2.6 The Carbon Cycle and Earth's Atmosphere

The carbon cycle moves carbon between the atmosphere, oceans, land, and living organisms. It's one of the main systems that regulates Earth's climate by controlling how much $CO_2$ ends up in the atmosphere. Human activities have disrupted this cycle, pushing atmospheric $CO_2$ to levels not seen in hundreds of thousands of years.

The Carbon Cycle

more resources to help you study

practice questions

Overview of the Carbon Cycle

The carbon cycle is the biogeochemical cycle that exchanges carbon among four major parts of the Earth system: the atmosphere, oceans, land (including rocks and soil), and living organisms. Carbon constantly moves between these parts through both fast processes (like breathing) and slow ones (like rock formation).

Carbon dioxide ($CO_2$) is a greenhouse gas, meaning it traps heat in the atmosphere. When $CO_2$ concentrations go up, more heat gets trapped, and global temperatures rise.

Here's how carbon moves through the cycle:

• Photosynthesis pulls $CO_2$ out of the atmosphere. Plants, algae, and other autotrophs convert it into organic carbon stored in their biomass.
• Cellular respiration and decomposition release that stored carbon back into the atmosphere as $CO_2$.
• Oceans absorb large amounts of $CO_2$ from the atmosphere, acting as a carbon sink. However, as ocean temperatures rise, their capacity to absorb $CO_2$ decreases.
• Volcanic eruptions release $CO_2$ from Earth's interior into the atmosphere.
• Weathering of silicate rocks removes $CO_2$ from the atmosphere, but this process operates over very long timescales (millions of years).

Regulating Earth's Climate

The balance between carbon sources (processes that release carbon) and carbon sinks (processes that absorb carbon) is what keeps Earth's climate relatively stable over long timescales. When that balance gets disrupted, global temperatures shift.

Atmospheric $CO_2$ has risen from about 280 ppm (parts per million) in pre-industrial times to over 420 ppm today, driven primarily by human activities. This increase strengthens the greenhouse effect, trapping more heat and raising global temperatures.

Positive feedback loops can make things worse. For example, as warming temperatures thaw permafrost, the organic carbon frozen inside it gets released as $CO_2$ and methane ($CH_4$). That extra greenhouse gas causes more warming, which thaws more permafrost, and the cycle accelerates.

Carbon Reservoirs and Fluxes

Major Carbon Reservoirs

A reservoir is anywhere carbon is stored for a period of time. The four major reservoirs are:

• Deep ocean: The largest carbon reservoir on Earth. Carbon is stored here as dissolved inorganic carbon and as sinking organic matter (sometimes called marine snow).
• Soil and permafrost: Significant reservoirs of organic carbon on land. Permafrost alone holds an estimated 1,500 billion metric tons of carbon, much of which could be released as it thaws.
• Fossil fuels: Coal, oil, and natural gas formed from the remains of ancient organisms buried and compressed over millions of years. This carbon was locked away underground until humans began extracting and burning it.
• Atmosphere and living organisms: The atmosphere holds carbon as $CO_2$ and $CH_4$, while living organisms store carbon in their tissues (biomass).

Carbon Fluxes

A flux is the movement of carbon from one reservoir to another. The major natural fluxes are:

• Photosynthesis: Removes $CO_2$ from the atmosphere and converts it to organic carbon in biomass.
• Respiration and decomposition: Return carbon to the atmosphere as $CO_2$.
• Ocean-atmosphere exchange: The ocean surface both absorbs and releases $CO_2$, depending on temperature, wind, and water chemistry.
• Weathering: Chemical reactions between $CO_2$, water, and silicate rocks slowly pull carbon out of the atmosphere and eventually deposit it in ocean sediments.
• Volcanic emissions: Release $CO_2$ stored in Earth's interior back into the atmosphere.

Anthropogenic (human-caused) fluxes have significantly altered this natural balance. Fossil fuel combustion and land use changes like deforestation now add carbon to the atmosphere far faster than natural sinks can remove it.

Human Impact on the Carbon Cycle

Fossil Fuel Combustion and Deforestation

• Burning fossil fuels releases $CO_2$ that had been locked in Earth's crust for millions of years, adding "new" carbon to the active cycle.
• Deforestation has a double effect: it removes trees that would otherwise absorb $CO_2$ through photosynthesis, and it releases the carbon stored in those trees back into the atmosphere (through burning or decomposition).
• Agriculture contributes too. Livestock farming and rice paddies release methane ($CH_4$), a greenhouse gas roughly 80 times more potent than $CO_2$ over a 20-year period.
• Cement production involves heating limestone ($CaCO_3$), which releases $CO_2$ as a byproduct. The cement industry accounts for about 8% of global $CO_2$ emissions.

Consequences of the Altered Carbon Cycle

Since the start of the Industrial Revolution (around 1750), human activities have driven a sharp increase in atmospheric greenhouse gas concentrations. The jump from ~280 ppm to over 420 ppm of $CO_2$ has enhanced the greenhouse effect and raised average global temperatures by roughly 1.1°C so far.

The consequences extend beyond temperature. Higher $CO_2$ levels lead to ocean acidification as seawater absorbs more $CO_2$ and forms carbonic acid. This threatens marine ecosystems, particularly organisms that build shells or skeletons from calcium carbonate, like corals and shellfish.

Carbon Cycle and Earth's Atmosphere

Interactions between Carbon Cycle and Atmosphere

The carbon cycle directly controls how much $CO_2$ sits in the atmosphere at any given time. When sources and sinks are roughly in balance, atmospheric $CO_2$ stays stable. Right now, they're not in balance: human emissions add about 36 billion metric tons of $CO_2$ per year, and natural sinks can only absorb roughly half of that.

The oceans act as a buffer, absorbing excess atmospheric $CO_2$. But this buffering capacity has limits. As ocean temperatures rise and the water becomes more acidic, the oceans absorb $CO_2$ less efficiently. This could accelerate the buildup of $CO_2$ in the atmosphere over time.

Why Understanding This Relationship Matters

Tracking atmospheric $CO_2$ concentrations (measured continuously at stations like Mauna Loa Observatory in Hawaii since 1958) helps scientists monitor how the carbon cycle is changing. By studying how carbon moves between reservoirs, researchers can build better climate models and predict future warming.

This understanding directly informs policy decisions about reducing greenhouse gas emissions, protecting carbon sinks like forests and oceans, and developing strategies to limit the worst impacts of climate change.