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How climate change spurs megadroughts

Posted on 5 September 2022 by Guest Author

This is a re-post from Yale Climate Connections by Shannon Osaka

On an afternoon in late June, the San Luis Reservoir – a nine-mile lake about an hour southeast of San Jose, California – shimmered in 102-degree heat. A dusty, winding trail led down into flatlands newly created by the shrinking waterline. Seven deer, including a pair of fawns, grazed on tall grasses that, in wetter times, would have been at least partially underwater. On a distant ridge, wind turbines turned languidly. 

That day, the reservoir, California’s sixth-largest and a source of water for millions of people, was just 40% full. Minerals deposited by the receding waters had turned the reservoir’s lower banks white, like the rings on a bathtub. Discarded clothing, empty bottles, and a lone shoe sat scattered across the newly exposed, parched ground. An interactive graphic in the visitor’s center reported that this year’s snowpack – which provides the water that travels from the Sacramento River Delta into the reservoir itself – was zero percent of the yearly average. 

Depending on how you look at it, California – and most of the American West – has either entered its third catastrophic drought of the past 10 years, or has been in a constant, unyielding “megadrought” since 2000. Reservoirs are emptying; lawns are turning brown; swaths of farmland that have coaxed lettuce, almonds, and alfalfa out of the dry ground for decades are going fallow. The Colorado River, which originates in the snow-capped Rocky Mountains and provides water to some 40 million people in the Southwest, has slowed to a trickle. That waterway also feeds the largest reservoir in the United States, Lake Mead, 40 miles east of Las Vegas, which in recent months has seen water levels so low that bodies have emerged from its shrinking, normally crystalline waters. The Bureau of Reclamation, the federal agency responsible for many supersized water projects, has asked states to cut their use of water from the Colorado River by 2 to 4 million acre-feet, an amount close to all the water that California receives from the Colorado in a single year.

Throughout the West, anxiety about drought is as palpable as the dryness of the air; talk of water fills newspapers and conversations alike. “Aridification kills civilizations. Is California next?” read one Los Angeles Times headline in June. In February, scientists confirmed that the current, decades-long “megadrought” is the worst in 1,200 years. They also confirmed that rising temperatures - driven by human consumption of fossil fuels - were partly to blame. 

In one sense, the climate change link seems obvious. Since 1850, global temperatures have climbed 1.2 degrees Celsius (2.2 degrees Fahrenheit); in areas of the U.S. hit hardest by drought, the increase is even higher. Temperatures in California have risen about 3 degrees F since 1896; in Arizona, they have gone up by 2.5 degrees.

But the connection between climate change and drought is not as straightforward as it seems. Some areas are likely to get wetter while others get drier. Still others may accumulate the same total rainfall, but in inconsistent patterns: More rain might fall in fewer, more intense bursts, followed by longer dry spells. “It’s complicated,” said Benjamin Cook, a climate scientist at NASA and the Lamont-Doherty Earth Observatory. 

But scientists can say some things with certainty. As the world gets hotter, soils are getting drier; it takes more and more precipitation to water the same crops and fill the same reservoirs. Rising temperatures, therefore, are digging the American West and other arid regions into a deeper and deeper hole. The more the world warms, the more rain will be needed to compensate, and that will force people to rethink how - and where - they will live and eat when the water dries up. 

One problem with linking drought and climate change is that there is little agreement on what drought actually is. “No two people - including no two scientists - really agree on even how to define drought,” said Daniel Swain, a climate scientist at the University of California, Los Angeles. A drought, in its most general sense, is simply a lack of water relative to some long-term average - but where that dearth of water appears can change how the drought is defined, studied, and managed. Climate scientists and meteorologists talk about “meteorological drought” (a lack of rainfall), farmers worry about “agricultural drought” (a lack of soil moisture), and water managers try to avoid “hydrological drought” (a lack of groundwater or water in reservoirs). 

, left image
, right image

Satellite images from NASA and the US Geological Survey's Landsat satellites show Lake Mead in 1972 (top) and Lake Mead in 2021 (bottom).

