The Earth’s water cycle is a constant circulation of water between the oceans, atmosphere, and land through evaporation, condensation, precipitation, and collection. This process is fundamental to life, but human activities are introducing measurable changes. These actions alter the movement, distribution, and chemical makeup of water on a global scale.
Altering Surface Water and Infiltration
Changes to the planet’s land surface interfere with the water cycle’s ground-level processes. Forests act as natural regulators, with tree canopies intercepting rainfall and root systems enhancing soil permeability. This allows water to seep into the ground, recharging groundwater aquifers. The process of transpiration, where trees release water vapor from their leaves, also contributes moisture to the atmosphere, which later falls as rain.
When forests are removed, these functions are lost. Without the tree canopy, rainwater strikes the ground directly, increasing surface runoff and soil erosion. This rapid runoff can overwhelm rivers, leading to more frequent and intense flooding. The reduction in transpiration means less moisture is returned to the atmosphere, which can lead to drier conditions and altered rainfall patterns in downwind regions.
Urban development replaces permeable soil with impervious surfaces like concrete and asphalt. These roads, rooftops, and parking lots prevent rainwater from infiltrating the ground, increasing the volume and velocity of surface runoff. While natural landscapes absorb much of the rainfall, in highly urbanized areas more than half can become surface runoff. This water is channeled into storm drainage systems that deliver it to local streams and rivers, increasing the risk of urban flooding.
Diverting and Storing Water Resources
Large-scale engineering projects have rerouted and contained the planet’s freshwater resources. The construction of dams creates artificial reservoirs that alter the natural flow of rivers. These structures regulate water release based on human needs for hydroelectric power or agriculture, rather than natural seasonal patterns. This disruption changes the temperature and chemistry of the water downstream.
Reservoirs create large, static bodies of water that increase the surface area exposed to the sun, leading to higher rates of evaporation, especially in arid regions. Globally, it is estimated that about 170 cubic kilometers of water evaporates from reservoirs annually. This amount is more than 7% of the total freshwater consumed by all human activities.
Beyond surface water, humans are mining groundwater from underground aquifers. Agriculture is the largest user, accounting for approximately 70% of global groundwater withdrawals for irrigation. This pumping often occurs faster than the natural recharge process, leading to the depletion of these resources. This over-extraction can cause the land to sink, a phenomenon known as subsidence, which permanently reduces the aquifer’s storage capacity.
Accelerating the Cycle Through Climate Change
Human-induced climate change is intensifying the entire water cycle. Rising global temperatures from the burning of fossil fuels increase the rate of evaporation from oceans, lakes, and soil. A warmer atmosphere holds more moisture, which can lead to more extreme precipitation events, causing heavy rain and snowfall that result in severe flooding in some regions.
This atmospheric dynamic contributes to greater imbalances in water distribution. While some areas experience more intense storms, others face prolonged and more severe droughts. The warmer climate alters established weather patterns, shifting where and when precipitation falls, causing wet areas to become wetter and dry regions more arid.
The warming climate is also reducing the amount of fresh water stored in solid form. Glaciers and ice sheets are melting at an accelerated rate, releasing large quantities of water that flow into the oceans. This contributes to sea-level rise and represents a loss of long-term freshwater storage. The melting alters seasonal water availability for communities and ecosystems that depend on glacial runoff.
Degrading Water Quality
Human activities are altering the chemical composition of water as it moves through its cycle. Industrial processes and the burning of fossil fuels release pollutants like sulfur dioxide and nitrogen oxides into the atmosphere. These chemicals react with water vapor to form acid rain, which can harm forests and acidify lakes and streams, impacting aquatic life.
Water quality is also degraded by runoff from agricultural and urban areas. In farming, the use of fertilizers and pesticides can lead to these chemicals being washed into nearby rivers and groundwater systems. This nutrient pollution can cause harmful algal blooms that deplete oxygen in the water, creating “dead zones” where fish and other organisms cannot survive. Similarly, runoff from cities carries pollutants from roads and other surfaces directly into waterways.