5.6 - Climate change and the hydrological cycle
Learn how warming and climate variability alter water inputs, outputs, stores and flows, and how uncertainty affects decisions about drought, floods and reliable supply.
Changing inputs and outputs
Warming changes the energy available for evaporation and the amount of water vapour the atmosphere can contain. Where water is available, higher temperatures can increase evaporation from oceans, lakes and wet soils. Potential evapotranspiration represents atmospheric demand; actual evapotranspiration is constrained by soil water, vegetation and other conditions. A dry soil cannot supply unlimited evaporation simply because the air is warmer.
More atmospheric moisture can support heavier precipitation when air rises and condenses, but circulation determines where and when it falls. A stronger global water cycle does not mean every location becomes wetter. Some regions gain precipitation while others lose it, and the seasonal distribution can change. More rain falling in intense bursts may generate runoff without restoring soil moisture as effectively as gentle, sustained rain.
The IPCC's 2021 assessment identifies increasing water-cycle intensity and changing precipitation variability with warming. Its Chapter 8 distinguishes globally robust changes from less certain regional outcomes. IPCC AR6, Chapter 8.
A basin water balance can be written , where is change in storage, precipitation, evapotranspiration and runoff leaving the basin. Express all terms as water depth over the same area and period; include other transfers when present.
In an illustrative basin with no external transfers, annual precipitation is 900 mm, evapotranspiration 500 mm and runoff 350 mm. Storage increases by . In a warmer illustrative year, values of 850, 550 and 340 mm give : storage declines. This accounting describes the measured/supplied balance; it cannot identify which climatic cause changed each term without further evidence.
Cryosphere and soil stores
Snow and glacier ice store precipitation and delay its release. A warmer winter can turn snowfall into rainfall, reducing the snow store and raising winter runoff. Earlier snowmelt shifts a spring peak forward, potentially leaving less late-summer water for irrigation and ecosystems. Glacier melt may initially increase runoff, but a shrinking glacier eventually supplies less meltwater after peak water, when annual glacier-derived runoff reaches its maximum. The timing depends on glacier size, elevation and warming; different basins reach it at different times.
The IPCC's 2022 impacts assessment describes the consequences of cryosphere change for downstream water availability. IPCC AR6 Working Group II, water chapter. Loss of a seasonal or long-term store changes reliability as well as annual volume: water arriving outside the growing season may not substitute for water needed during it.
Thawing permafrost changes drainage pathways. Melting ground ice can create depressions and lakes; new connections through previously frozen ground can also drain lakes and increase subsurface flow. The direction of local change depends on ice content, topography and connectivity. Warmer conditions can lower soil moisture where evapotranspiration increases more than replenishment, weakening vegetation and reducing resistance to erosion. Soil moisture and vegetation then influence infiltration, evaporation and runoff in return, producing feedbacks within the basin.
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