Droughts lasting decades that make the look like a picnic will begin to hit south-western and central regions of the US within the next 50 to 100 years. And the primary cause of the droughts will be global warming.
The impact on agriculture could be huge, say the researchers who made the predictions, and could trigger 鈥�water wars鈥� as farmers and large settlements squabble over water resources that become ever more scarce.
鈥淚nstead of lasting maybe 10 years, these mega-droughts will last 20, 30, maybe even 40 years,鈥� says Benjamin Cook at the NASA Goddard Institute for Space Studies in New York, who led the analysis.
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鈥淢ost importantly, they are droughts that, once started, will last a really long time, longer than ever seen in the US,鈥� says Cook. 鈥淚t鈥檚 not going to be transient either. There will be new average drier conditions.鈥�
Cook and his colleagues made their predictions after analysing over the past 2000 years, based on tens of thousands of tree-ring samples collected throughout the country. 鈥淚f there鈥檚 a wide ring, it was wet, and a narrow one shows it was dry,鈥� explains Cook.
Parched projections
The team used this to evaluate the probability of droughts and their likely severity over the next 100 years. Their work factored in 17 different climate projections, and compared two global warming scenarios 鈥� a 鈥渂usiness-as-usual鈥� scenario and one in which moderate action is taken to curb emissions of greenhouse gases like carbon dioxide.
To predict the severity of future droughts, the team also estimated soil moisture levels, and used the 鈥� which reflects how soil moisture content varies with rainfall and how much moisture the soil retains.
A PDSI score of zero means that soil moisture is in perfect balance. Positive scores indicate flooding or waterlogging, while negative scores indicate drought. 鈥淥nce you鈥檙e below -1, that鈥檚 a hard, serious drought,鈥� says Cook.
The analysis suggests things will get much worse than that before 2100 鈥� even with moderate action to reduce emissions. Under many of the 17 climate projections, PDSI scores are predicted to plummet inexorably downward to -3 for both the Central Plains and the Southwest US, with the latter particularly hard hit. Scores for soil moisture are equally precipitous, closely tracking the collapse in PDSI.
If the predictions are correct, the droughts will be worse than anything seen over the past millennium, including a 200-year period of intense drought between 1100 and 1300, called the Medieval Climate Anomaly.
鈥淭he paper sounds very plausible, and sounds an ominous warning,鈥� says Kevin Trenberth of the US National Center for Atmospheric Research in Boulder, Colorado. 鈥淚n the absence of moisture, drought is exacerbated, plants wilt, the risk of heatwaves increases and the odds of wildfires sky-rocket,鈥� he says.
Water = tight
And the key cause will be global warming. 鈥淚t will still rain, but what鈥檚 driving a lot of the drought is drying of the soils through evaporation,鈥� says Cook.
This makes sense, says Trenberth. 鈥淲hat is very robust is the expected increase in drying, through evapotranspiration, simply because there鈥檚 more heat available from global warming that has to go somewhere.鈥�
鈥淎s scientists develop better ways to uncover the occurrence of such events in distant history, while also improving models capable of projecting future climate conditions, both threads of research are saying the same thing鈥� that we are likely underestimating the risks,鈥� says Douglas Kenney of the University of Colorado at Boulder. 鈥淚t鈥檚 sobering news, but it deserves our attention.鈥�
Worst hit will be agriculture. 鈥淚t鈥檚 the dominant user of water in North America,鈥� says Cook. 鈥淚t may be necessary to give up on some areas, and there will be decisions about drought-resistant crops, and whether to import water from elsewhere,鈥� he says.
The omens from the current drought across much of the south-west are not good. Aquifers are being depleted at unsustainable rates, and surface reservoirs are already down to historically low levels. 鈥淢uch of the groundwater is non-renewable, so once you use it, it鈥檚 gone,鈥� says Cook. Likewise the snowpack on the Sierra mountains is only a quarter of its usual volume.
Conflict could mount over the water sources still available. It may mean coastal cities turning to solutions like desalination of seawater, which is highly energy-intensive.
鈥淭he question is, to what extent will these current strategies work with the much more serious droughts that are outside our experience?鈥� says Cook.
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