Freshwater Boundary
Despite water covering 70% of our planet, only 3% is freshwater, with two-thirds trapped in glaciers or otherwise inaccessible. Threats to this available freshwater need to be taken seriously.
Boretti and Rosa (Boretti & Rosa, 2019) note in the opening sentences of their abstract:
"The 2018 edition of the United Nations World Water Development Report stated that nearly 6 billion peoples will suffer from clean water scarcity by 2050. This is the result of increasing demand for water, reduction of water resources, and increasing pollution of water, driven by dramatic population and economic growth. It is suggested that this number may be an underestimation, and scarcity of clean water by 2050 may be worse as the effects of the three drivers of water scarcity, as well as of unequal growth, accessibility and needs, are underrated."
The freshwater planetary boundary was recently revised (Gleeson et al., 2020; Wang-Erlandsson et al., 2022), and green water — terrestrial precipitation, evaporation and soil moisture — was explicitly added. This addition is the boundary for anthropogenic modifications of Earth system functions of freshwater. The boundaries are based on deviations from preindustrial variability, with control variables representing the percentage of global land area experiencing these deviations. The revised boundary considers changes in the entire water cycle over land, accounting for both increases and decreases in water on a monthly scale. These changes capture the Earth system impacts of water on terrestrial ecosystems, climate, and biogeochemical processes. The current boundary settings are considered preliminary and highly precautionary due to potential risks to freshwater's Earth system functions. Green water is fundamental to Earth system dynamics and is now extensively perturbed by human pressures at continental to planetary scales.
In a recent global review of groundwater, Jasechko (Jasechko et al., 2024) noted that these resources are essential for ecosystems and livelihoods. Taking out too much groundwater can lead to problems like seawater intrusion, land sinking, reduced stream flows, and dried-up wells. The speed and extent of local groundwater declines globally are not well understood because global groundwater levels have not been studied comprehensively. A recent analysis of 170,000 monitoring wells and 1,693 aquifers in countries that account for about 75% of global groundwater use shows that rapid declines in groundwater levels are common, especially in dry regions with lots of farming. The study indicates that the rate of groundwater level drops has increased significantly over the last forty years in 30% of the world's aquifers. This widespread deepening of groundwater levels emphasizes the urgent need for more effective actions to combat groundwater depletion. Some cases have shown that depletion trends can be reversed with policy changes, managed recharge of aquifers, and diverting surface water, indicating that depleted aquifers can recover under the right conditions.
A recent study by Ki-Weon Seo and colleagues (Seo et al., 2023) noted that groundwater depletion poses significant threats, with rapid declines in many aquifers showing the urgent need for effective conservation measures. Additionally, the study revealed that human activities like groundwater pumping have caused the Earth's axial shift by almost a meter through water redistribution between 1993 and 2010 alone.
Main Threats
Drought and Aridification: Global warming is causing prolonged droughts, notably in East Africa and the Western US, exemplifying climate change. The UN reports that over a third of the global population lives in water-scarce regions. Simultaneously, global warming also leads to severe flooding, affecting farmers, as seen in India and Bangladesh.
Mismanagement of Groundwater: Groundwater, crucial for 43% of irrigation, is over-extracted due to improved drilling technologies, particularly in India. This unsustainable use threatens global grain production, with 10% reliant on depleting groundwater. Experts advocate for better management practices and technologies like drip irrigation to mitigate this issue.
Saltwater Intrusion: Intensive irrigation can raise water tables, causing salt to infiltrate soils and harm plants. Overusing groundwater alongside rising sea levels can lead to saltwater penetrating coastal aquifers, damaging crops and drinking water supplies. Salinity pollution affects about 10% of the world's rivers.
Pollution: In arid regions, wastewater, often reused for crops, can carry pathogens like cholera. Flooding can exacerbate this by contaminating water sources with sewage and fertilizers, causing harmful algal blooms. Pollution seeping into groundwater poses long-term risks to crops and health.
Land Degradation: Human activity has degraded over 70% of the Earth's land, diminishing soils' capacity to store and filter water. This change hampers agriculture and livestock rearing, potentially disrupting food supplies. Climate change-induced extreme weather accelerates land degradation, threatening food security.
As we can see, water used by plants and creatures (including people) is scarce. This scarcity means that over 1 billion people currently lack access to clean water, and 2.7 billion face water shortages at least one month a year. Poor sanitation affects 2.4 billion people, leading to diseases like cholera and typhoid, and causing 2 million deaths annually, mostly among children. Water systems supporting ecosystems and human populations are under stress, with rivers, lakes, and aquifers drying up or polluted. Over half of the world's wetlands have vanished. Agriculture, the largest water consumer, is highly inefficient, and climate change exacerbates water issues by causing droughts and floods.
Some estimate that by 2025, two-thirds of the world's population may face water shortages of some type, with a growing number having extreme conditions.
References:
Boretti, A., & Rosa, L. (2019). Reassessing the projections of the World Water Development Report. npj Clean Water, 2(1), 15. doi:10.1038/s41545-019-0039-9
Gleeson, T., Wang-Erlandsson, L., Zipper, S. C., Porkka, M., Jaramillo, F., Gerten, D., . . . Famiglietti, J. S. (2020). The Water Planetary Boundary: Interrogation and Revision. One Earth, 2(3), 223-234. doi:https://doi.org/10.1016/j.oneear.2020.02.009
Jasechko, S., Seybold, H., Perrone, D., Fan, Y., Shamsudduha, M., Taylor, R. G., . . . Kirchner, J. W. (2024). Rapid groundwater decline and some cases of recovery in aquifers globally. Nature, 625(7996), 715-721. doi:10.1038/s41586-023-06879-8
Seo, K.-W., Ryu, D., Eom, J., Jeon, T., Kim, J.-S., Youm, K., . . . Wilson, C. R. (2023). Drift of Earth's Pole Confirms Groundwater Depletion as a Significant Contributor to Global Sea Level Rise 1993–2010. Geophysical Research Letters, 50(12), e2023GL103509. doi:https://doi.org/10.1029/2023GL103509
Wang-Erlandsson, L., Tobian, A., van der Ent, R. J., Fetzer, I., te Wierik, S., Porkka, M., . . . Rockström, J. (2022). A planetary boundary for green water. Nature Reviews Earth & Environment, 3(6), 380-392. doi:10.1038/s43017-022-00287-8