How climate change affects precipitation and evaporation

The hydrological cycle is a closed system, meaning the total amount of water circulating within it remains constant. Climate change does not add or remove water from the system. However, it significantly alters the movements and transfers within the cycle, changing where, when, and how water moves between stores.

Changing precipitation patterns

A warmer atmosphere has an increased capacity to hold water vapour. This fundamental change drives a range of shifts in global precipitation patterns.

Expected changes in wet and dry regions:

• Wet regions (e.g. the tropics) - These areas are expected to become wetter. Higher temperatures cause rapid and increased evaporation, generating more low-pressure systems and producing more intense rainfall events.
• Dry regions (e.g. areas around 30° latitude) - These areas are expected to become drier. An increased frequency of high-pressure systems reduces the amount of water vapour in the air, limiting the potential for rainfall.

This pattern suggests a growing disparity between water-rich and water-scarce regions across the globe.

Changes to the form and intensity of precipitation

• Extreme weather events - Intense storms, floods, heatwaves, and droughts are expected to become both more frequent and more severe.
• Changes in precipitation form - In glacial and polar areas, rising temperatures mean precipitation is more likely to fall as rain rather than snow, affecting the replenishment of ice and snow stores.

Increasing rates of evaporation and transpiration

As air temperatures rise, there is more energy available in the atmosphere to convert liquid water into water vapour. This leads to higher rates of evaporation from water surfaces and soils. Transpiration — the release of water vapour from vegetation — also increases. Together, these processes contribute to denser cloud cover and a more active atmospheric water cycle.

Changes to the size of water stores

Climate change is expected to alter the volume of water held in several key stores within the hydrological cycle. Some stores will grow, while others will shrink.

Oceanic stores

• Glacial meltwater addition - As glaciers and ice sheets melt, the resulting meltwater flows into the oceans, increasing the volume of the oceanic store and raising sea levels.
• Thermal expansion - Warmer ocean temperatures cause seawater to expand in volume. This thermal expansion contributes further to sea level rise, independent of any additional meltwater input.

Cryospheric stores (ice, snow, and permafrost)

• Glacial ice - Glacial stores are likely to shrink as warming temperatures increase ablation — the loss of ice through melting, sublimation, and calving. This reduces the long-term capacity of glaciers to act as water stores.
• Snow cover - Snow cover is expected to become shallower and persist for a shorter season due to rising temperatures.
• Permafrost - The active layer — the layer of ground above the permafrost that thaws seasonally — will deepen as warmer temperatures cause more permafrost to thaw. This releases additional meltwater into the soil.

Surface water stores

• Initial increase, then decline - In the short term, increased meltwater from glaciers and ice sheets will add to rivers, lakes, and reservoirs. Over time, however, as these ice stores diminish in size, the volume of meltwater feeding surface water stores will decrease.
• Greater fluctuations - Lakes, reservoirs, and wetlands may experience more extreme variations in water levels, swinging between full capacity after intense rainfall events and very low levels during prolonged heatwaves when evaporation is high and precipitation is absent.

How climate change alters rates of flow

Changes in precipitation patterns and ground conditions directly affect how water moves across and through the landscape.

Runoff and streamflow

Rates of direct runoff and streamflow are closely linked to precipitation levels. During and immediately after intense rainfall, these flows become fast and high in volume, increasing the risk of flooding.

Soil moisture and infiltration

Between periods of rainfall, soil moisture levels may decrease as flow rates slow and the ground dries out. Prolonged dry conditions can cause the ground surface to harden, reducing its permeability. When rainfall returns, this hardened surface prevents water from infiltrating the soil effectively, causing water to flow over the surface at greater speeds as overland flow. This creates a cycle where dry periods intensify the impact of subsequent rainfall events.

Uncertainty in the hydrological system

Despite advances in climate science, there remains a high degree of uncertainty about the precise ways in which climate change will affect the hydrological cycle. Several factors contribute to this uncertainty.

Incomplete understanding and data gaps

The hydrological cycle is complex, and the interactions between its different components are only partially understood. Incomplete data records from many parts of the world make it even harder to build accurate models of future change.

Difficulty separating human and natural causes

It is challenging to distinguish between changes caused by human activity and those resulting from natural climate variability. The climate reacts to human interference, and humans then respond to the changing climate. Some responses aim to restore balance — such as reducing dependence on fossil fuels — while others heighten the imbalance, such as artificially irrigating barren regions.

Interaction of short-term and long-term causes

Climate change involves both short-term climate events and long-term climate adjustments. These interact with and influence each other, making precise forecasting difficult.

Short-term events - the ENSO:

The El Niño-Southern Oscillation (ENSO) is a short-term climate phenomenon that causes increased rainfall and flooding in some regions while bringing drought to others. Its exact impact, timing, and intensity vary with each occurrence, making it difficult to predict. In the long term, climate change is expected to make El Niño and La Niña events last longer and become more intense.

Long-term climate warming:

The warming of the global climate is a long-term process whose impacts are difficult to measure and forecast because the rate of change varies across different locations. There is also a significant lag time between changes in global atmospheric temperature and the resulting changes in the hydrological cycle, making it hard to know exactly when specific impacts will occur.

Feedback systems within the hydrological cycle

The hydrological cycle contains feedback mechanisms that can either amplify or counteract changes, giving the system a degree of self-regulation. This makes predicting the net effect of climate change particularly challenging.

Positive feedback (amplifying change):

1. Temperatures rise
2. Evaporation increases
3. The amount of water vapour in the atmosphere increases
4. Water vapour is a greenhouse gas, so the greenhouse effect intensifies
5. Temperatures rise further, reinforcing the cycle

Negative feedback (counteracting change):

1. Temperatures rise
2. Evaporation increases
3. The amount of water vapour in the atmosphere increases, causing more clouds to form
4. Increased cloud cover reflects more of the Sun's energy back into space
5. Temperatures fall, partially offsetting the initial warming

Changes that might push a water store in one direction may therefore be balanced by changes operating in the opposite direction, creating uncertainty about the overall outcome.

The impact of climate change on water supply security

Water security refers to the reliable availability of an adequate quantity and quality of water for communities. Climate change threatens water security in several ways.

Unpredictable rainfall patterns

Rainfall is expected to become more unpredictable and unreliable, making it difficult to plan water use — particularly in agricultural areas. Communities that depend on seasonal rainfall, such as the monsoons in Asia, to replenish depleted water stores may experience greater water stress as these patterns shift.

Contamination from storm surges

An increase in the frequency and intensity of tropical storms and cyclones may lead to more storm surges and inland flooding by seawater. Saline water — water with a high salt content — can contaminate groundwater supplies, severely affecting water security for coastal and low-lying communities.

Glacial meltwater communities

Communities in upland areas that rely on glacial meltwater for their water supply face a dual threat.

Key risks for glacial meltwater-dependent communities:

• Deluges rather than steady flow - There is an increased likelihood that meltwater will arrive as sudden, large volumes rather than as a constant, manageable feed. This means some water may be wasted because it cannot be captured and stored.
• Loss of the meltwater source - Once glaciers have fully melted, the meltwater supply will be permanently lost, drastically reducing long-term water security for these communities.

Increased evaporation from water stores

Higher temperatures drive increased evaporation from lakes, reservoirs, and other stores that supply communities with water. This can reduce the volume of available water, particularly during prolonged warm periods, further decreasing water security.