Water Cycle

Why is it important?

Water is vital for life on Earth, and it plays an essential role in regulating climate. When water evaporates, it absorbs heat and cools Earth’s surface. That vapor then travels into the atmosphere where it releases heat as it condenses into clouds. Water vapor also traps the sun’s heat, creating a greenhouse effect that helps warm the planet. Without it, Earth would be a ball of ice.

On regional and local scales, the availability of water supports ecosystems and human activities such as agriculture and manufacturing. Changes to the water cycle can have profound consequences for society. Major shifts in precipitation patterns likely led to the downfall of ancient civilizations around the world, from Angkor to Mesoamerica to the Indus Valley.

How does climate change affect the water cycle?

A warming climate has begun to shift some well-established patterns of precipitation. Warmer temperatures increase evaporation, adding more water vapor to the atmosphere. At the same time, a warmer atmosphere can hold more water vapor, increasing downpours when precipitation occurs. Under the right conditions, this can lead to heavy rainfall events and catastrophic flooding. This is particularly true of tropical cyclones, which have grown in intensity over recent years, but also applies to temperate regions. Recurring atmospheric rivers dumping massive amounts of rain on the west coast of North America are one example. Unusually large swings in precipitation tied to the South Asian monsoon have also caused significant flooding events in recent years.

While some regions are inundated with rain, others experience drought, as a warming climate alters atmospheric circulation patterns. Prolonged drought can have profound impacts, particularly on agriculture, and it contributes to extensive wildfires and loss of vegetation.

How do human activities alter the water cycle?

As we have modified waterways to better accommodate our needs, we have had a significant impact on the water cycle at the regional level. Dams and reservoirs hinder the return flow of water to the ocean. Clear-cutting forests can decrease local precipitation and even change the location of developing storms.

Rain that falls over urban areas can’t soak into the ground when it encounters impermeable surfaces, such as roads, sidewalks, and buildings. Instead, rainwater runs over surfaces and overwhelms storm sewers, creating dangerous flash floods or more widespread flooding events that can damage urban infrastructure.

Even air pollution affects the water cycle. Water molecules attach to particles in the atmosphere, condensing into tiny droplets that make up clouds. In areas with air pollution, clouds can contain many smaller droplets—too small to fall as rain—which can limit precipitation in the area. Pumping groundwater from aquifers for use in agriculture is removing water faster than it can replenish in many areas, leading to significant regional water shortages.

How do scientists study the ocean water cycle?

The ocean water cycle is the marine branch of Earth’s water cycle. As the primary source of atmospheric moisture, it plays a crucial role in shaping global precipitation patterns and regulating climate.

In addition to on-the-ground measurements of precipitation and water flow, scientists use satellite data to track precipitation, evaporation, and freshwater movement. Some NASA satellites use changes in ocean salinity to detect shifts in the water cycle. Others track freshwater flows into the ocean and measure changes in ocean mass caused by influxes of freshwater from ice melt or runoff.

Because the ocean water cycle drives evaporation, influences cloud formation, and interacts with radiation balance, scientists track its changes to understand how climate change can lead to heavier rainfall events—or disrupt rainfall patterns altogether.