This complexity has resulted in conflicting messages about the role of human-caused global warming in the droughts that have ravaged the American West and the rest of the world. Thanks to the science of extreme event attribution, which connects weather extremes to global warming, it has become commonplace to cite climate change as a factor in devastating heat waves or torrential floods. But droughts are trickier. Drought depends on both the rain that falls and how quickly it is evaporated and used.

On the rainfall side of things, climate change’s influence in California, Nevada, Arizona, and other Western states remains murky. In recent years, rain and snowfall in California have become more variable; the dry years are drier, the wet years wetter. In 2017, Lake Oroville served as a sobering illustration of this whiplash when - in the span of less than four months - the reservoir north of Sacramento went from less than half full to nearly overflowing, causing the main spillway to collapse. Some 188,000 local residents were evacuated. Swain and his colleagues estimate a 25- to 100-percent increase in such “extreme dry-to-wet precipitation events” in California over the next century. 

But even with this volatility, total precipitation in the West is expected to stay roughly the same. Swain said scientists expect the Pacific Northwest to get somewhat wetter; Arizona and New Mexico somewhat drier. The clearest link between drought and climate change right now, therefore, is not a lack of rainfall - it’s rising temperatures. 

The atmosphere is like a sponge: It sucks up water from soils, plants, rivers, oceans, and lakes. Any time rain falls, some of it will evaporate, returning back into the sky before it can be piped into homes, fields, or aqueducts. Scientists have a measure for how “thirsty” the atmosphere is, or how much water the sky absorbs: evaporative demand. As temperatures go up, evaporative demand increases. The sky gets thirstier.

“A very basic rule is that if you’re going to have a warmer atmosphere then you need more precipitation to compensate,” said Park Williams, a hydroclimatologist at the University of California, Los Angeles. “If you turn the heater up in your house and you don’t give your plants extra water, you see the same thing.” 

Christine Albano, an ecohydrologist at the Desert Research Institute in Reno, Nevada, studies evaporative demand and how it might change under global warming. “A warmer atmosphere can hold more water,” she explained. And, she added, the changes are nonlinear - a small change in temperature could lead to a much larger change in how thirsty the sky is. In a paper published earlier this year, Albano and her co-authors found that evaporative demand has increased over the past 40 years, most dramatically in the U.S. Southwest around the Rio Grande River. In that region, evaporative demand increased by 8 to 15% - meaning that the area would require 8 to 15% more rainfall to maintain the same water levels. 

And as temperatures warm, the situation will get even worse. “For every raindrop, we’re going to get less of that going into our streams and rivers,” Albano said. 

That thirsty atmosphere has been behind most of the studies that have found a clear link between global warming and persistent droughts. The last catastrophic drought in California, which stretched from 2011 to 2017, drained reservoirs and forced farmers to pump groundwater from the state’s disappearing underground aquifers. Some scientists looked for a direct link between climate change and the lack of rainfall but did not find convincing evidence. Those who looked at the effect of temperature on soil moisture and general aridity, however, found something more interesting: that human-caused climate change had turned what would have been a more moderate drought into a devastating one. In a paper published in 2015, Williams, Cook, and others found that skyrocketing temperatures, brought on by human-caused global warming, had made the drought 15 to 20% more intense.

Similar results have been found all over the globe. A few years ago, scientists analyzed the European drought of 2016 to 2017 - which helped spark deadly wildfires in Portugal - and found that it had been made worse by high evaporative demand. To the south, the Horn of Africa has been ravaged by a series of droughts over the last decade, causing successive crop failures and threatening millions with severe hunger and starvation. In 2015, scientists searching for ties to climate change found no connection to the region’s low rainfall. They did, however, find a link between rising greenhouse gas emissions and the high temperatures that have helped to desiccate the landscape of Kenya, Somalia, and Ethiopia. 

Temperature has also been implicated in the study of the decades-long “megadrought” in the American West, a loosely defined term that has been used to indicate droughts that last two decades or more. Scientists have spent decades drilling holes in trees to collect tree ring records, a science known as dendrochronology, which can be used to estimate soil moisture levels going back for millennia. (Some records have even been collected from ancient wooden ladders in the cliff dwellings of Chaco Canyon.) According to those records, 19 of the last 23 years were drier than the average over the past millennium. 

