Tropical rainforests are often called the “lungs of the Earth,” a reputation stemming from the enormous amount of oxygen produced by their abundant vegetation. Many people believe the sheer size and productivity of the Amazon and other tropical forests suggest they are responsible for the majority of the air we breathe. This common understanding drives a fundamental question: exactly how much oxygen does the rainforest produce for the global atmosphere?
Addressing the Popular Claim
The popular belief that the Amazon rainforest produces 20% or more of the world’s atmospheric oxygen is a misconception. The scientifically accurate answer regarding the rainforest’s net contribution to the global oxygen supply is surprisingly low. A mature, stable tropical rainforest, despite its incredible production capacity, adds very little new oxygen to the air we breathe because it is in a state of balance.
The confusion arises from conflating the forest’s massive Gross Primary Production (GPP) with its Net Ecosystem Exchange. GPP is the total oxygen produced by plants through photosynthesis, making the rainforest one of the most photosynthetically active environments on the planet. However, in a mature forest, the oxygen consumed by the ecosystem nearly balances the oxygen produced, resulting in a net atmospheric contribution close to zero.
Understanding the Carbon Cycle in Mature Ecosystems
The neutral balance is explained by the ecosystem’s internal carbon and oxygen cycles. Plants perform photosynthesis, generating oxygen as a byproduct while converting carbon dioxide into energy. Simultaneously, these plants and all other living organisms consume oxygen through cellular respiration to fuel their maintenance, releasing carbon dioxide back into the air.
The second significant draw on oxygen comes from decomposition. In a mature forest, organic matter constantly dies and falls to the forest floor. Microbes, fungi, and insects break down this dead material, a process that requires substantial oxygen and releases carbon dioxide. Since the rainforest environment is warm and wet, this decomposition happens quickly, consuming most of the oxygen that was produced when the material was first created.
In a climax, or fully grown, forest, new growth that permanently stores carbon is offset by the respiration from the immense biomass and the decay of older organic material. This continuous cycle of production and consumption means the Net Ecosystem Exchange—the net flow of oxygen—hovers near equilibrium. The system is essentially recycling the oxygen it creates, maintaining a stable internal balance.
Earth’s Primary Oxygen Producers
The majority of the oxygen that makes up the air we breathe is produced by tiny, single-celled marine organisms known as phytoplankton. These microscopic, plant-like organisms float near the ocean surface and perform photosynthesis.
Scientists estimate that marine photosynthesizers, including algae and cyanobacteria, are responsible for between 50% and 80% of the oxygen production on Earth. For example, a single species of cyanobacteria, Prochlorococcus, is thought to generate up to 20% of the oxygen in the biosphere. Unlike the rainforest, the ocean provides a mechanism that allows for a long-term net oxygen gain.
When these oceanic organisms die, a significant portion of their carbon-rich organic matter sinks to the deep ocean floor. This sinking prevents the material from being immediately decomposed by oxygen-consuming microbes. By burying the carbon, the oxygen released during the initial photosynthesis remains in the atmosphere, resulting in a net atmospheric surplus over geologic time.
The Essential Global Roles of Rainforests
The finding that rainforests contribute little net oxygen should not diminish their importance to global ecology and human civilization. Their primary roles center on regulating the Earth’s climate and maintaining biological diversity.
The dense vegetation and extensive root systems of tropical forests act as immense carbon sinks. These forests hold billions of tons of carbon within their living biomass and soils, preventing this amount from being released into the atmosphere as carbon dioxide. Protecting this stored carbon is a primary strategy for mitigating global climate change.
The rainforests also play a fundamental part in regulating global weather patterns through the water cycle. Through transpiration, rainforest trees release enormous quantities of water vapor into the atmosphere. This moisture contributes to the formation of clouds and rainfall, influencing precipitation patterns thousands of miles away.
Finally, tropical rainforests are unparalleled centers of biodiversity, housing an estimated majority of the world’s plant and animal species. This genetic library represents a reservoir of unique life forms and potential resources for medicine and agriculture.