Transpiration moves water from the soil into the atmosphere through plants, acting as a critical pump in the water cycle. About 10% of all moisture in the atmosphere comes from plant transpiration, with the remaining 90% from evaporation off oceans, lakes, and rivers. That 10% figure understates how important transpiration is on land, though, where plants recycle enormous volumes of water that fuel rainfall over continents.
How Water Moves Through a Plant
Transpiration starts underground. Roots absorb water from the soil, and that water crosses several cell layers before reaching the xylem, the plant’s internal plumbing system. Along the way, water travels either through cell walls or directly through the insides of cells. At a key checkpoint called the endodermis, a waterproof band forces all water to pass through living cells rather than simply flowing between them. This acts as a natural filter, controlling what enters the plant’s transport system.
Once inside the xylem, water moves upward through open tubes with relatively little resistance. The driving force is evaporation at the leaves. Plants have tiny pores on their leaf surfaces called stomata, which open to let carbon dioxide in for photosynthesis. When stomata are open, water vapor escapes into the air. As water evaporates from the leaf surface, it pulls more water up through the xylem from the roots, creating a continuous chain of water molecules stretching from soil to sky. This pull is strong enough to move water to the top of the tallest trees.
Recycling Rainfall Over Land
Once water vapor leaves a plant’s leaves, it doesn’t just disappear into the broader atmosphere. Globally, about 70% of the water that evaporates from land surfaces (including transpiration) falls back as rain over continents. That recycled moisture accounts for roughly 40% of all rainfall on land. Without plants pumping water vapor into the air, inland areas would be significantly drier.
The scale of this effect varies by region. Across Africa, nearly 50% of annual rainfall originates from transpiration, though that number ranges from 5% in coastal areas to 68% in some inland watersheds. The Congo basin is a striking example: local transpiration sustains year-round precipitation and also feeds rainfall in surrounding watersheds. Coastal regions depend more on moisture blowing in from the ocean, but large inland basins like the Congo and Nile rely heavily on vegetation to keep the rain coming. Remove those forests, and you don’t just lose trees. You lose the rain that depends on them.
Cooling the Air and the Plant
Evaporating water requires energy. When water transpires from a leaf, it absorbs heat from the leaf surface and surrounding air, converting that heat into the energy needed for evaporation. This is the same principle that makes sweating cool your skin. For the plant, transpiration prevents overheating during photosynthesis. For the surrounding environment, it pulls heat out of the air and lowers local temperatures.
This cooling effect is why forested areas and parks feel noticeably cooler than paved surfaces on a hot day. Urban trees, for instance, reduce nearby air temperatures through transpiration in a way that shade alone cannot fully explain. The heat energy that would otherwise warm the air gets locked up in water vapor instead, carried upward and released only when that vapor condenses into clouds higher in the atmosphere. This transfer of energy from the ground level to the upper atmosphere is a major component of Earth’s energy balance.
Pulling Water From the Ground
Transpiration also shapes what happens below the surface. Most plant roots sit above the water table and depend on rainwater that soaks into the upper soil layers. But where the water table is shallow, such as near lakes, rivers, or coastlines, roots can reach directly into the saturated zone and pull groundwater up through the plant and into the air. When this happens, transpiration draws down the local water table in much the same way a pumped well does, creating a small depression in groundwater levels around the root zone.
This means transpiration doesn’t just add water to the atmosphere. It actively depletes soil moisture and, in some settings, groundwater reserves. During dry periods, this drawdown can be significant. It also explains why removing vegetation sometimes causes water tables to rise and why planting trees in arid regions can lower already-scarce groundwater supplies.
What Controls the Rate of Transpiration
Plants don’t transpire at a constant rate. Several environmental factors speed it up or slow it down. When soil is relatively moist, the biggest drivers are sunlight intensity and vapor pressure deficit, which is essentially how dry the air is. Bright sun powers photosynthesis, which requires open stomata, and dry air pulls moisture out of leaves faster. Wind speed and humidity also play a role, with wind whisking away the thin layer of moist air that clings to leaf surfaces.
When soil dries out, the picture shifts. Air temperature and vapor pressure deficit become the dominant factors, and the plant begins to ration its water. Stomata close partially or fully to prevent water loss, which also slows photosynthesis. This is a tradeoff plants constantly manage: open stomata to take in carbon dioxide for growth, or close them to conserve water and survive drought.
How Climate Change Affects Transpiration
Rising carbon dioxide levels complicate this balance. Higher CO2 concentrations allow plants to take in the same amount of carbon with their stomata open less wide, which reduces water loss per leaf. In theory, this could mean less transpiration globally. At the same time, warming temperatures increase the dryness of the air, which pushes transpiration rates up. These two forces pull in opposite directions, and the net effect depends on region and vegetation type.
Widespread land drying under climate change is expected to further limit transpiration in water-stressed ecosystems. Since transpiration feeds back into rainfall, especially over inland regions, any sustained reduction in plant water cycling could weaken precipitation patterns that depend on moisture recycling. The relationship runs in both directions: less rain means less transpiration, and less transpiration means less rain.