Greenhouse effect

3.4 Earth’s natural carbon cycle

Carbon dioxide (CO2) is just one of many chemical forms of carbon on the earth. All of these forms of carbon play an important role in the circle of life on earth. Research about how life on earth began is still in full process, but most of life as we know it is based on the so called cycle of carbon.

Our carbon cycle of life starts about a billion years ago. This is when the first algae and later other plants started to use the energy in sunlight to convert water and CO2 from earth’s atmosphere in food (sugars) to grow. During this proses, that’s called photosynthesis, oxygen is released back into the air. Animals (and even plants) need oxygen to breathe. But each time those animals, including us, exhale we are releasing CO2 back into the atmosphere.

Through food chains, the carbon that is in plants moves to the animals that eat those plants. When those animals are eaten by other animals the carbon moves further up the food-chain. The animals and plants that die will decay, bringing the carbon into the ground. Some is buried and millions of years later will become fossil fuels. When humans burn fossil fuels to power factories, power plants, cars and trucks, most of this stored carbon quickly re-enters the atmosphere as CO2.

Carbon also moves from the atmosphere into the oceans and other bodies of water. The carbon dioxide that dissolves into the ocean is like the fizz in a can of soda when it reacts with water molecules and produces carbonic acid. Part of the carbon will be used by marine life to produce bones, shells and coral. When this marine life dies it settles on the bottom of the oceans to form limestone, one of earth’s natural CO2 sinks. Increasing acidity of the oceans (among other things) interferes with the ability of marine life to extract calcium from the water to build their shells and skeletons. When the ocean’s acidity increases, its capability of absorbing CO2 will therefore decrease. Interesting enough the amount of CO2 that will be deposited into this limestone sinks depends also on the temperature of the water. This is thanks to the prolific limestone-producing organisms such as corals which live mainly in tropical, shallow and warm oceans. 

Additional information about earths carbon cycle

II- Earth’s carbon cycle

Carbon moves through our planets environment over relative short, but also much longer time scales. This carbon cycle takes place in the near-surface environment of the earth. This environment contains approximately 121,000,000 gigatonne of carbon (or 121 million GtC). One gigaton is equivalent to a billion metric tons. To get a better impression, only one gigatonne is already the weight of more than 100 million African elephants! Indeed, these are numbers that are difficult to grasp. But these numbers are important to try to understand the differences between earth’s CO2 emission and the CO2 emission caused by humans and tourism. It is the complex interactions involving carbon, rock, soil, water, air, and all living organisms on earth together that determine the availability of life-sustaining resources on our planet.

The carbon in earth’s environment can be divided into three types based on its availability to the atmosphere. Those three types are:
• Carbon that is locked away in permanent storage and is not available to combine with oxygen to form CO2 in the atmosphere*
• Carbon that is in relatively long-term storage in the land and the oceans**
• And carbon that is already in the atmosphere, mainly as CO2 gas.***.

* About two-thirds of the near-surface carbon on earth occurs in nearly permanent storage in fossil fuels, limestone rocks, or sediments. This carbon was originally in the atmosphere but has gone into storage underground over millions of years. Like the earlier mentioned plants and trees that turned into fossil fuels.

** Most of the remaining one-third is in relatively long-term storage in the ocean and at the surface of the land. In the ocean, this carbon occurs as dissolved CO2 gas, as lime in seashells, and in the organic tissues of small marine creatures (i.e., plankton).
About 2000 GtC of carbon is held on the land, where it occurs primarily in plants, animals, humans, and decaying organic matter.

***For centuries only a small part of the carbon, around 594 Giga tons, less than 1% of all the near-surface carbon on the earth occurred in the form of a gas in the atmosphere. Note: In 2019 this amount has already gone up to more than 873 GtC!
Each year somewhere between 215 and 260 Gt from this carbon moves from the land and oceans into the atmosphere. And a nearly equal amount moves from the atmosphere into temporary storage in the oceans and the land. The difference stays in the air.

When carbon is exposed to the atmosphere, it can combine with oxygen to make CO2. Based on the ratio of the weights of the atoms of carbon and oxygen, 1 ton of carbon would combine with 2.667 tons of oxygen to form 3.667 tons (or 3667 kg) of CO2. Therefore 215 gigatonne of carbon make around 788 gigatonne of CO2 to move freely around. This CO2 cycle has been relatively constant, but there have been times in the past when CO2 levels in the atmosphere have been relatively high. There have also been periods when the amount of CO2 in the atmosphere has been relatively low.

• Seasonal CO2 emissions

The importance of plants in relation to CO2 absorption can be shown by looking at the difference in CO2 emissions which you can witness in each year’s seasonal variations. Carbon dioxide is lower in the northern hemisphere summer when plants use CO2 from the atmosphere for photosynthesis and higher in the winter when little photosynthesis occurs, and respiration releases CO2 back to the atmosphere. This seasonal trend correlates to the northern hemisphere seasons because the majority of the land and plants are in the northern hemisphere. The total CO2 level in the atmosphere drops by about 6 or 7 parts per million (ppm) during the Northern Hemisphere’s growing season and then goes up by about 8 or 9 ppm during autumn and winter when part of the vegetation dies. Notable in these cold months humans also produce more CO2 to keep warm, hence the difference in emission and absorption of CO2. Remember that a difference of 7 ppm equals 7 times 2.13 Gt of Carbon. If we multiply this with 3.667 this means that earths growing season can absorb about 55 gigatonne of CO2 from the atmosphere.

In the southern hemisphere, there is almost no seasonal cycle, and the average CO2 concentration is a little bit lower than the concentration in the northern hemisphere.
The extra CO2 produced in the northern hemisphere takes about a year to mix into the southern hemisphere. The further north you look the more the seasonal oscillation is noticeable.

III – pH value of seawater

The carbon dioxide that it dissolves into the ocean like the fizz in a can of soda reacts with water molecules. This produces carbonic acid and lowers the ocean’s pH. In chemistry, pH stands for the ‘power of hydrogen’. Its scale is used to specify how acidic or basic (or alkaline) a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature (25°C or 77°F), pure water is neither acidic nor basic and has a pH of 7.

Since the start of the Industrial Revolution, the pH of the ocean’s surface waters has dropped from 8.21 to 8.10. This drop in pH is called ocean acidification.
A drop of 0.1 may not seem like a lot, but the pH scale is logarithmic; a 1-unit drop in pH means a tenfold increase in acidity. A change of 0.1 means a roughly 30% increase in acidity. Increasing acidity (among other things) interferes with the ability of marine life to extract calcium from the water to build their shells and skeletons. When the ocean’s pH value decreases, its capability of absorbing CO2 will also decrease.