Consequently, the researchers warn the observed changes are likely to be \u201cpermanent\u201d if current warming trends continue.<\/p>\n
An accompanying\u00a0perspective article<\/a>\u00a0says the study provides \u201crobust evidence\u201d of an \u201cirreversible shift\u201d in terrestrial water sources under climate change.<\/p>\n The drying out of soil \u201cincreases the severity and frequency\u201d of major droughts, with consequences for humans, ecosystems and agriculture, explains\u00a0Dr Benjamin Cook<\/a>, an interdisciplinary Earth system scientist working at the\u00a0NASA Goddard Institute for Space Studies<\/a>\u00a0and\u00a0Columbia University<\/a>, who was not involved in the research.<\/p>\n He tells Carbon Brief:<\/p>\n \u201cDroughts are one of the most impactful, expensive natural hazards out there, because they are typically persistent and long lasting. Everything needs water \u2013 ecosystems need water, agriculture needs water. People need water. If you don\u2019t have enough water \u2013 you\u2019re in trouble.\u201d<\/p><\/blockquote>\n Contents<\/p> Every year, around 6tn tonnes of water cycles through Earth\u2019s land<\/a> surface. When rain falls on land it gets held up in soil<\/a>, wetlands, groundwater, lakes and reservoirs on its journey back to the oceans.<\/p>\n Soil moisture forms a critical part of the Earth\u2019s system, helping to irrigate soil, cycle nutrients<\/a> and regulate the climate.<\/p>\n The amount of water contained in the soil is sensitive to a range of factors, including changes in rainfall, evaporation, vegetation and climate \u2013 as well as human activity<\/a>, such as intensive agriculture.<\/p>\n The research points to a \u201cgradual decline\u201d in soil moisture levels in the 21st century, kickstarted by a period of \u201csharp depletion\u201d in the three years over 2000-02.<\/p>\n Specifically, the researchers find the depletion of soil moisture resulted in a total loss<\/a> of 1,614bn tonnes (gigatonnes, or Gt) of water over 2000-02 and then 1,009Gt between 2002 and 2016.<\/p>\n (For context, ice loss in Greenland resulted in 900Gt of water loss over 2002-06.)<\/p>\n Soil moisture has not recovered as of 2021, according to the research, and is unlikely to pick up under present climate conditions.<\/p>\n Joint-lead author\u00a0Prof Dongryeol Ryu<\/a>, professor of hydrology and remote sensing at the\u00a0University of Melbourne<\/a>, explains to Carbon Brief:<\/p>\n \u201cWe observed a stepwise decline [in soil moisture] twice in the past two decades, interspersed within a continuously declining trend in soil moisture. We haven\u2019t seen this trend earlier, so that is why this is very concerning.\u201d<\/p><\/blockquote>\n Ryu explains the decision to analyse changes to soil moisture on a global scale meant the researchers could confirm<\/a> trends difficult to see in smaller geographic datasets:<\/p>\n \u201cThe unique thing we found through analysing these larger-scale measures is that \u2013 even if we have seen widely fluctuating ups and downs in precipitation and increasing temperature \u2013 the total water contained in the soil<\/a>, as soil moisture and groundwater, has been declining gradually from around the beginning of this century.\u201c<\/p><\/blockquote>\n The maps below illustrate soil moisture changes in 2003-07 and 2008-12 against a 1995-99 baseline, as estimated by the\u00a0ERA5-Land<\/a>\u00a0reanalysis dataset. The areas marked on the map in brown saw a drop in soil moisture and the areas marked in blue an increase in soil moisture.<\/p>\n The top map shows soil moisture depletion across large regions in eastern<\/a> and central Asia, central Africa and North and South America over 2003-07. The lower map shows that \u201creplenishment\u201d in the years that followed occurred in relatively small parts of South America, India, Australia and North America<\/a>.<\/p>\n Ryu says the researchers \u201csuspect that increasing temperature played an important role\u201d in the decline in terrestrial water storage and soil moisture in the 21st century.<\/p>\n The study points to two factors driving<\/a> gradual depletion of soil moisture over the last quarter century: fluctuations to rainfall patterns and increasing \u201cevaporative demand\u201d.