Changes to the Global Water Cycle

Mark Altaweel

Updated:

Few global-scale processes are as important as the global water cycle. We may have noticed in recent years how some regions appear to have become more dry or have prolonged drought while at the same time we hear about places experiencing major deluges descried as ‘once in a century’ even if they seems increasingly common.

Research is indicating that what might be going on is a major shift in the global water cycle and this can have major social repercussions for us. Fortunately we are better prepared than we were only a decade ago, but the question remains if we have created effective management strategies to mitigate the worst impacts. 

A diagram showing the flow of water on Earth.
The global water cycle. Image: NASA/JPL-Caltech, public domain.

In 2022, a third consecutive La Niña year affected the Pacific Rim with a lot of rain in Australia and Southeast Asia but dry conditions prevailed in the Americas.

Using 40 different satellites monitoring rain and thousands of weather ground stations’ data, researchers have begun to conclude that this may foretell large shifts in the Earth’s water system.


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Difference in oceanic temperatures from the average during a La Niña year in September of 2020. Image: NASA.
Difference in oceanic temperatures from the average during a La Niña year in September of 2020. Image: NASA.

A key conclusion in a recent 2022 report on Global Water is that the Earth’s air is not only getting hotter and drier but droughts and fire conditions are now developing faster and frequently even while some regions are experiencing more common torrential rains.

Warmer than normal waters in the Western Pacific continue to dominate, while cooler waters in the eastern part of the Pacific have formed in the last few years. Additionally, a negative Indian Ocean Dipole with warm sea water in the eastern and northern Indian Ocean but cooler in the western part of the Indian Ocean have led to this continual La Niña conditions for three years.

A dry river bed during a drought.
Two miles below the Morelos Dam, the Colorado River runs dry at the border between the United States and Mexico during a drought in 2009. Photo: Pete McBride, USGS, public domain.

This resulted in powerful monsoons in Southeast Asia and the Australian continent, while droughts were severe in the Americas and Africa. Heatwaves and drought were noticeably unusual in Europe and China in the Summer of 2022.

Even at the end of 2022, conditions appear to continue in some places, although in some regions changes witnessed in recent years are somewhat waning and would suggest conditions are potentially more akin to El Niño for the current year. In other words, above-average sea-surface temperatures are now increasingly likely to develop across the east-central equatorial Pacific for 2023.[1]

What are the long-term trend of climate conditions?

Beyond these recent years, the question is if these conditions prevalent over the last few years are likely to persist over the long-term.

Current estimates is that the rate of increasing extreme precipitation events and longer dry spells will continue in coming years. Recent work has shown that rainfall events will become larger for regions experiencing increasing rain (ranges of +11.5% and +18.5%) and the duration of droughts longer in dry regions by the end of the century, with the largest changes noticeable in the northern latitudes.

An oak tree that has split and fallen over.
The stress of extreme drought followed by an increase in atmospheric rivers in Northern California has resulted in an increase in downed trees with each storm system. A fallen oak tree in the Santa Cruz mountains after a recent storm on January 28, 2023. Photo: Caitlin Dempsey.

The problem is that fluctuations from year-to-year will not be consistent for regions but regions will swing from drought to surplus rain at more extreme levels. In other words, it may shift more drastically between El Niño and  La Niña rather than just persist in one condition, even if one does prevail for a few years.

In other words, water cycles will stay in one scenario longer before switching to another extreme.[2] 

Better satellite monitoring capabilities to predict changes in the Earth’s water cycle

The good news is because there are increasing satellite monitoring capabilities, including missions such as the Surface Water and Ocean Topography (SWOT) or the NASA-ISRO Synthetic Aperture Radar (SAR) mission (NISAR), or even the upcoming SMall Altimetry Satellite for Hydrology (SMASH) constellation and Sentinel-3 Topography New Generation (S-3 Topo NG) satellite programs, we will have much better monitoring and forecasting capabilities starting this decade.

A diagram of a Earth observation satellite showing the swaths of data it will collect.
Earth observation satellite like the Surface Water and Ocean Topography (SWOT) can continuously collect data to track changes in the global water cycle. Diagram: Yeosang Yoon, NASA, public domain.

Models such as the Agricultural Catchment Research Unit (ACRU) model and Hybrid Atmospheric and Terrestrial Water Balance (HATWAB) model are now also better able to handle and produce more accurate forecasts using updated satellite and weather station data. What this will mean is that although swings in hydrology and global water cycle will be more extreme, we will be better able to forecast these swings.

Regions could be better prepared through better management of extremes such that extreme rainfall or water events could potentially store or capture excess rain for anticipated periods of increasing drought. Flood management may need to be better created to handle extreme flooding relative to what was expected in the past.

Vulnerable regions that depend on steady or more predictable rainfall, such as parts of Africa, will need careful monitoring and management in particular because large populations depend on vulnerable water resource conditions. Data will need to be better shared and planned across large regions rather than for only specific countries as has been in the past.[3]

A need for effective water management on a regional level

We are now in a period where our previous planetary cycles have begun to change at rates not witnessed since perhaps many millennia ago. While this does make us more vulnerable, given how our societies have adapted to more expected rainfall and water conditions in recent centuries, the good news is we now have much better monitoring and forecasting tools thanks in large part to open data sets, new satellite systems, and improved modeling.

Although we have better capabilities, we will still need good water management to avert potentially major disasters. This will require global-scale cooperation and coordination that helps to avert the worst disasters as we begin to witness more extreme weather.

References

[1]    For more on the water cycle change and report about changing global conditions see:  Van Dijk, A.I.J.M., H.E. Beck, R.A.M. de Jeu, W.A. Dorigo, J. Hou, W. Preimesberger, J Rahman, P.R. Rozas Larraondo, R. van der
Schalie (2022) Global Water Monitor 2022, Summary Report. Global Water Monitor (www.globalwater.online).

[2]    For more on fluctuation and changes in dry and wet conditions across continents, see:  Ficklin DL, Null SE, Abatzoglou JT, et al. (2022) Hydrological Intensification Will Increase the Complexity of Water Resource Management. Earth’s Future 10(3). DOI: 10.1029/2021EF002487.

[3]    For more on satellites and expectations of hydrologic changes across regions, particularly in Africa, see:  Papa F, Crétaux J-F, Grippa M, et al. (2022) Water Resources in Africa under Global Change: Monitoring Surface Waters from Space. Surveys in Geophysics. DOI: 10.1007/s10712-022-09700-9.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.