Williams, the UCLA scientist, says that this megadrought is being made even worse by climate change. “Forty percent of the severity of the drought conditions in this megadrought is attributable to human-caused climate trends,” largely from rising temperatures, Williams said. 

Sixty percent of the megadrought, Williams cautioned, could simply be seen as simply bad luck; even without humans burning fossil fuels, megadroughts have endured for decades in the past, starving the landscape and local species of water. But what was previously just bad luck is now getting a boost from climate change. “It’s only going to get warmer,” Williams said. “It’s going to take more and more good luck to bail us out of drought - and less and less bad luck to fall back in.” 

Climate change is also undermining one of the American West’s most treasured tools for managing drought: snowpack. In the Sierra Nevadas of California and in the Colorado Rockies, snow falls during the winter and then acts as a natural reservoir, slowly releasing water as it thaws during the hot, dry summer season. But as temperatures rise, more precipitation is falling as rain instead of snow, and any remaining snow is melting more quickly and earlier in the season. By 2050, scientists estimate that the mountains of the Western U.S. will lose around 25% of their snowpack. In 60 years, they warn, there may be no snowpack at all.  

And as the planet heats up, megadroughts such as the one raging in California, Arizona, and New Mexico are expected to return again. And again. According to one study by Cook, the NASA scientist, and others, the risk of a 35-year-long drought hitting the American Southwest was less than 12% between 1950 and 2000. But if countries fail to take aggressive action to combat climate change, and the world continues to warm, the risk of such a drought will climb to more than 80%. 

Landsat satellite images of Lake Powell.These Landsat satellite images of Lake Powell, a reservoir on the Colorado River in Utah and Arizona, were taken in the spring when lake levels are traditionally at their lowest before mountaintop snow begins to melt and run into the watershed. The images capture years with the two highest and lowest levels over the past 22 years.

The American West is built on a strange, hodgepodge system of water that, for the last century, has somehow sustained millions of residents in the most arid parts of the country. Reservoirs, dams, and aqueducts carry water from where it is plentiful - the peaks of the Sierra Nevadas, the banks of the Colorado River - and deliver it to where it is scarce: fast-growing metropolises like Phoenix, Salt Lake City, and Los Angeles. In California, 75% of the state’s rain and snow falls north of Sacramento, but 80% of its water demand comes from the southern two-thirds of the state. This imbalance is corrected artificially: A long cement aqueduct carries water from the north of the state to the south, shuttling through the dry, crackling Central Valley. More comes from the Colorado River, which brings water from the east to Los Angeles and Southern California. 

This system has faced numerous droughts before. In dry times, policymakers call for cutbacks and march down the list of water rights-holders and inform each how their supply will be curtailed. The last big drought in California, which reached its peak in 2014 and 2015, saw residents “drought-shaming” one another for maintaining lush lawns (Los Angeles Mayor Eric Garcetti called such shaming a civic duty) and an enormous backlash against almond growers, after news broke that it takes a gallon of water to produce a single almond. 

But the sheer longevity of the current dry period has even the most experienced water managers worried. That complex system of dams, aqueducts, and reservoirs that funnels water to Western states for lawns, golf courses, and farms is cracking under the strain. “We built these amazing places based on the promise of water,” said John Fleck, a professor of water policy and governance at the University of New Mexico. “And they’re good things - I don’t want to demonize what we did. But they were based on the promise of water that wouldn’t be there.” 

California aqueductThe California Aqueduct - pictured here in 2010 near Palmdale - carries water from Northern California's mountains to communities in the south. (Photo credit: Ben+Sam / CC BY-SA 2.0)

To be sure, the current drought and even the overlapping, decades-long “megadrought” will eventually end. “I don’t expect it to be as dry as it has been the past few years forever,” Williams said. But the slow-moving disaster has demonstrated just how shaky the West’s foundation is. And it is a warning that the water system of the present may not hold for the future. 

