Respiration is the process that returns carbon to the atmosphere. Every living organism, from bacteria in the soil to trees in a forest to you reading this, breaks down carbon-based molecules for energy and releases carbon dioxide (CO₂) as a byproduct. If photosynthesis is the carbon cycle’s inhale, pulling CO₂ out of the air and locking it into organic matter, respiration is the exhale, sending it back. Together, these two processes move enormous quantities of carbon between the atmosphere, land, and oceans every year.
How Respiration Releases Carbon
At the molecular level, respiration is the reverse of photosynthesis. Cells take in glucose (a sugar built from carbon, hydrogen, and oxygen) along with oxygen and break the glucose apart. The carbon atoms that were locked inside the sugar molecule combine with oxygen to form CO₂, which is released as waste. Water and usable energy are the other products. Every one of the six carbon atoms in a single glucose molecule ends up in a separate CO₂ molecule by the time the process is complete.
This happens inside nearly every living cell on Earth. The energy released powers everything from muscle contractions to root growth to bacterial reproduction. The CO₂, meanwhile, diffuses out of the cell and eventually reaches the atmosphere or dissolves into water. That carbon is now available to be captured again by a photosynthesizing organism, completing one turn of the cycle.
Plant Respiration: The Biggest Surprise
People often think of plants as pure carbon absorbers, but plants respire too. Every leaf, root, and stem cell burns sugars for energy around the clock, releasing CO₂ in the process. Photosynthesis only runs during daylight hours when energy from the sun is available. Respiration never stops.
The numbers are striking: plants release back 40 to 60 percent of all the carbon they fix through photosynthesis. That means roughly half the CO₂ a forest pulls from the air goes right back out through the trees’ own respiration. This “autotrophic” respiration (respiration by organisms that make their own food) represents about half of the total annual CO₂ input to the atmosphere from terrestrial ecosystems. The carbon a plant keeps after respiration is what actually builds wood, leaves, and roots, the material we think of as plant growth.
Soil Microbes and Decomposition
The other major category is “heterotrophic” respiration, carried out by organisms that consume organic matter rather than making it. Soil microbes are the heavyweights here. Bacteria and fungi break down dead leaves, roots, animal waste, and other organic debris, using the carbon compounds for energy and exhaling CO₂ in return. This is why a compost pile heats up and shrinks over time: microbes are actively respiring, converting solid carbon into gas.
Global soil respiration (plant roots plus microbes combined) releases an estimated 93 billion metric tons of carbon per year. To put that in perspective, human fossil fuel emissions are roughly 10 billion metric tons annually. The difference is that soil respiration is part of a balanced loop: the carbon being released was recently pulled from the atmosphere by plants. Fossil fuel burning adds carbon that has been locked underground for millions of years, which is why it shifts the balance.
Microbial respiration can restart remarkably fast. When dry soil is rewetted, microbes resume high rates of CO₂ release within minutes. Even enzymes left behind by dead microbial cells can continue breaking down organic matter and releasing CO₂ outside of living cells, contributing roughly as much CO₂ as the living microbes themselves in the first 24 hours after rewetting.
Respiration in the Ocean
Marine respiration follows the same basic chemistry but operates in water. Tiny organisms near the ocean surface, including bacteria, single-celled algae, and zooplankton, respire carbon that was originally captured by marine photosynthesizers (phytoplankton). Surface microbial respiration rates in productive ocean regions like the North Atlantic run about 40 percent higher than in less productive waters like the North Pacific. Below the sunlit zone, respiration rates drop by roughly threefold because there is less organic material sinking down to fuel it.
This matters for the carbon cycle because CO₂ produced by respiration deep in the ocean can stay dissolved for centuries before ocean circulation brings it back to the surface. Respiration near the surface, on the other hand, can release CO₂ back to the atmosphere relatively quickly.
Anaerobic Respiration and Methane
Not all respiration uses oxygen. In waterlogged soils, wetlands, and oxygen-poor sediments, microbes use alternative chemical pathways to break down carbon. This anaerobic respiration still releases CO₂, and in some environments it actually produces more CO₂ than aerobic respiration does. But it can also generate methane (CH₄), a greenhouse gas roughly 80 times more potent than CO₂ over a 20-year period.
Methane production tends to occur only under fully water-saturated conditions. The quantities are much smaller than CO₂ output, but methane’s outsized warming effect makes anaerobic respiration in wetlands, rice paddies, and thawing permafrost a significant factor in climate science.
Why Temperature Changes the Equation
Respiration is highly sensitive to temperature. Scientists use a metric called Q10 to express this: it measures how much respiration speeds up for every 10°C rise in temperature. Most climate models use a Q10 value between 1.5 and 2.0, meaning a 10°C warming would increase respiration rates by 50 to 100 percent. Measured values range from 1.2 to 2.8 depending on soil conditions, moisture, and the type of organic matter available.
This creates a feedback loop. As global temperatures rise, soil microbes respire faster, releasing more CO₂, which drives further warming. One of the most concerning examples is permafrost, permanently frozen ground in Arctic regions that stores vast amounts of ancient organic carbon. As it thaws, microbes gain access to material that has been locked away for thousands of years and begin respiring it. Climate simulations project that thawing permafrost could release an average of 0.3 to 0.7 billion metric tons of carbon per year depending on how much warming occurs, with about 75 percent of that reaching the atmosphere as CO₂ by the year 2300. At its peak, permafrost emissions could temporarily reach about half of current annual fossil fuel emissions.
Respiration at the Human Scale
Your own body is part of the carbon cycle too. An average adult doing office work produces about 0.005 liters of CO₂ per second, which works out to roughly 200 liters (about 400 grams) of CO₂ per day. That carbon came from the food you ate, which traces back to plants that pulled it from the atmosphere. Unlike fossil fuel emissions, human breathing is carbon-neutral: you are returning recently captured carbon, not adding new carbon to the system.
This is the key distinction in understanding respiration’s role in the carbon cycle. Natural respiration, whether from forests, soils, ocean microbes, or your own lungs, recycles carbon that is already part of the active cycle. The system stays roughly in balance as long as the total amount of carbon in circulation does not change. Problems arise when carbon stored outside that active cycle (in fossil fuels, permafrost, or deep ocean sediments) gets added to it faster than natural processes can absorb it.