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Paleoclimatic records indicate that the Earth's water cycle has undergone natural fluctuations since Earth's formation, however, current changes in the water cycle can be primarily attributed to a changing climate as a result of anthropogenic emissions. The effects of climate change on the water cycle are profound and have been described as an intensification or a strengthening of the water cycle (also called hydrologic cycle). This effect has been observed since at least 1980. The global water cycle encompasses the continuous circulation of water through the Earth's surface, atmosphere, subsurface and stores such as glaciers, oceans and ground water. It is an essential mechanism for maintaining freshwater resources, as well as other water reservoirs such as oceans, Ice sheet s, atmosphere and land surface. The water cycle is essential to life on Earth and plays a large role in maintaining a stable global climate. The warming of our planet is expected to cause changes in the water cycle for various reasons. Changes is the water cycle can have global, regional and local impacts, impacting water-resource availability, the frequency and severity of storms, droughts and floods, and further increases in global warming through increased water vapor in atmosphere.

Causes
The underlying cause of water cycle intensification, is the release of greenhouse gases, increasing the amount of heat stored trapped by Earth's atmosphere, known as the greenhouse effect. Physics dictates that saturation vapor pressure increases by 7% when temperature rises by 1 °C (as described in the Clausius-Clapeyron equation).

Global warming leads to changes in the global water cycle, often resulting in increased atmospheric water vapor pressure. Changes in the atmospheric water vapor content leads to shifts in the frequency and intensity of rainfall events, as well as changes in groundwater and soil moisture. Local and regional climates can be altered resulting in droughts, floods, tropical cyclones, glacier retreat, ice jam floods and other extreme weather events.

The saturation Vapor pressure of air increases with temperature, which means that warmer air can contain more Water vapor. Increases in air temperature increases both the rate of evaporation and the amount of water the air molecule can hold, resulting in more intense rainfall events.

This relation between temperature and saturation vapor pressure is described in the Clausius–Clapeyron equation, which states that saturation pressure will increase by 7% when temperature rises by 1 °C. This is visible in measurements of the tropospheric water vapor, which are provided by satellites, radiosondes and surface stations. The IPCC AR5 concludes that water vapor in the Troposphere has increased by 3.5% over the last 40 years, which is consistent with the observed temperature increase of 0.5 °C.

'''Anthropogenic emissions leading to the overall warming of the earth's climate has global consequences, changing climates around the world, having significant impacts on frequency and intensity of rainfall. However, localized impacts on rainfall can be caused by anthropogenic emissions that release sulfates, soot and mineral dust. These pollutants reflect and absorb short wave light and reduce the amount of solar irradiance that reaches the earth's surface. Less solar energy striking the earth results in lower rates of evaporation and therefore, less rainfall. However, the primary effect of these aerosols on impacting rainfall is their ability to increase cloud concentration nuclei, reducing cloud droplet size, resulting in reduced coalescence of rain drops. Recent studies have shown these effects have began to diminish, leading to increases in rainfall. '''

Observations and predictions
Since the middle of the 20th century, human-caused climate change has resulted in observable changes in the global water cycle. The IPCC Sixth Assessment Report in 2021 predicted that these changes will continue to grow significantly at the global and regional level. The strength of the water cycle and its changes over time are of considerable interest, especially as the climate changes.

State of Earths Climate Based on 2022 Reports
Concentrations of greenhouse gases, ocean temperature and sea level have reached record levels. Annual surface temperatures worldwide were on average half a degree Fahrenheit above averages measured between the years 1991-2020, making it the sixth warmest year recorded. Temperatures in the Arctic were the fifth highest seen in the last 123 years, continuing to indicate the Arctic amplification anomaly in which polar regions are increasing in temperature at faster rates than areas of lower latitude. This has led to significant increases in the rates of precipitation in the Arctic, with 2022 being the third wettest year since 1950. Other regions were remarkably dry, with 29% of land undergoing moderate and worse droughts. Droughts were especially devastating in Chile where it experienced drought conditions for a 13th consecutive year.

On a global scale the rate of precipitation over land has seen an increase of 2% between 1900-1998, varying significantly depending on the region. Precipitation increases between 30oN and 85oN were between 7-12%, while precipitation between 0oS and 55oS increased by 2%. Other regions have experienced significant decreases in the amount of precipitation. Water vapor in the atmosphere (in particular the Troposphere) has increased since at least the 1980s. It is expected that over the course of the 21st century, the annual global precipitation over land will increase due to a higher global surface temperature.

Considerable changes in atmospheric circulation are most notable on a regional level, while large-scale global changes such as the weaking of tropical circulation and shift in climate regimes are expected across most latitudes. Local changes can in precipitation and atmospheric circulation can be initiated by land use changes that alter moisture levels and surfaces energy balances. A warming climate makes extremely wet and very dry occurrences more severe. There can also be changes in atmospheric circulation patterns. This will affect the regions and frequency for these extremes to occur. In most parts of the world and under all emission scenarios, water cycle variability and accompanying extremes are anticipated to rise more quickly than the changes of average values.

