Carbon Cycle and Greenhouse Gases

How carbon moves among the atmosphere, ocean, land, and rocks, and how human emissions of carbon dioxide and other gases accumulate to drive warming.

Definition

The carbon cycle is the set of exchanges that move carbon among the atmosphere, ocean, land biosphere, soils, and rocks, and greenhouse gases are the atmospheric constituents, chiefly carbon dioxide, methane, and nitrous oxide, whose concentrations this cycle regulates.

Scope

This topic covers the global carbon cycle and the budget of greenhouse gases. It treats the natural exchanges of carbon among the atmosphere, ocean, and land biosphere on timescales from years to millennia, the geological cycle of weathering and volcanism, the uptake of human emissions by ocean and land sinks, and the resulting rise in atmospheric carbon dioxide, methane, and nitrous oxide, along with the concept of a remaining carbon budget for limiting warming.

Core questions

• How does carbon move among the atmosphere, ocean, land, and rocks?
• What fraction of human emissions stays in the atmosphere?
• How do ocean and land carbon sinks respond to warming?
• How much carbon can still be emitted within a given warming limit?

Key theories

Airborne fraction and carbon sinks

Only about half of human carbon emissions remain in the atmosphere because the ocean and land biosphere absorb the rest, and how these sinks evolve under warming governs future concentrations.

Carbon-cycle feedbacks

Warming can weaken carbon uptake and release carbon from soils, permafrost, and vegetation, a feedback that leaves more emitted carbon in the atmosphere and amplifies warming.

Mechanisms

Carbon cycles rapidly between the atmosphere, surface ocean, and vegetation through photosynthesis, respiration, and air-sea exchange, and slowly through ocean mixing, sedimentation, weathering, and volcanism. Human emissions add carbon faster than the slow geological removal can act, so concentrations rise; the ocean and land currently absorb roughly half, but warming and changing conditions can weaken these sinks, increasing the airborne fraction.

Clinical relevance

Because cumulative carbon emissions largely determine peak warming, understanding the carbon cycle defines the remaining carbon budget consistent with temperature targets and the role of natural and engineered carbon removal.

Evidence & guidelines

The IPCC Sixth Assessment Report finds a near-linear relationship between cumulative carbon emissions and global warming and quantifies the remaining carbon budgets for limiting warming to specified levels.

History

Keeling's continuous measurements begun in 1958 first revealed the steady rise of atmospheric carbon dioxide, and subsequent decades of ocean and land studies, synthesized in the annual global carbon budget, quantified the sinks and established the link between cumulative emissions and warming.

Debates

Future strength of land and ocean carbon sinks

How long the natural sinks will keep absorbing about half of emissions, and whether warming will sharply weaken them, is a major uncertainty in projecting future concentrations.

Key figures

• Charles David Keeling
• Pierre Friedlingstein
• Wallace Broecker
• Corinne Le Quere

Seminal works

• keeling1960
• friedlingstein2022

Frequently asked questions

Where does the carbon dioxide we emit go?

Roughly half stays in the atmosphere, while the ocean and land vegetation each absorb about a quarter, though these sinks may weaken as the climate warms.

What is a carbon budget?

It is the total amount of carbon dioxide that can still be emitted while keeping warming below a chosen limit, based on the near-linear link between cumulative emissions and temperature.