Soil moisture loss is driven by changes in evapotranspiration and precipitation, influenced by climatic changes. Rising atmospheric and oceanic temperatures have changed terrestrial water circulation and surface water fluxes, leading to changes in evapotranspiration and precipitation. Terrestrial water storage has been affected on a global scale. Basically, a warmer atmosphere draws more water from the surface. This is known as evaporative demand, and it has increased over the past two decades. Changes in rainfall patterns also play a role. According to the abstract of a new paper in Science: Abrupt sea level rise and Earth’s gradual pole shift reveal permanent hydrological regime changes in the 21st century:
“The European Centre for Medium-Range Weather Forecasts
(ECMWF) Reanalysis v5 (ERA5) soil moisture (SM) product reveals a sharp
depletion during the early 21st century. During the period 2000 to 2002, soil
moisture declined by approximately 1614 gigatonnes, much larger than
Greenland’s ice loss of about 900 gigatonnes (2002–2006). From 2003 to 2016, SM
depletion continued, with an additional 1009-gigatonne loss. This depletion is
supported by two independent observations of global mean sea level rise (~4.4 millimeters)
and Earth’s pole shift (~45 centimeters). Precipitation deficits and stable
evapotranspiration likely caused this decline, and SM has not recovered as of
2021, with future recovery unlikely under present climate conditions.”
It seems they are suggesting that the soil moisture loss is permanent unless the climate cools.
The study utilized satellite
data, sea level measurements, and observations of polar motion to derive the
soil moisture balance quantities. In another article in Science about the new
research, Permanent shifts in the global water cycle, Luis
Samaniego writes:
“How climate warming affects Earth’s hydrological
cycle—the continuous water movement between Earth and the atmosphere—is a key
question for managing water resources and making weather predictions.”
Soil drying can increase the
severity and frequency of droughts. Joint-lead author Prof Dongryeol Ryu,
professor of hydrology and remote sensing at the University of Melbourne,
explained to Carbon Brief:
“The unique thing we found through analysing these
larger-scale measures is that – even if we have seen widely fluctuating ups and
downs in precipitation and increasing temperature – the total water contained
in the soil, as soil moisture and groundwater, has been declining gradually
from around the beginning of this century.“
As long as the evaporative
demand remains high and rainfall doesn’t increase, the soil moisture cannot be
replenished. Since soil moisture loss is driven by both evaporative demand and
changes in rainfall, any changes in rainfall can alter soil moisture loss. It
is thought that the sharp drop observed in 2000-2002 was driven by drops in
global precipitation. The other main drop, observed in 2015-2016, can be
attributed to the 2014-16 El Niño event,
according to Ryu.
The use of a variety of data
sources, including ERA5’s land surface modelling system, corroborates results
and suggests that even though this water budgeting is correct, it is generally
difficult to lower error margins with global-scale datasets. According to Ryu,
the use of global average sea level rise and “Earth wobble” data in this
research is the innovative part of the paper, which offers corroborating
evidence that the loss of soil moisture is genuine, rather than a data
artifact. NASA’s Dr. Benjamin Cook cautions that while the paper offers
convincing evidence that there has been a loss of soil moisture, it does not
imply any specific causes since even natural variability in moisture could be
high over the very short time scales with which we have data.
Carbon Brief’s summary of the
research also notes the limitations of the modeling and the need for
improvements:
“The paper notes that land surface and hydrological
models require “substantial improvement” to accurately simulate changes in soil
moisture in changing climate.”
“Current models do not factor the impacts of
agricultural intensification, nor the ongoing “greening” of semi-arid regions –
both of which “may contribute” to a further decline in soil moisture, it states.”
The effects of dams and irrigation systems also need to be
factored in and quantified better.
In the peer review forum, the
following comment offers a summary of soil moisture loss and some possible
solutions, such as reforestation, water retention strategies, regenerative
agriculture, and soil conservation. Soil drying is also known as desiccation. (I took out the references for the sake of
brevity)
Reversing desiccation: cooler, moister, greener
Douglas Sheil Professor Forest Ecology and Forest
Management, Wageningen University & Research, Wageningen, the Netherlands
Pierre Ibisch Professor Econics Institute; Eberswalde
University for Sustainable Development, Germany
The Earth is drying. Seo et al. highlight an alarming
shift: while for most of the planet’s history, a warming climate brought a
wetter, greener world, it now brings desiccation. Our biosphere’s
water-regulating functions are broken.
