The planet regulates itself: yes and no.

I. Natural cycles

Many natural cycles help the planet to self-regulate to sustain ecosystems. They are called "biogeochemical cycles".

These cycles are all based on the same principle: the elements, such as water and carbon, circulate continuously in various forms called “flows” between the different reservoirs in the environment capable of storing them. These different reservoirs are called "wells” or “sinks" and are found in:

• the atmosphere;
• the hydrosphere (sea, ocean - anywhere there is water);
  
• the lithosphere (soil); and
  
• the biosphere (living organisms).
  

We’ve all heard of the water cycle: water evaporates from oceans and vegetation, forms clouds, then falls back as rain or snow, feeds streams and vegetation, and then the cycle begins again.

The carbon cycle plays a particularly vital role in climate change. It has continued to function since the Industrial Age began in the 1850s, but the cycle has been disrupted by human activities.

New elements have been released into the natural flows: human emissions. These additional - and unnatural - elements enter the cycle, joining existing, natural flows. As a result, they change natural balances which leads to significant climate change.

II. Understanding CO2 and the natural carbon cycle

As explained earlier, CO2 is one of the main GHGs responsible for global warming. However, CO2 is part of the carbon cycle (C).

The flows in the classic carbon cycle.

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The carbon gigaton (GtC), or one billion tons of the element carbon, is used as a unit to analyze the carbon cycle. Note that it is not the same unit used for GHG emissions, which are in gigatons of CO2 (or CO2 equivalent), or billions of tons of carbon dioxide. In reality, the two units are proportional, since 1kg of CO2 contains 272.7g of carbon, the remainder being due to oxygen atoms.

Physical exchanges: carbon is exchanged between the atmosphere and the oceans.

• CO2 dissolves in water, like sugar in coffee. The lower the water
  temperature, the better the dissolving process. As a result, CO2 uptake
  occurs when ocean currents are cool, for example when they flow from the
  tropics to the poles. Such exchanges account for 70 GtC/year.
• When ocean currents are warm, part of the CO2 in the water is released to the atmosphere.

Biological exchanges: between plant and animal life.

We'll work from the ground up:

• Photosynthesis: soil and vegetation absorb CO2 from the atmosphere. This
  process divides the CO2 molecule into compounds containing carbon (C)
  that integrate the biosphere (plant, soil) and oxygen (O2) that is
  released into the atmosphere.

On the other hand, two types of biological flow release CO2:

• When animals and humans breathe, they inhale oxygen and release CO2.
• Decomposing plants and animals release carbon, which, when associated with oxygen, forms CO2 in the atmosphere.

When the carbon cycle functions correctly, all the flows between the atmosphere and the rest of the planet are balanced. That is an exchange of approximately 190 GtC per year.

Note: there are also flows in the water (50 GtC/year).

• Marine plants - mainly algae and phytoplankton - grow by absorbing CO2
  in the water. They are then eaten by marine animals which absorb their
  carbon content.
• In the other direction, marine animals breathe in oxygen and discharge
  CO2 into the water. Microorganisms also feed on dead seabed plants and
  breath out CO2.

Finally, as we saw earlier, carbon fossilizes to create hydrocarbons, which, when burned, release CO2 into the atmosphere.

How the carbon cycle has been disrupted.

The balance of the carbon cycle has been disrupted since the Industrial Revolution began in 1850.

As we explained earlier, the stock of fossil fuels (coal, gas and oil) was created by the slow decomposition of living organisms over hundreds of millions of years. Those fossil resources are being transformed by human activity. Their use suddenly releases large amounts of CO2 into the atmosphere from carbon that was underground for millions of years.

Human activity has therefore produced an additional flow that is disrupting the carbon cycle.

The Earth’s reaction: threatened by this unnatural flow in the atmosphere, the Earth activates all the natural, physical and biological mechanisms described above. Unfortunately, that’s not enough.

The breakdown of human-generated CO2 flow:

• around 35% is absorbed by the ocean;
• around 15% is absorbed by continental ecosystems (soils and plants); and
  
• around 50% is directly added to the CO2 already present in the atmosphere, and increases the greenhouse effect.

Human activity has a strong impact on the carbon cycle which can no longer regulate excessive emissions.

The total greenhouse gas emissions generated by human activity since 1850 is already more than 2,000 Gt CO2 equivalent. Half of that was emitted since 1980. Not only do human activities disrupt the carbon cycle, but the effect is increasing radically each year, with soaring acceleration over the past three decades.

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It took humans almost 130 years to issue 1,000 Gt CO2 equivalent, then only 30 years to issue the same amount again.

At that rate, humans will emit another 1,000 Gt within the next 20 years.

Another telling figure: 50% of the annual emissions by human activities (over 50Gt CO2 equivalent) takes up residence in the atmosphere as GHGs. The natural regulation of the planet has been disrupted and the atmosphere’s composition has been modified. This has a direct impact on the greenhouse effect, global temperatures, and more generally, the Earth’s climate.