Biogeochemical cycles are the pathways by which chemical elements move through the Earth’s living (bio) and non-living (geo) components. These cycles involve the continuous redistribution of matter, often changing forms and locations, across the atmosphere, oceans, land, and living organisms. The carbon and water cycles represent two fundamental processes that shape the planet’s environment and provide conditions necessary for life. These global cycles move vast quantities of elements and compounds across the Earth system.
Understanding the Carbon Cycle
Carbon, a fundamental building block of life, continuously moves through various reservoirs on Earth. The atmosphere holds carbon primarily as carbon dioxide (CO2), while oceans contain dissolved CO2, carbonates, and bicarbonates. On land, carbon is stored in living organisms, dead organic matter in soils, and in geological formations like fossil fuels.
Photosynthesis is a primary process where plants and other photosynthetic organisms absorb atmospheric carbon dioxide. They convert CO2, along with water and sunlight, into organic compounds like glucose, releasing oxygen as a byproduct. This process transfers carbon from the atmosphere into biomass, forming the base of most food webs.
Organisms release carbon back into the atmosphere through respiration, a process where they break down organic compounds for energy. Both plants and animals perform respiration, releasing carbon dioxide. When organisms die, decomposers such as bacteria and fungi break down their organic matter, releasing carbon back into the soil and atmosphere through decomposition.
Oceans play a role in carbon cycling, absorbing amounts of atmospheric CO2 through direct gas exchange at the surface. This dissolved carbon can then be incorporated into marine organisms or transported by ocean currents. Colder ocean waters absorb more CO2, and deep ocean currents can store carbon for centuries.
Combustion, both natural and human-induced, also releases carbon. Natural events like wildfires release carbon stored in vegetation into the atmosphere as CO2. The burning of fossil fuels—coal, oil, and natural gas—which are stored carbon from ancient organic matter, releases vast quantities of carbon dioxide into the atmosphere. This process has accelerated due to human activities.
Understanding the Water Cycle
Water, another fundamental element for life, moves continuously through a global cycle, transforming between liquid, solid, and gaseous states. The largest reservoir of water on Earth is the oceans, holding about 97% of the planet’s water. Other reservoirs include ice caps and glaciers, groundwater, lakes, rivers, and the atmosphere.
Evaporation is a primary process where liquid water absorbs energy and transforms into water vapor, rising into the atmosphere. This process occurs extensively from ocean surfaces, lakes, and moist soil. As water vapor rises into the cooler upper atmosphere, it undergoes condensation, changing back into liquid water droplets or ice crystals. These aggregate to form clouds.
When these water droplets or ice crystals in clouds become too heavy, they fall back to Earth as precipitation. This can take various forms, including rain, snow, sleet, or hail, replenishing water sources on land and in oceans. Once precipitation reaches the land surface, it can follow several paths.
Some water infiltrates into the ground, becoming soil moisture or recharging groundwater aquifers. Other water flows over the land as runoff, collecting in streams, rivers, and eventually flowing into lakes or oceans. Plants also contribute to the water cycle through transpiration, absorbing water through their roots and releasing water vapor into the atmosphere through small pores on their leaves, linking the biosphere directly to atmospheric moisture.
The Interconnectedness of Carbon and Water
The carbon and water cycles are intertwined, with processes in one cycle directly influencing the other. Water is a component for photosynthesis, the process by which plants convert atmospheric carbon dioxide into organic matter. Without sufficient water, plants cannot take up CO2, limiting carbon transfer from the atmosphere into the terrestrial biosphere.
Plants connect these cycles through transpiration, where they release water vapor into the atmosphere as a byproduct of photosynthesis. This release contributes to atmospheric moisture and cloud formation, while also aiding the plant’s uptake of carbon dioxide. The amount of water available to plants impacts their ability to fix carbon, which in turn influences atmospheric CO2 levels.
Atmospheric water vapor acts as a greenhouse gas, influencing Earth’s temperature. Changes in atmospheric water vapor concentrations, often influenced by temperature shifts caused by carbon cycle imbalances, can amplify warming or cooling trends. This shows how alterations in carbon levels affect the water cycle’s role in climate regulation.
Ocean circulation, a component of the water cycle, distributes carbon. Ocean currents transport dissolved carbon dioxide across the globe, moving it from areas of high absorption to areas of release. Colder deep ocean waters can store large quantities of dissolved CO2, and their movement influences carbon distribution and storage.
Human Influence on These Cycles
Human activities have altered the natural balance of both the carbon and water cycles. For the carbon cycle, the burning of fossil fuels—coal, oil, and natural gas—for energy is the largest contributor to increased atmospheric carbon dioxide. These fuels release carbon that has been stored underground for millions of years, adding an estimated 9 to 10 billion tons of carbon per year to the atmosphere from fossil fuel combustion and cement production.
Deforestation further impacts the carbon cycle by removing trees, which are natural absorbers of atmospheric CO2. When forests are cleared, either through burning or decomposition, the carbon stored in the trees is released back into the atmosphere. This land-use change contributes roughly an additional 1 billion tons of carbon per year to the atmosphere.
Human activities also influence the water cycle. Land-use changes, such as urbanization and agriculture, alter natural water flows. Urban areas, with their extensive impervious surfaces like roads and buildings, reduce the infiltration of rainwater into the ground, increasing surface runoff and altering local evaporation patterns. Agriculture requires irrigation, leading to the depletion of groundwater aquifers and surface water bodies.
Water extraction for drinking, industrial processes, and agriculture can deplete natural reservoirs, affecting water availability. Pollution from industrial discharge, agricultural runoff, and domestic waste contaminates water sources, impacting water quality. These changes disrupt the natural movement and purity of water.
These human-induced alterations to both the carbon and water cycles are directly linked to climate change. Increased atmospheric carbon dioxide and other greenhouse gases, largely from human emissions, trap more heat within the Earth’s atmosphere. This warming leads to altered precipitation patterns, more frequent extreme weather events, the melting of glaciers and ice sheets, and rising sea levels.