While climate science and policy focus on greenhouse gases,
they often neglect vegetation’s role in keeping the planet cool and hydrated.
Forests, wetlands, and other ecosystems regulate temperatures and drive the
water cycle — but degradation has impaired these services. Feedbacks
from droughts, heatwaves, and declining vegetation now amplify local and
regional warming. Nonlinear responses risk abrupt shifts and catastrophic
tipping points.
Solutions become clear when we recognise water and
vegetation as partners in climate regulation. Protecting and restoring forests
and wetlands does more than sequester carbon — it rebuilds the processes that
keep landscapes cool, moist, and productive. Managing land to increase
infiltration, reduce runoff, and restore soil water storage helps sustain
transpiration and cool the land. We need to revive a “sponge
planet” and support place-based innovations like “sponge cities”
that enhance water retention where it's most needed.
Policymakers must act boldly to safeguard “green water”.
Land-use decisions must prioritise ecosystems that regulate moisture and
climate. Strong incentives are essential: those who degrade should pay; those
who protect and restore must be rewarded. The message is simple and urgent: a
cool, moist, green planet is our best defence against a drier, warmer world. It
remains possible. The time to act is now.
Moisture
uptake comes directly from soil (evaporation) or indirectly through vegetation
(transpiration). A February 2022 study in the Journal of Hydrometeorology
utilized the ratio of transpiration to evaporation across China to calculate
soil moisture loss or gain.
“The results show that across China, the ratio of
vegetation transpiration to soil evaporation has generally increased across
vegetated land areas, except in grasslands and croplands in north China.”
“Major contributions come from the increases in
vegetation transpiration over the semiarid and subhumid grasslands, croplands,
and forestlands under the influence of increasing temperatures and prolonged
growing seasons (with twin peaks in May and October). The future increasing
vegetation transpiration ratio in soil moisture loss implies the potential of
regional greening across China under global warming and the risks of
intensifying land surface dryness and altering the coupling between soil
moisture and climate in regions with water-limited ecosystems.”
Certainly, soil moisture
balance is complex and variable by region and locality. Changing rainfall
patterns seem to further complicate things. However, the new study suggests
that global water budgeting models can show and track global-scale soil moisture
changes.
References:
The
Increasing Role of Vegetation Transpiration in Soil Moisture Loss across China
under Global Warming. Mingxing Li, Peili Wu, Zhuguo Ma, Zhihua Pan, Meixia Lv,
Qing Yang, and Yawen Duan. Journal of Hydrometeorology. Volume 23: Issue 2. February
15, 2022. DOI: https://doi.org/10.1175/JHM-D-21-0132.1.
Page(s): 253–274.
Global
soil moisture in ‘permanent’ decline due to climate change. Carbon Brief. March
27, 2025. Global soil moisture in 'permanent' decline due to climate
change - Carbon Brief
Officials
sound alarm over soil collapse that could threaten food security for millions:
'We better be prepared earlier than later.’ Margaret Wong. The Cool Down. April
27, 2025. Officials sound alarm over soil collapse that could threaten
food security for millions: 'We better be prepared earlier than later'
Abrupt
sea level rise and Earth’s gradual pole shift reveal permanent hydrological
regime changes in the 21st century. Ki-Weon, Dongryeol Ryu, Taehwan Jeon, Kookhyoun
Youm, Jae-Seung Kim, Earthu H. Oh, Jianli Chen, James S. Famiglietti, and Clark
R. Wilson. Science. March 27, 2025. Vol 387, Issue 6741. pp. 1408-1413. Abrupt sea level rise and Earth’s gradual pole shift reveal
permanent hydrological regime changes in the 21st century | Science
Permanent
shifts in the global water cycle: Decades of terrestrial water-storage changes
reveal an irreversible decline in soil moisture. Luis Samaniego. Science. 27
Mar 2025. Vol 387, Issue 6741pp. 1348-1350. Permanent shifts in the global water cycle | Science
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