Biogeochemical cycles are the complex pathways through which chemical elements move between living organisms and the non-living components of Earth, including the atmosphere, lithosphere, and hydrosphere. These cycles recycle and regulate the supply of elements needed for biological processes, making them fundamental to sustaining life. The global water cycle and the global carbon cycle are two of the most significant of these systems, continuously exchanging matter and energy across the planet. Their constant movement, in various forms and phases, ultimately regulates Earth’s climate and maintains the conditions necessary for all ecosystems.
The Global Movement of Water
The hydrological cycle involves the continuous movement of water ($\text{H}_2\text{O}$) and its phase changes across the globe. Earth’s water is primarily stored in major reservoirs. The oceans hold approximately 97% of the free water, while the remaining fresh water is found in ice caps, glaciers, groundwater, lakes, rivers, and the atmosphere. The cycle begins with solar energy driving evaporation, where liquid water turns into water vapor and rises into the atmosphere from the surfaces of oceans, lakes, and soil.
As the air rises, it cools, causing the water vapor to undergo condensation and form clouds. The water droplets in the clouds eventually return to the Earth’s surface as precipitation, such as rain, snow, or hail. Once on land, this water follows two main paths: it can either become runoff, flowing over the surface into rivers and eventually back to the oceans, or it can seep into the ground through infiltration to replenish soil moisture and groundwater stores. Plants also play a role through transpiration, releasing water vapor from their leaves into the atmosphere.
The Global Movement of Carbon
The carbon cycle describes the movement of carbon through four main reservoirs: the atmosphere, the biosphere, the hydrosphere (oceans), and the lithosphere (rocks and sediments). This cycle operates on both fast timescales, involving rapid exchanges between the atmosphere and living things, and slow timescales, which include geological processes. In the fast cycle, the primary biological mechanism is photosynthesis, where plants, algae, and phytoplankton absorb atmospheric carbon dioxide ($\text{CO}_2$) and convert it into organic matter.
Carbon is returned to the atmosphere through respiration, a process used by nearly all living organisms to release energy by breaking down organic carbon and emitting $\text{CO}_2$. When organisms die, decomposers break down the organic matter, releasing the stored carbon back into the soil or the atmosphere. The oceans serve as a major carbon sink, absorbing atmospheric $\text{CO}_2$ through simple diffusion. Some of this dissolved $\text{CO}_2$ forms carbonic acid. Over the long term, geological processes, such as the chemical weathering of rocks, transport carbon into the oceans, where it can be incorporated into marine sediments and eventually into crustal rocks.
The Fundamental Interplay Between Carbon and Water
The two cycles are linked through various physical and biological mechanisms, where a change in one system influences the other. The availability of water controls the rate of carbon uptake in the terrestrial biosphere, as photosynthesis requires water to proceed. When precipitation is limited, plant growth and the corresponding absorption of atmospheric $\text{CO}_2$ are reduced, limiting the capacity of land ecosystems to act as a carbon sink.
Conversely, the concentration of carbon in the atmosphere influences the water cycle through its effect on global temperature. Carbon dioxide is a greenhouse gas; higher atmospheric concentrations trap more heat, causing the Earth’s surface and ocean temperatures to rise. This warming leads to increased evaporation from water bodies and greater transpiration from plants, transferring more water vapor into the atmosphere. Water vapor itself is a powerful greenhouse gas, and its increased presence creates a feedback loop that further amplifies the warming effect initiated by the carbon cycle.
The oceans act as the largest active reservoir for both water and carbon. The ocean’s ability to absorb atmospheric $\text{CO}_2$ is dependent on temperature; warmer water holds less dissolved gas. Ocean warming driven by increased carbon emissions thus reduces the effectiveness of the ocean as a carbon sink. Ocean circulation patterns, like the thermohaline circulation, are driven by temperature and salinity changes. These currents transport both heat and dissolved carbon throughout the deep ocean.
Human Alteration of Global Cycles
Anthropogenic activities are simultaneously disturbing the natural balance of both the carbon and water cycles, primarily through the combustion of fossil fuels and changes in land use. The burning of coal, oil, and natural gas releases carbon previously sequestered in the lithosphere back into the atmosphere as $\text{CO}_2$. This rapid increase in atmospheric carbon raises global temperatures, which then accelerates the water cycle by increasing evaporation and intensifying precipitation events.
Land-use changes, particularly large-scale deforestation, disrupt the local balance of both cycles. Removing forests diminishes the capacity of the terrestrial biosphere to absorb $\text{CO}_2$ through photosynthesis, reducing the carbon sink. The absence of trees also alters local water dynamics, reducing the amount of water returned to the atmosphere through transpiration and increasing surface runoff. This combined effect can lead to soil erosion, reduced local rainfall, and a higher risk of flooding.