The biosphere, encompassing all life on Earth, and the atmosphere, the enveloping layer of gases, are often viewed as separate environmental components. This perspective is incomplete, as the two represent a single, deeply interconnected system where biological processes fundamentally shape the air’s chemical composition and physical properties. Life actively engineers the atmosphere, influencing everything from global climate regulation to the formation of clouds and precipitation. The exchange of matter and energy between living organisms and the air drives Earth’s life-support systems in a co-evolving relationship.
The Exchange of Major Atmospheric Gases
The most profound interaction between life and air occurs through the biogeochemical cycles that regulate the planet’s bulk atmospheric gases. This chemical exchange is dominated by the simultaneous movements of carbon and oxygen. Photosynthesis, primarily carried out by plants and phytoplankton, extracts carbon dioxide (\(\text{CO}_2\)) from the air and releases oxygen (\(\text{O}_2\)) as a byproduct, building organic compounds that form the basis of all life.
In contrast, respiration and decomposition complete the loop by consuming \(\text{O}_2\) and returning \(\text{CO}_2\) to the atmosphere through metabolic processes. Terrestrial ecosystems alone exchange an estimated 120 gigatons of carbon annually with the atmosphere, demonstrating biological control over the two most abundant reactive gases. The balance between these two biological processes determines the net global carbon sink, which directly impacts the concentration of atmospheric \(\text{CO}_2\), a major greenhouse gas.
Beyond the carbon-oxygen cycle, the biosphere manages atmospheric nitrogen (\(\text{N}_2\)), which makes up nearly 78% of the air. This inert gas must be converted into reactive forms like ammonium or nitrate before it can be used by most organisms, a process known as nitrogen fixation performed by certain soil and aquatic microbes. Other specialized microbial communities, through a process called denitrification, complete the cycle by converting nitrate compounds back into gaseous \(\text{N}_2\), returning it to the atmosphere.
Microbial activity also results in the release of trace gases that have disproportionate atmospheric effects, such as nitrous oxide (\(\text{N}_2\text{O}\)). This gas, produced as an intermediate during denitrification, is a potent greenhouse gas with a warming potential approximately 300 times greater than \(\text{CO}_2\) over a 100-year period. Similarly, the marine sulfur cycle is biologically driven, with oceanic phytoplankton releasing a volatile compound called dimethyl sulfide (DMS). DMS is the largest natural source of sulfur to the global atmosphere, acting as a precursor to non-sea-salt sulfate particles.
Biological Influence on Earth’s Energy Balance
The biosphere exerts physical control over Earth’s energy balance by managing the exchange of heat and water with the atmosphere. One primary mechanism is evapotranspiration, where plants move water from the soil through their leaves, releasing water vapor into the air. This process absorbs heat energy from the surface as latent heat, which cools the local environment.
The water vapor released through evapotranspiration influences regional humidity and precipitation patterns, particularly over large vegetated areas like rainforests. Deforestation disrupts this biological cooling, leading to warmer surface temperatures and altered rainfall regimes in the surrounding area. The water cycled by plant life, moving it from the surface to the atmospheric boundary layer, is a major component of the regional hydrothermal cycle.
Another significant physical influence is albedo regulation, the measure of a surface’s reflectivity of solar radiation. Different biomes exhibit different albedo values, directly affecting how much solar energy is absorbed and converted into heat. For instance, a dense forest canopy is dark, reflecting only about 8 to 15% of incoming sunlight, whereas a snow-covered field can reflect up to 90%.
The presence of a dark forest canopy in high-latitude, snow-covered regions can lead to a localized warming effect by absorbing more sunlight than the reflective snow or bare ground it covers. Conversely, land use changes, such as the desertification of the Sahel region, caused the surface albedo to increase from a vegetated value of 0.14 to a bare-soil value of 0.35, coinciding with a significant decrease in regional rainfall. The physical structure of vegetation also influences wind patterns through surface roughness, as a forest canopy creates aerodynamic drag that slows wind speeds and affects the turbulent mixing of heat, moisture, and gases.
The Role of Life in Atmospheric Particle Formation
Living organisms contribute significantly to the non-gaseous components of the atmosphere, particularly aerosols and particulate matter, which are essential for cloud formation. Terrestrial plants release Biogenic Volatile Organic Compounds (BVOCs), such as isoprene and terpenes, which are responsible for the distinct scent of forests. These gaseous compounds react with atmospheric oxidants, like ozone, to form less volatile substances.
These reaction products condense, creating Secondary Organic Aerosols (SOA) that exist as tiny airborne particles. Sesquiterpenes, a type of BVOC with a higher molecular weight, are particularly effective at forming these particles, even though they are emitted in smaller quantities than other compounds like isoprene. The efficiency of this process is often amplified when plants are under environmental stress, such as high heat or drought.
These biogenic aerosols, alongside sulfate particles derived from marine DMS, act as Cloud Condensation Nuclei (CCN). CCN are microscopic platforms on which water vapor condenses to form cloud droplets. The concentration and properties of these particles directly govern cloud formation, which in turn influences the amount of solar radiation reflected back into space. Biological activity contributes to the atmospheric load of dust and bioaerosols, with terrestrial microbes being lofted into the atmosphere, often protected within dust particles that act as additional CCN.