Intermittency in precipitation
Climate models do not simulate the water cycle very well, and make it difficult to predict changes due to climate change. Precipitation is a difficult quantity to measure because it is inherently intermittent. Changes in Earth's precipitation patterns includes changes in the amount of rainfall, as well as intensity, frequency, duration, and type (whether rain or snow). New Zealand climatologist Kevin E. Trenberth and former NCAR scientist, researched the characteristics of precipitation and found that changes in frequency and intensity are most, and those are difficult to calculate in climate models.

Water Vapor
Water vapor is an important greenhouse gas for maintaining Earth's energy balances and shaping atmospheric circulation, thus playing a major role in regulating Earth's climate. Water vapor is at the center of natural disasters such as hurricanes, thunderstorms and flood causing rainfall events. '''The release of latent heat through the phase changes of water leads to the heating and cooling of air, driving convection dynamics, most prominently at equatorial regions. Recognizing and understanding the changes that global warming is having on water vapor in atmospheric water vapor is essential for predicting changes in the global water cycle.'''

Increasing mean tropospheric water vapor has been measured in most regions of the world, with highest rates of increase in the Arctic. Increases in water vapor is expected to increase surface radiation by 5-70W/m2 depending on the location, increasing global and regional temperatures. Water vapor is part of a positive feed-back loop, in which warming temperatures increases evaporation and thus water vapor, which in turn increases warming.

Changes in ocean salinity
One way in which changes in the Earth's water cycle can be detected is through the monitoring of the ocean's surface salinity and the "precipitation minus evaporation (P–E)" patterns over the ocean. Both are elevated. Increases in both precipitation and evaporation leads to ocean conditions becoming increasingly saline in areas with high evaporation rates, and less saline in areas experiencing increasing rainfall.

Due to global warming and increased glacier melt, thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as upwelling. The advantage of using surface salinity is that it is well documented in the last 50 years, for example with in-situ measurement systems as ARGO. 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. The tropic regions are relatively fresh, since these regions are dominated by rainfall. The subtropics are more saline, since these are dominated by evaporation, these regions are also known as the 'desert latitudes'. The latitudes close to the polar regions are then again less saline, with the lowest salinity values found in these regions. This is because there is a low amount of evaporation in this region, and a high amount of fresh meltwater entering the Arctic Ocean.

The long-term observation records show a clear trend: the global salinity patterns are amplifying in this period. This means that the high saline regions have become more saline, and regions of low salinity have become less saline. The regions of high salinity are dominated by evaporation, and the increase in salinity shows that evaporation is increasing even more. The same goes for regions of low salinity that are become less saline, which indicates that precipitation is intensifying only more. This spatial pattern is similar to the spatial pattern of evaporation minus precipitation. The amplification of the salinity patterns is therefore indirect evidence for an intensifying water cycle.

To further investigate the relation between ocean salinity and the water cycle, models play a large role in current research. General Circulation Models (GCMs) and more recently Atmosphere-Ocean General Circulation Models (AOGCMs) simulate the global circulations and the effects of changes such as an intensifying water cycle. The outcome of multiple studies based on such models support the relationship between surface salinity changes and the amplifying precipitation minus evaporation patterns.

A metric to capture the difference in salinity between high and low salinity regions in the top 2000 meters of the ocean is captured in the SC2000 metric. The observed increase of this metric is 5.2% (±0.6%) from 1960 to 2017. But this trend is accelerating, as it increased 1.9% (±0.6%) from 1960 to 1990, and 3.3% (±0.4%) from 1991 to 2017. Amplification of the pattern is weaker below the surface. This is because ocean warming increases near-surface stratification, subsurface layer is still in equilibrium with the colder climate. This causes the surface amplification to be stronger than older models predicted.

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

Between 1994 and 2006, satellite observations showed an 18% increase in the flow of freshwater into the world's oceans, partly from melting ice sheets, especially Greenland and partly from increased precipitation driven by an increase in global ocean evaporation.

Convection-permitting models to predict weather extremes
Convection-permitting models (CPMs) are able to better simulate the diurnal cycle of tropical convection, the vertical cloud structure and the coupling between moist convection and convergence and soil moisture-convection feedbacks in the Sahel. The benefits of CPMs have also been demonstrated in other regions, including a more realistic representation of the precipitation structure and extremes. 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. In other words: "both ends of Africa's weather extremes will get more severe".

Impacts on water management aspects
Climate change related shifts in the water cycle will have regional and global impacts, impacting water availability (water resources), water supply, water demand, water security and water allocation.