Changes in the Carbon Cycle Over Time: Physical Causes

Introduction

The carbon cycle has never been fixed. Over Earth’s history, it has shifted dramatically, even long before human influence. Carbon constantly moves between the atmosphere, biosphere, oceans and lithosphere, but the magnitude of these stores changes due to natural processes such as climate fluctuations, volcanic activity and wildfire events. These drivers can alter the amount of carbon stored and the rate at which carbon is transferred between stores, influencing the global climate over both short and long timescales.

1. Natural Climate Change

During the Quaternary period (the last 2.6 million years), Earth’s climate has swung between cold glacial periods and warmer interglacial periods. These temperature shifts altered the size and activity of major carbon stores.

Glacial periods (cold conditions)

• Vast ice sheets spread across continents, locking up water and slowing the movement of carbon into the oceans.
• Lower global temperatures reduced photosynthesis and respiration.
• Decomposition became much slower due to frozen or waterlogged soils, meaning more carbon was stored in soil and peat.
• Carbon stored in vegetation was lower because forests and grasslands contracted.

Interglacial periods (warm conditions)

• Melting ice released carbon-rich meltwater into the oceans.
• Higher temperatures increased photosynthesis, respiration and decomposition, returning more CO₂ to the atmosphere.
• Forests expanded and vegetation biomass increased, acting as a larger biospheric store.

Key idea: Over hundreds of thousands of years, temperature and atmospheric CO₂ rise and fall together. Warmer conditions lead to higher CO₂ concentrations due to faster biological activity and ocean outgassing, creating a positive feedback loop.

2. Wildfires

Wildfires can be natural (lightning strikes) or caused by humans. Either way, they have a significant impact on carbon transfers.

How do wildfires affect carbon?

• Stored carbon in vegetation is released rapidly to the atmosphere as CO₂.
• If soils burn, stored carbon in humus or peat can also be released.
• Wildfires can turn forests from a carbon sink into a temporary carbon source.

After the fire

• Regrowth can slowly rebuild carbon stores through photosynthesis.
• Fire-adapted ecosystems (e.g. savannas) can return to being net sinks within years.
• More intense or frequent fires, often linked to drought and climate change, release more carbon and slow recovery.

Example:
In 2019, severe dry-season fires in Indonesia released over 700 million metric tons of CO₂ into the atmosphere. Much of this came from burning peat soils, showing how fire can rapidly transfer long-term carbon stores into the atmosphere.

3. Volcanic Activity

Volcanoes release carbon stored in rocks deep within the Earth’s crust.

Key processes

• Magma contains dissolved CO₂ from the mantle. When volcanoes erupt, this carbon is emitted into the atmosphere.
• Lava flows contain minerals that slowly react with atmospheric CO₂ through weathering, locking carbon away again over long timescales.
• Although volcanic CO₂ emissions are much smaller each year than those from human activity, over millions of years, they play a key role in maintaining the natural carbon balance.

Example:
The 1991 eruption of Mount Pinatubo injected ash and sulphate aerosols into the stratosphere, temporarily cooling the planet by about 0.5°C. Although volcanic gases add CO₂, the short-term cooling effect from ash and aerosols can offset this for a few years.

Volcanoes vs human activity:
Volcanoes emit an estimated 130–380 million tonnes of CO₂ per year, compared with over 35 billion tonnes from human activity today.

The Overall Picture: Physical Causes

Natural processes can either increase or reduce atmospheric CO₂:

• Cold periods → larger soil and ocean stores, lower atmospheric CO₂
• Warm periods → faster decomposition and ocean outgassing, higher atmospheric CO₂
• Wildfires → rapid release of carbon
• Volcanoes → return carbon from rock stores to the atmosphere
• Even without human intervention, the carbon cycle is constantly changing.