis.

Although man-made carbon dioxide emissions represent only a few percent of natural streams, they have led to a measurable change in the amount of carbon in the atmosphere, the oceans and on land. In those 200 years some 440 gigatonnes of carbon have been added to the atmosphere in the form of carbon dioxide, about half of which have been captured by the oceans and the trees and plants on land. To get an idea of how much extra carbon dioxide is now in the atmosphere since the industrial revolution: suppose you can cleverly convert these extra 440 gigatonnes of carbon into diamond – another form of carbon – then you end up with a high ‘mountain’ measuring 5 km x 5 km x 5 km.

The impact on climate
Higher CO₂ concentrations affect the processes that take place in the atmosphere, and therefore the climate. More CO₂ retains more heat, resulting in higher temperatures. We are already noticing this because the average temperatures over the last few decades are among the highest since measurements began. And this heat has two extremes: on the one hand, it is obvious that extra heat in dry places causes even more heat and drought. On the other hand, more water evaporates and warm air can contain more water (vapour), which means that much more rain can fall in humid areas.
But heat also affects seawater, because water expands at higher temperatures. In addition, more land ice melts at higher temperatures. Both phenomena will result in a higher seawater level, and therefore possible flooding of low-lying areas of land. More heat also supplies more energy to the oceans and the atmosphere, which can discharge in the form of more violent storms.

Solutions for CO₂ reduction
There is currently not one ‘best’ solution that can reduce CO₂ on its own. So the strategy is: try them all. Should it turn out in the future that one of these technologies does not live up to expectations, the others will be able to cope. Just as we have artificially increased the concentration of CO₂ in the atmosphere by burning fossil fuels, we can also artificially reduce this concentration. On the one hand, it is a matter of emitting less CO₂ by using less energy or by switching to a non-fossil energy supply such as solar energy or nuclear energy, and on the other hand, of reducing the ‘mountain of CO₂‘ above our heads. Reducing this amount of CO₂ in the atmosphere from 410 ppm back to 280 ppm poses a major challenge.
Just as large-scale deforestation was one of the causes of the increase in CO₂, large-scale forestry is a way of extracting CO₂ from the air. You can also use energy from the sun to electrochemically convert CO₂ from the air or seawater into usable chemicals and fuels. Just a wild idea: suppose you could convert this CO₂ into graphite, then perhaps you could use it as an electrode in batteries or fuel cells – for sustainable energy storage or conversion. Another type of carbon cycle …