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 the hydrologic cycle).Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, and O. Zolina, 2021: Water Cycle Changes. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 1055–1210, doi:10.1017/9781009157896.010. This effect has been observed since at least 1980. One example is when heavy rain events become even stronger. The effects of climate change on the water cycle have important negative effects on the availability of freshwater resources, as well as other water reservoirs such as oceans, ice sheets, the atmosphere, and soil moisture. The water cycle is essential to life on Earth and plays a large role in the global climate system and ocean circulation. The warming of our planet is expected to be accompanied by changes in the water cycle for various reasons. For example, a warmer atmosphere can contain more water vapor which has effects on evaporation and rainfall.
The underlying cause of the intensifying water cycle is the increased amount of greenhouse gases (GHGs) in the atmosphere, which lead to a warmer atmosphere through the greenhouse effect. Fundamental laws of physics explain how the saturation vapor pressure in the atmosphere increases by 7% when temperature rises by 1 °C. This relationship is known as the Clausius-Clapeyron equation.
The strength of the water cycle and its changes over time are of considerable interest, especially as the climate changes. Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License The hydrological cycle is a system whereby the evaporation of moisture in one place leads to precipitation (rain or snow) in another place. For example, evaporation always exceeds precipitation over the oceans. This allows moisture to be transported by the atmosphere from the oceans onto land where precipitation exceeds evapotranspiration. The runoff from the land flows into streams and rivers and discharges into the ocean, which completes the global cycle. The water cycle is a key part of Earth's energy cycle through the evaporative cooling at the surface which provides latent heat to the atmosphere, as atmospheric systems play a primary role in moving heat upward.
The availability of water plays a major role in determining where the extra heat goes. It can go either into evaporation or into an increase in air temperature. If water is available (like over the oceans and the tropics), extra heat goes mostly into evaporation. If water is not available (like over dry areas on land), the extra heat goes into raising air temperature. Also, the water-holding capacity of the atmosphere increases proportionally with temperature increase. For these reasons, the temperature increases dominate in the Arctic (polar amplification) and on land but not over the oceans and the tropics.
Several inherent characteristics have the potential to cause sudden (abrupt) changes in the water cycle.Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V. Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, et al., 2021: Technical Summary. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 33−144. doi:10.1017/9781009157896.002. However, the likelihood that such changes will occur during the 21st century is currently regarded as low.
Overview
Heating of the Earth leads to more energy cycling within its climate system, causing changes to the global water cycle. These include first and foremost an increased water vapor pressure in the atmosphere. This phenomenon causes changes in precipitation patterns with regards to frequency and intensity, as well as changes in groundwater and soil moisture. Taken together, these changes are often referred to as an "intensification and acceleration" of the water cycle. Key processes that will also be affected are droughts and floods, tropical cyclones, glacier retreat, snow cover, ice jam floods and extreme weather events.
The increasing amount of GHGs in the atmosphere leads to extra heating of the lower atmosphere, also known as the troposphere. The saturation vapor pressure of air rises along with its temperature, which means that warmer air can contain more water vapor. Transfers of heat to land, ocean, and ice surfaces additionally promote more evaporation. The greater amount of water in the troposphere then increases the chances for 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 tropospheric water vapor has increased by 3.5% over the last 40 years, which is consistent with the observed temperature increase of 0.5 °C.
The human influence on the water cycle can be observed by analyzing the ocean's surface salinity and the "precipitation minus evaporation (P–E)" patterns over the ocean. Both are elevated. Research published in 2012 based on surface ocean salinity over the period 1950 to 2000 confirms this projection of an intensified global water cycle with salty areas becoming more saline and fresher areas becoming more fresh over the period. IPCC indicates there is high confidence that heavy precipitation events associated with both tropical and extratropical cyclones, and atmospheric moisture transport and heavy precipitation events will intensify.
Intermittency in precipitation Climate models do not simulate the water cycle very well. One reason is that precipitation is a difficult quantity to deal with because it is inherently intermittent. Often, only the average amount is considered. People tend to use the term "precipitation" as if it was the same as "precipitation amount". What actually matters when describing changes to Earth's precipitation patterns is more than just the total amount: it is also about the intensity (how hard it rains or snows), frequency (how often), duration (how long), and type (whether rain or snow). 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.
Observations and predictions
Since the middle of the 20th century, human-caused climate change has included 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 report also found that: Precipitation over land has increased since 1950, and the rate of increase has become faster since the 1980s and in higher latitudes. Water vapour 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.
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 climate change scenarios, water cycle variability and accompanying extremes are anticipated to rise more quickly than the changes of average values.
In 2024 the World Meteorological Organization published a report saying that climate change had severely destabilized water cycle during the year 2023, causing both stronger rainfall and stronger drought. The world's rivers had their driest year in at least 30 years and many of the world's major river basins were drying up like the basins of the Mississippi, Amazon, Ganges, Brahmaputra, and Mekong. For 3 years in a row, more than 50% of global catchment areas had lower than normal river discharges. Glaciers lost more than 600 gigatons of water – the biggest water loss in the last 50 years. It was the second year in a row in which all glaciated regions had ice loss.
Changes to regional weather patterns
Regional weather patterns across the globe are also changing as a result of tropical ocean warming. The Indo-Pacific warm pool has been warming rapidly and expanding during the recent decades, largely in response to increased carbon emissions from fossil fuel burning. The warm pool expanded to almost double its size, from an area of 22 million km2 during 1900–1980, to an area of 40 million km2 during 1981–2018. 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.
Potential for abrupt change Several characteristics of the water cycle have the potential to cause sudden (abrupt) changes of the water cycle. The definition for "abrupt change" is: a regional to global scale change in the climate system that happens more quickly than it has in the past, indicating that the climate response is not linear. 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. The initiation or termination of solar radiation modification could also result in abrupt changes in the water cycle.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.
The scientific understanding of the likelihood of such abrupt changes to the water cycle is not yet clear. Sudden changes in the water cycle due to human activity are a possibility that cannot be ruled out, with current scientific knowledge. However, the likelihood that such changes will occur during the 21st century is currently regarded as low.
Measurement and modelling techniques Changes in ocean salinity