Thursday 25 June 2015

The drought in the western US

Century of Water Shortage Ahead? Lake Mead Drops Below Rationing Line For First Time in Its History.

24 June, 2015
1075 feet. That’s the water level Lake Mead must stay above before mandatory multi-state water rationing goes into effect. A level just 25 feet above the highest intake pipe used to supply cities across the Desert Southwest. Last night water levels at the key national water storage facility fell below that hard line to 1074.99 feet — a record low never before seen in all of its history.
lake meade water levels
(Lake Meade water levels hit below the 1075 hard line yesterday, the lowest level ever recorded. Image source: Lake Mead Water Data.)

If water levels remain below the 1075 foot mark through January of 2016, then a multi-state rationing will go into effect (with most acute impacts for Arizona and Nevada). A rationing that will have serious consequences for desert cities across the Southwest, cities like Las Vegas which rely on Lake Mead for so much of their water.
Despite Lake Mead hitting the 1075 hard line, it appears that rationing may be forestalled through 2016. It’s a silver lining of all the severe summer storms that have rolled through the Colorado River Basin this spring and summer — pumping up water flows to Lake Mead and Lake Powell. A flush of much needed moisture that will, hopefully, prevent water rationing from going into effect during 2016. But prospects for the future, despite this temporary respite, are starting to look a bit grim.
Risk of Future Megadrought

The trend set in place by a human-forced warming of the Desert Southwest has resulted in an increasing number of dry years. The added heat forces water to evaporate more rapidly. So even when it does rain an average amount, moisture levels still fall. The result is not only an increase in single year droughts, but an increased risk of decadal droughts (called megadroughts).

As the years progress and more of the impacts of human-forced global warming become apparent, the drought impacts and severe drought risks are only expected to rise. For according to a recent Cornell University report (2014) the chance of a 10 year drought for the US Southwest under a moderate warming scenario (RCP 4.5) is 50% this century (greater for states like Texas, Oklahoma, New Mexico, Arizona, and Nevada — see graphic below). The chances of a 30 year drought range from 20-50 percent depending on the severity of the human greenhouse gas emission.

Megadrought risk

(Risk of an individual State experiencing a 10 year or longer drought as a result of global warming due to human fossil fuel emissions over the course of the next century. Note that Lake Meade watershed states show the highest risk for periods of terrible drying. Image source: Southwest May Face Megadrought this Century.)
Toby Ault, lead author of last year’s Cornell Paper noted:
For the southwestern U.S., I’m not optimistic about avoiding real megadroughts. As we add greenhouse gases into the atmosphere – and we haven’t put the brakes on stopping this – we are weighting the dice for megadrought conditions.”
For reference, the current historic California drought is, so far, a four-year affair. So, as difficult and damaging as that drought has been, a 10 year or a thirty year drought may be seem comparatively unimaginable by today’s standards. In other words, though it’s been rather dry for the U.S. West over the past 15 years, an impact likely already worsened by human-caused climate change, we haven’t seen anything yet.
Early Warning and A Call For Necessary Action

In addition to increasing drying and severe drought risks, growing cities throughout the U.S. West have put greater and greater strains on water stores like Lake Mead. As a result of the combined human-forced drought and increased water consumption, levels at Lake Mead have been dropping since 1999. Back then, water levels averaged around 1200 feet. And since that time we’ve seen an average 8 foot drop each year.
It’s a trend that, unless it changes, will almost certainly mean water rationing in 2017, 2018 or 2019, if not 2016.  Water resource officials are notably concerned. Water-policy manager Drew Beckwith of Western Resource Advocates noted in USA TODAY:
This is the check-engine light. It really does (make critical) the fact that we have to start changing.”

And that’s absolutely true. We need to change how we manage and use water in the US Southwest and we need to do absolutely everything we can to prevent as much future warming as possible to reduce the risk and intensity of the future megadroughts that are a likely upshot of human-forced warming. The crossing of the 1075 line for Lake Mead yesterday should thus be viewed as a climate change shot across the bow. If we want to husband our resources wisely, we need to look both toward conservation and toward making certain conditions do not spiral beyond even the ability of responsible resource management to make a difference. That’s the basic lesson of climate change — there are simply some conditions that are impossible to adapt to. And the goal of every rational person should be to do everything possible to prevent and reduce the intensity of those conditions. The water security of the U.S. Southwest depends on it.
Hat Tip to Andy in San Diego

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