<\/p>\n Evaporative demand refers to the atmosphere\u2019s \u201cthirst\u201d for water, or how much moisture it can take from the land, vegetation and surface water.<\/p>\n Studies have highlighted how global evaporative demand has been\u00a0increasing<\/a>\u00a0over the last two decades globally, impacting water availability,\u00a0hurting crops<\/a>\u00a0and\u00a0causing drought<\/a>.<\/p>\n The new study notes that \u201cincreasing evaporative demand driven by a warming climate\u201d suggests a \u201cmore consistent and widespread trend toward drying as temperatures rise\u201d.<\/p>\n Ryu says the \u201cvery unusual\u201d drop in water moisture observed over 2000-02 could be attributed to low levels of rainfall globally, which coincided with the \u201cperiod when evaporative demand started increasing\u201d.<\/p>\n Another \u2013 less pronounced \u2013 period of rapid soil moisture decline seen over 2015-16 can be attributed to droughts triggered by the 2014-16 El Ni\u00f1o event, Ryu notes.<\/p>\n Ryu says the study findings indicate that soil moisture can no longer bounce back from a dry year, as it has in the past:<\/p>\n \u201cIt used to be that when precipitation goes up again, we recover water in the soil. But because of this increasing evaporative demand, once we have strong El Ni\u00f1o years \u2013 which lead to much less rainfall for a year or two \u2013 it seems that we are not recovering the water fully because of increasing evaporative demand. Because of that \u2013 even if we have a wet year following dry years \u2013 the water in the soil doesn\u2019t seem to recover.\u201d<\/p><\/blockquote>\n Measuring changes in global soil moisture has historically presented a challenge<\/a> to scientists, given the lack of comprehensive and direct observations of water in soil.<\/p>\n The researchers attempt to reduce this uncertainty by corroborating the ERA5-Land\u00a0reanalysis dataset<\/a>\u00a0from the\u00a0European Centre for Medium-Range Weather Forecasts<\/a>\u00a0(ECMWF) with three geophysical measurement datasets.<\/p>\n ERA5\u2019s land surface modelling system uses meteorological and other input data<\/a> to estimate water within the upper few metres of the soil.<\/p>\n These figures were compared with data collected by the Gravity Recovery and Climate Experiment (GRACE<\/a>) mission \u2013 a joint satellite mission between\u00a0NASA<\/a>\u00a0and the\u00a0German Aerospace Center<\/a>.<\/p>\n Running since 2002, the\u00a0GRACE mission tracks changes<\/a>\u00a0to the Earth\u2019s gravity by collecting data on groundwater depletion, ice sheet loss and sea level rise. These observations have revealed a persistent loss of water from land to the ocean.<\/p>\n The scientists also cross-reference the ERA5 reanalysis data with a\u00a0century-old dataset<\/a>\u00a0that measures fluctuations in the rotation of the Earth as the distribution of mass on the planet changes.<\/p>\n (The redistribution of ice and water, such as melting ice sheets and depleting groundwater, causes the planet to wobble as it spins and its axis to shift slightly. This is known as \u201cpolar motion<\/a>\u201d.)<\/p>\n The third set of measurements the scientists use is global mean sea level height, which is collected by satellites.<\/p>\n To extract soil moisture changes from this set of data<\/a>, the researchers subtracted other components of sea level rise from the overall total \u2013 including Greenland ice melt, Antarctica ice melt, the impact of increasing sea surface temperature (which expands water volume) and the contribution of groundwater.<\/p>\n This process of elimination left researchers with an estimate of the contribution of soil moisture to global<\/a> sea level rise.<\/p>\n The study notes that both the sea surface height and polar motion observations \u201csupport the conclusion that the abrupt change in soil moisture is genuine\u201d.<\/p>\n Ryu says using global average sea level rise and \u201cEarth wobble\u201d to track water redistribution on land<\/a> is the \u201cmain innovation\u201d applied in the paper.