Effects of climate change on the water cycle

The water cycle is essential to life on Earth and plays a large role in the global climate system and ocean circulation.

[3] Fundamental laws of physics explain how the saturation vapor pressure in the atmosphere increases by 7% when temperature rises by 1 °C.

The runoff from the land flows into streams and rivers and discharges into the ocean, which completes the global cycle.

If water is not available (like over dry areas on land), the extra heat goes into raising air temperature.

For these reasons, the temperature increases dominate in the Arctic (polar amplification) and on land but not over the oceans and the tropics.

[6] Several inherent characteristics have the potential to cause sudden (abrupt) changes in the water cycle.

[4] This is visible in measurements of the tropospheric water vapor, which are provided by satellites,[11] radiosondes and surface stations.

[6]: 50  Scientists have researched the characteristics of precipitation and found that it is the frequency and intensity that matter for extremes, and those are difficult to calculate in climate models.

[7]: 85  The IPCC Sixth Assessment Report in 2021 predicted that these changes will continue to grow significantly at the global and regional level.

[21] This expansion of the warm pool has altered global rainfall patterns, by changing the life cycle of the Madden Julian Oscillation (MJO), which is the most dominant mode of weather fluctuation originating in the tropics.

[7]: 1148  There may be "rapid transitions between wet and dry states" as a result of non-linear interactions between the ocean, atmosphere, and land surface.

For example, a collapse of the Atlantic meridional overturning circulation (AMOC), if it did occur, could have large regional impacts on the water cycle.

[7]: 1149  The initiation or termination of solar radiation modification could also result in abrupt changes in the water cycle.

[7]: 1151 There could also be abrupt water cycle responses to changes in the land surface: Amazon deforestation and drying, greening of the Sahara and the Sahel, amplification of drought by dust are all processes which could contribute.

[7]: 1151  Sudden changes in the water cycle due to human activity are a possibility that cannot be ruled out, with current scientific knowledge.

Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling.

[26] Another advantage is that oceanic salinity is stable on very long time scales, which makes small changes due to anthropogenic forcing easier to track.

The oceanic salinity is not homogeneously distributed over the globe, there are regional differences that show a clear pattern.

[28] The long-term observation records show a clear trend: the global salinity patterns are amplifying in this period.

To further investigate the relation between ocean salinity and the water cycle, models play a large role in current research.

[26] The outcome of multiple studies based on such models support the relationship between surface salinity changes and the amplifying precipitation minus evaporation patterns.

This is because ocean warming increases near-surface stratification, subsurface layer is still in equilibrium with the colder climate.

[33] An instrument carried by the SAC-D satellite Aquarius, launched in June 2011, measured global sea surface salinity.

[40][41] Precipitation on the one hand, only has long term accurate observation records over land surfaces where the amount of rainfall can be measured locally (called in-situ).

[42] There have been limited changes in regional monsoon precipitation observed over the 20th century because increases caused by global warming have been neutralized by cooling effects of anthropogenic aerosols.

A convection-permitting (4.5 km grid-spacing) model over an Africa-wide domain shows future increases in dry spell length during the wet season over western and central Africa.

The scientists concludes that, with the more accurate representation of convection, projected changes in both wet and dry extremes over Africa may be more severe.

They include more frequent and intense heavy precipitation which affects the frequency, size and timing of floods.

There is now ample evidence that greater hydrologic variability and climate change have had a profound impact on the water sector, and will continue to do so.

[62] The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration.

Extreme weather (heavy rains, droughts , heat waves ) is one consequence of a changing water cycle due to global warming . These events will become more and more common as the Earth warms. [ 1 ] : Figure SPM.6
The water cycle
Predicted changes in precipitation event intensity and evapotranspiration under the SSP2-4.5 scenario. [ 17 ]
Predicted changes in average soil moisture for a scenario of 2°C global warming. This can disrupt agriculture and ecosystems. A reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.
The yearly average distribution of precipitation minus evaporation. The image shows how the region around the equator is dominated by precipitation, and the subtropics are mainly dominated by evaporation.
The global pattern of the oceanic surface salinity. It can be seen how the by evaporation dominated subtropics are relatively saline. The tropics and higher latitudes are less saline. When comparing with the map above it can be seen how the high salinity regions match the by evaporation dominated areas, and the lower salinity regions match the by precipitation dominated areas. [ 25 ]