What will happen next? Nearly 40 million people live in California alone; another 12 million reside in New Mexico, Arizona, and Nevada. And in the wake of the pandemic, southwestern states are growing fast, as people look for more affordable housing, strong job markets, and warmer weather. But that warmer weather has a darker side. Not far from Phoenix, Arizona - one of the fastest-growing cities in the U.S. - one community is already running out of water. As the Colorado River and the snowcaps of the Sierra Nevadas continue to dry up, the water flowing to the West’s sprawling suburbs and millions of acres of farmland will slow to a crawl. When that happens, communities will need to adapt. Agricultural water use will have to decline - even if that means destroying livelihoods that have continued uninterrupted for decades. Lawns will dry up; lush golf courses will disappear. The very character of the West - and of many arid parts of the globe - will be transformed. “In some ways it’s really simple,” Fleck said, of the climate-changed drought future. “The West will be less green.”

This piece, first posted at Grist.org, is reposted here through the Covering Climate Now collaboration.

This story is part of the Grist series Parched, an in-depth look at how climate change-fueled drought is reshaping communities, economies, and ecosystems.

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Comments

Comments 1 to 16:

  1. The connection between drought intensity and global warming is clear.  However droughts start and end naturally and they should mention that there has been persistent La Nina since 1999.  There have been four major La Nina and one major (super) El Nino.  From 1980 to 1998 the numbers were reversed: four major El Nino and one major La Nina.

    While persistent La Nina creating the present drought in the western US, it may be also be true that persistent El Nino and it's excess moisture masked the drying effects of global warming in 80's and 90's.  Possibly separate from ENSO (not sure), there was a lack of monsoon rains starting in 2000. There was substantial monsoon this year which tempers evaporation in some locations.

    Lawns will dry up; lush golf courses will disappear. The very character of the West - and of many arid parts of the globe - will be transformed

    I would say "revert" and mostly for the better.

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  2. Eric,

    Please share more details of the basis for your comments regarding El Nino and La Nina. The presentation of the history of El Nino and La Nina by Jan Null (here) based on the NOAA ONI evaluations does not appear to show the dramatic difference before and after 2000 that you infer. I am also curious about the choice of 2000 for the dividing year and the limit of 1980, note the significance of La Nina in the 1970s).

    Also, what is the basis for the claim about monsoons. My understanding is that changes of monsoon timing and intensity affect agriculture that has developed in monsoon affected regions (earlier or later can be a problem, significantly more or less rain can be a problem).

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  3. Some analysis of whether ENSO has changed here. "There is no clear evidence that any changes since 1950 in ENSO are all that unusual." Even more so if proxy records are used. Also, it is incredibly unclear how global warming will change ENSO. "But regardless of any changes in ENSO sea surface temperatures, in intermediate to very high GHG scenarios, it is very likely that rainfall variability over the east-central tropical Pacific will increase significantly (4). Basically, we may expect El Niño to be wetter in this region and La Niña may be drier. "

    That prediction seems to be holding.

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  4. Review the last half dozen or so "Hydrology, hydrometeorology & climate change" sections of our weekly new research feature to get an initial grip on the complexity of the topic, Eric. "It's not that simple."

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  5. Thanks for the feedback and suggestions.  I want to clear up the dates for weaker monsoon since 2000 and La Nina since 1999.  They are not related as far as I know, but they could be.  The Pacific is cooler during La Nina and it is one of the two monsoon moisture sources for the SW US monsoon.  Could be related in some other ways.

    Monsoon weaker starting in 2000: NWS Phoenix monsoon data. Predominant La Nina starting in 1999: MEI Time Series (NOAA) from psl.noaa.gov/enso/mei/ Note that the monsoon statistics are only Phoenix AZ so a somewhat limited depiction.   Also note that the MEI is one of several ENSO indexes.  The Jan Null link provided above uses Nino 3.4 ocean temperatures.  The regime change from El Nino (red in MEI link) to La Nina (blue in MEI link) doesn't show up nearly as much in the Nino 3.4 data.

    I acknowldege my data is limited.  But I believe drought is a natural occurrence affected by global warming.  Drought will be more intense since warmer air holds more moisture from evaporation and transpiration.  It can start sooner and end later for the same reason.