<\/p>\n He adds the value<\/a> of \u201creverse engineering\u201d the ERA5 dataset is to understand how to enhance land surface modelling in the future:<\/p>\n \u201cBy explaining all the contributing factors to this measurement, you can understand the process. And if you understand the process, you can actually predict what\u2019s going to happen in the future if any of these factors change in a certain manner.\u201d<\/p><\/blockquote>\n NASA\u2019s Dr Cook says the \u201ccorroborating evidence\u201d supplied by the paper offers a \u201creally strong case that there has been a large-scale decline in soil moisture in recent decades\u201d.<\/p>\n However, he says the relatively short reference period of the study means that identifying the cause of the decline is less clear cut:<\/p>\n \u201cWhether [the decline] is permanent or not\u00a0 is much more uncertain\u2026On these timescales, internal natural variability can be really, really strong. Attributing this decline to something specific \u2013 either climate change<\/a> or internal variability \u2013 is much much more difficult.\u201d<\/p><\/blockquote>\n A notable finding in the study\u2019s sea level rise<\/a> analysis is that terrestrial water storage may have been the dominant driver of sea level rise in the early 21st century.<\/p>\n Specifically, the paper notes that the decline in terrestrial water storage over 2000-02 \u2013 when soil moisture plummeted \u2013 led to global average sea level rise of almost 2mm annually.<\/p>\n The researchers note<\/a> this rate of sea level rise is \u201cunprecedented\u201d and \u201csignificantly higher\u201d than the rate of sea level rise attributed to Greenland ice mass loss, which they note is approximately 0.8mm a year.<\/p>\n Prof Reed Maxwell<\/a>, a professor at the\u00a0High Meadows Environmental Institute<\/a>\u00a0at\u00a0Princeton University<\/a>, who was also not involved in the study, says the researchers\u2019 efforts to compare soil moisture with other global water stores was \u201cnovel\u201d and \u201copens the door to future study of a more holistic global water balance\u201d.<\/p>\n The paper notes that land surface and hydrological models require \u201csubstantial improvement\u201d to accurately simulate changes in soil moisture in changing climate<\/a>.<\/p>\n Current models do not factor the impacts of agricultural intensification, nor the ongoing \u201cgreening<\/a>\u201d of semi-arid regions \u2013 both of which \u201cmay contribute\u201d to a further decline in soil moisture, it states.<\/p>\n Writing in a perspectives article published in\u00a0Science<\/a>,\u00a0Prof Luis Samaniego<\/a>\u00a0from the department of computational hydrosystems at the\u00a0Helmholtz Centre for Environmental Research<\/a>\u00a0says that it is \u201cessential\u201d that next-generation models incorporate human-caused influences such as farming, large dams and irrigation systems.<\/p>\n The study posits that the \u201cinnovative methods\u201d for estimating changes in global<\/a> soil moisture presented in the study provide opportunities to \u201cimprove the present state of modelling at global and continental scales\u201d.<\/p>\n More broadly, advances in scientific understanding of changes to soil moisture can help improve the world\u2019s preparedness for drought.<\/p>\n Drought is often described as a \u201ccreeping disaster<\/a>\u201d \u2013 because by the time it is identified, it is usually already well under way,<\/p>\n Paper author Ryu explains:<\/p>\n \u201cUnlike a flood and heatwaves, drought comes very very slowly \u2013 and has prolonged and delayed consequences. We better be prepared earlier than later, because once drought comes you can expect a long period of consequences.\u201d<\/p><\/blockquote>\n Dr Shou Wang<\/a>, associate professor at the Hydroclimate Extremes Lab and the\u00a0Hong Kong Polytechnic University<\/a>, who was not involved in the study, says the research findings are \u201ccrucial\u201d for advancing understanding of the \u201cpotential drivers and dynamics\u201d of \u201cunprecedented hydrological extremes in a warming climate\u201d. He tells Carbon Brief:<\/p>\nDrying soil<\/span><\/h2>
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Climate change<\/span><\/h2>\n
Cross-validation<\/span><\/h2>\n
Sea level rise<\/span><\/h2>\n
\u2018Creeping disaster\u2019<\/span><\/h2>\n