Natural processes that release large amounts of CO₂ include respiration, decomposition, ocean outgassing, wildfires, and volcanic activity, among others.

Key natural CO₂ sources (besides fossil fuels)

1. Respiration by plants, animals, and microbes

All aerobic organisms “burn” organic molecules with oxygen to release energy, producing CO₂ as a waste product.

  • This includes:
    • Animals (including humans) breathing.
    • Plants at night (and even in the day, though photosynthesis usually outweighs it).
    • Microorganisms in water and soil.
  • Globally, respiration from plants and animals emits on the order of hundreds of billions of tonnes of CO₂ per year, making it one of the largest natural fluxes in the carbon cycle.

You can think of respiration as nature’s internal combustion: instead of engines burning gasoline, cells burn sugars and fats.

2. Decomposition and soil respiration

When dead plants, animals, and organic matter break down, decomposer microbes respire and release CO₂.

  • Key processes:
    • Decay of leaf litter and dead wood in forests.
    • Breakdown of organic matter in grasslands, wetlands, and agricultural soils.
    • Soil microbes constantly consuming organic carbon and emitting CO₂.
  • Soil respiration and decomposition are another major natural CO₂ source, comparable in scale to biological respiration.

3. Ocean–atmosphere exchange

The ocean both absorbs and releases CO₂, depending on local temperature, chemistry, and circulation.

  • In warmer regions, CO₂ tends to outgas from surface waters to the atmosphere.
  • In cooler regions or areas with strong biological uptake (lots of photosynthesis), oceans often act as a sink.
  • On a global average, ocean–atmosphere exchange accounts for a large fraction of natural CO₂ emissions and absorptions; it is one of the dominant natural fluxes in the carbon cycle.

A simple analogy is a carbonated drink: when it warms or is stirred, it releases dissolved gas into the air.

4. Natural wildfires

Even without human ignition, lightning-caused fires in forests, grasslands, and savannas burn vegetation and release CO₂.

  • Burning biomass rapidly converts organic carbon in plants and surface litter to CO₂, water vapor, and other gases.
  • Some carbon remains as charcoal or ash and can be stored in soils for long periods, but a substantial portion goes straight to the atmosphere as CO₂.
  • Natural fire regimes (e.g., periodic savanna fires) have been part of Earth’s carbon cycle for millions of years.

5. Volcanic eruptions and degassing

Volcanoes and related geological processes emit CO₂ stored in Earth’s interior.

  • Sources include:
    • Explosive volcanic eruptions.
    • Continuous degassing from volcanic vents, fumaroles, and geothermal areas.
    • CO₂-rich fluids emerging through hot springs.
  • Compared with other natural sources, volcanic CO₂ is relatively small : estimates place it at a tiny fraction of total natural emissions each year.

6. Weathering of carbonate rocks and geologic processes

The chemical weathering and dissolution of carbonate rocks (like limestone) can release CO₂ in some contexts.

  • When acidic water reacts with carbonate minerals, it can free CO₂ that may eventually reach the atmosphere.
  • Over geologic timescales, metamorphism of carbonate rocks and degassing along tectonic plate boundaries contribute additional CO₂.
  • These fluxes are generally slow but important for the long-term carbon cycle.

7. Natural emissions from ecosystems and animals

Several specific ecosystem and biological processes act as natural CO₂ sources:

  • Ruminant animals (e.g., wild cattle, deer) exhale CO₂ via respiration; in addition, they emit methane, which later oxidizes to CO₂ in the atmosphere.
  • Peatland and wetland decay where organic matter decomposes, sometimes emitting CO₂ (and methane that later becomes CO₂).
  • Natural disturbance events like insect outbreaks or storms that kill trees can indirectly increase CO₂ as dead biomass decomposes.

8. Balance with natural sinks

Although natural processes emit huge amounts of CO₂, they are largely balanced by natural sinks such as photosynthesis on land and in the ocean and long‑term storage in soils and sediments.

  • Before large‑scale human industrial activity, this balance kept atmospheric CO₂ relatively stable for thousands of years.
  • Human activities (especially fossil fuel combustion and deforestation) now add extra CO₂ on top of this natural cycling, upsetting the previous equilibrium.

TL;DR

Besides fossil fuel combustion and other human activities, the main natural processes that produce large quantities of CO₂ are:

  1. Respiration by plants, animals, and microbes.
  1. Decomposition and soil respiration.
  1. Ocean outgassing (ocean–atmosphere exchange).
  1. Natural wildfires.
  1. Volcanic eruptions and continuous degassing.
  1. Weathering and geologic processing of carbonate rocks.

Information gathered from public forums or data available on the internet and portrayed here.