    The climate.gov ENSO change analysis is very helpful.  I believe however they should say the "switches" are affected in various ways by warming.  It's not particularly useful to use "climate change" to describe cause and effect since climate change (not further defined) can be either cause or effect.  The increase in amplitude is notable but their caveat is noted.   I agree that the modeling is difficult and adding warming doesn't make it any easier.  I think that added warmth doesn't necessarily change the sign of ENSO but wil enhance precipitation as they note, without adding much drought over the ocean (except possibly drier in the mid and upper levels).  That could increase ENSO intensity since it is primary driven by precipitation.

    I'll look at the hydrology material next.

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  6. Sorry, left out the link: Google Scholar Link. Multiyear La Nin ̃a events and persistent drought in the contiguous United States

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  7. Germane by topic and extra useful thanks to its nature and hence collection of citations, Eric:

    A history of ENSO events since AD 1525: implications for future climate change

    Actually, not only Eric ought to take a look. We've got hydrometeorological problems. "People gotta eat." Breakfast, lunch and dinner are under threat. 

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  8. Doug @7,

    That link did not work for me.

    The link (here) got me to a page where a pdf of "A history of ENSO events since AD 1525: implications for future climate change" could be downloaded for Free.

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  9. Doug's original link @ 7 also got me to an "Oops. It looks like you are in the wrong aisle" page. (Custom error 404 page...)

    OPOF's link works for me.

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  10. Doug Bostrom,

    The potential ENSO problem is indeed a significant concern, and is well summarized in the following quote from the abstract:

    "Although extreme ENSO events are seen throughout the 478-year ENSO reconstruction, approximately 43% of extreme and 28% of all protracted ENSO events (i.e. both El Niño and La Niña phase) occur in the 20th century. The post-1940 period alone accounts for 30% of extreme ENSO years observed since A.D. 1525. These results suggest that ENSO may operate differently under natural (pre-industrial) and anthropogenic background states. As evidence of stresses on water supply, agriculture and natural ecosystems caused by climate change strengthens, studies into how ENSO will operate under global warming should be a global research priority."

    Of course, a major part of the problem is the powerful developed bias against any research that is considered to:

    • be less likely to produce short-term economic benefits than other research
    • potentially require external governing to limit harm done by desired activity that is beneficial to some but harmful to others (popular or profitable among beneficiaries who are able to compromise 'governing actions' - misguiding research funding and other ways of penalizing 'Institutions of Learning' to make them more compliant 'pursuers of desired types of learning')
    • potentially require the rapid ending of a developed popular or profitable activity by external governing and penalty to bring about a more rapid ending of the activity to limit the total harm done.

    Undeniably climate change research unintentionally produced these 'learning and research fears and threats' triggering a massive harmful aggressive response from people who like to benefit from:

    • harmful unsustainable beliefs and actions
    • a lack of awareness of the harm done
    • excusing the harm done because of the benefits obtained
    • claiming that the future is always better so future generations will naturally create amazing improvements in spite of the harm being done to the ability of others, especially future generations, to live better lives.

    Human willingness to be harmfully misled, or excuse and ignore the harm done, puts more than "Breakfast, lunch and dinner are under threat".

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  11. The second and third figures in the post show pretty dramatic changes. I have visited both areas. The satellite images don't really show the full drama of the change that you see from ground level, though.

    The first of the two is just east of Lost Wages Las Vegas. I visited the area in 2009, and the drop in water level was very dramatic. Quite the sequence of bathtub rings.

    Lake Mead in 2009

     

    The second satellite image is from the top of Lake Powell. The Colorado River flows from east to west across the image, and enters Lake Powell at Hite. Or, what used to be Hite, which disappeared under the lake when it was formed. If you look on Google Earth, you can still see an abandoned marina area marked, which was shut down in the mid-2000s due to low water levels. Google Earth historical images are also quite interesting.

    Highway 95 crosses the Colorado River just before the bend in the satellite image. I visited there in 2008, and there was enough water to provide for some lush vegetation. Not so lush now.

    ColoradoRiver at Hite 2008

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  12. Thanks for the reference to the "History of ENSO..." and for the link that worked for me too.  The paper references some of the same authors earlier work (e.g. Stahle 1998) for part of the reconstruction: Google Scholar for Stahle 1998. In that 1998 paper they say: 

    The reconstruction model relies heavily (though not exclusively; Table 1) on the tree-growth response in subtropical North America to wet conditions during El Nino years and dry conditions during La Nina years. However, this model does not uniquely fingerprint ENSO extremes because other circulation patterns can bring drought or wetness to the Texas-Mexico region

    It is complicated and the correlation is being used for a reconstruction, not a study of North American drought or causation.   As Stahle 1998 points out the drought and wetness has other causes.

    I looked through some of the references from History of ENSO and found there were suggestions of more intense and/or prolonged ENSO as early as 1996 (Trenberth and Hoar):

    The late 20th century contained a number of extreme and prolonged ENSO episodes (Trenberth and Hoar 1996), including the two most intense El Niños (1982– 1983 and 1997–1998) and La Niñas (1988–1989, 1973–1974).

    I think the evidence for prolonged and more intense ENSO cycles is stronger now.   Some papers that cite the History of ENSO paper and use that same history to study other drought and rainfall effects.  One is Multicentury Evaluation of Recovery from Strong Precipitation Deficits in California which looks at the role of very strong El Nino (e.g. 2016) in drought recovery (2012-15 drought) in California.   Note that paper does not look at what role if any that La Nina had, just the role of strong El Nino in drought recovery.

    We know from further experience that the 2016 recovery didn't last long.  Any effect of global warming on the sign of ENSO will be important if ENSO is an important factor in drought and drought recovery.  Also prolonged La Nina like the current one will be problematic if it causes drought and is prolonged by global warming.

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  13. The loss of volume in lake meade is one of the most worrying results of climate change that I have witnessed.  While storms are indeed getting stronger, we as humans are able to willfully dismiss this as natural and with a mindset of storms happen.  In a sense, we can bury our head in the sand because we can say storms come and go, but life goes on.  However, with lake meade disappearing, it makes it extremely difficult to do this.  The lake is going but its not coming back and most of all, life won't go on without water.  The droughts that are related to climate change has to start somewhere, and to me atleast, it appears like it has started there.

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  14. A recent article over at CNN gives an interesting discussion of the issues of water management in the Colorado River basin. Talks mostly about Lake Powell and Flaming Gorge reservoirs, but Lake Mead gets a mention. It's a basin-wide management issue.

    https://www.cnn.com/2022/09/16/us/colorado-river-water-lake-powell-flaming-gorge-climate/index.html

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  15. Bob Loblaw @14,

    That CCN item provides an interesting presentation of the ways that the water of the Colorado Basin are managed.

    The following NPR presentation from August 27, 2022 that explains that a major part of the problem is a lack of collaboration among the users of water in the Colorado Basin.

    7 states and federal government lack direction on cutbacks from the Colorado River

    It is like a regional version of the ways that different global groups of people who benefit from fossil fuel use fight, any way they can get away with, to maximize 'their benefit' in the face of the undeniable need to curtail global fossil fuel use to limit the harm done to the future of humanity.

    In the case of the lack of agreement to reduce water use from the Colorado Basin the Federal Government will likely end up being the "Bad Meany" imposing harmful restrictions on the water users whose leadership wouldn't agree to cut back their use.

    In the global fossil fuel case, external restrictions will also likely be required (like trade tariffs or other penalties for 'persistently harmful groups'). And some populist politicians in populations containing significant numbers of people who deserve to be penalized will rally their 'victims of persecution' to fight to obtain more personal benefit from harmful fossil fuel use.

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  16. OPOF:

    Yes, that NPR story is interesting, and jives with some of the other stories I've read recently on CNN. It's a classic "tragedy of the commons" scenario: nobody wants to be the one that goes without, and as long as everyone keeps trying to take as much as they can, everyone will lose in the end. Eventually, Mom and Dad need to decide who gets the window seat, and for how long.

    If the taps run dry in the U.S. SW, it will be difficult to pretend it isn't happening. Agriculture is a huge water user, and the effects of dropping agricultural productivity will extend well beyond the U.S. SW. What happens in Vegas will not stay in Vegas.

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