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What Happens When the World Dries Out

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By Tim Radford, Climate News Network

LONDON – A warmer, drier world will be bad news for those people who already live on the edge. Higher temperatures will do more than evaporate the soil moisture: they will alter the natural soil chemistry as well.

African bush elephants in Sumburu National Reserve, Kenya. How much aridity can they tolerate?
Credit: Vicente Polo via Climate News Network

Manuel Delgado-Baquerizo of the Universidad Pablo de Olavide, in Seville, Spain, and fellow scientists report in Nature that they looked at soil samples from 224 dryland ecosystem plots in every continent except Antarctica.

Drylands matter: they account for more than 40 percent of the planet’s land surface and they support more than 38 percent of its population. Drylands add up, in the dusty language of science, to the largest “terrestrial biome” of all.

And even though on average more warmth will mean more evaporation, and therefore more water vapor in the atmosphere and more precipitation in some of those zones that already have ample rainfall, the pattern could be different in the arid lands.

All the calculations so far indicate that these drylands will increase in area, and become drier with time. Already 250 million people are trying to scrape an increasingly meager living from lands which are degrading swiftly, either because they are turning to desert, or because they are overgrazed.

Hard on microbes

But to make things worse, climate scientists predict that between 2080 and 2099, soil moisture will decrease by between 5 percent and 15 percent worldwide. And that in turn could have a profound effect on the levels of carbon and nitrogen nutrients naturally in the top soils.

What keeps soils alive, and productive, is the compost or humus of leaf litter, animal dung, withered roots and other decaying vegetation in the first meter or so of topsoil: this in turn feeds an invisible army of tiny creatures that recycle the nutrient elements for the next generation of plant life.

But these microbes also need water to thrive. The consortium of researchers predicted that as the soils got drier, biological activity would decrease, but geochemical processes would accelerate. That is, nutrients that depended on little living things in the soil would drain away, but other elements – phosphorus among them – would increase, because they would be winnowed from the rock by mechanical weathering or erosion.

The research team tested this argument with samples from 16 countries, including the Negev desert in Israel, the woodlands of New South Wales in Australia, the Altiplano of Peru, and the Pampas lowlands of Argentina.

A dryland ecosystem in Peru that was one of the sites sampled by researchers in their global study of sites in 16 countries.
Credit: David Eldridge

Balance upset

These regions could all expect from 3.9 inches of rainfall a year to 31.5 inches; all soil samples were analyzed in the same laboratory in Spain.

And as predicted, they revealed an increasing imbalance: more phosphorus, less carbon and nitrogen as they became drier. Such a trend would actually feed back into global warming: ideally, more vigorous plant growth would absorb more carbon dioxide.

But if vegetation wilts, and soils turn to dust over large areas of already parched land, then the carbon dioxide levels in the atmosphere will increase even more.

“Plants need all of these elements, in the correct amounts, and at the right times, but increasing aridity will upset this balance, leading to a breakdown in essential soil processes,” said David Etheridge, of the University of New South Wales, one of the authors.

“As the world’s population grows, people will increasingly rely on marginal lands – particularly drylands – for production of food, wood and biofuels. But these ecosystems will be severely affected by imbalances in the cycle of carbon, nitrogen and phosphorus.”

Tim Radford is a reporter for Climate News Network. Climate News Network is a news service led by four veteran British environmental reporters and broadcasters. It delivers news and commentary about climate change for free to media outlets worldwide.

Comments

By Eric Peterson (Front Royal, VA 22630)
on November 4th, 2013

We are told elsewhere that the a warming world will be wetter.  If that is not the case, then the world won’t be much warmer at all since positive water vapor feedback is necessary for warming above roughly 1C (total including warming up to the present).  Without positive water vapor feedback a lot of things will not happen such as melting of Greenland’s ice.  A warmer world will warm much more if is wetter, not drier,  One argument is that the wetness will be concentrated and extreme, but there is really no solid science to back that up.  What seems to be missing from these stories is a discussion of regions and unreliability of model predictions for regions.  It is unlikely that such predictions will be accurate since ENSO dictates most dry patterns in these areas of interest like Africa and ENSO predictions are very poor even in a static world never mind a warming world.

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By James (Ireland)
on November 4th, 2013

Plants need water, but is a flood a good way to water your crops? Too much carbon dioxide is bad news. http://clmtr.lt/cb/zfp0AU

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By Neil (Kenosha, WI 53143)
on November 4th, 2013

From your words earlier, Eric, the positive water vapor must exist because the melting is occurring already.  The oceans are warming too, massive ice sheets are breaking free, most glaciers are in retreat, and the chemical changes that don’t just exist, but are increasing, too, are alarming.
This also poisons the oceans and begins to dismantle the food chain.
And if you think about it, we haven’t even addressed the number one issue is to mans existence… 

Food for thought.

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By kermit
on November 5th, 2013

Eric, where are we told else that the world will be “wetter”?  The world is more complicated than a flask on a table. Warmer water evaporates more readily, and warmer air holds more water vapor. As dry regions heat up they will get drier. When it rains, it will rain more. Some areas will *average the same, but it will be in the form of droughts punctuated by torrential downpours. Why is more water vapor necessary for temps over 1°C? We have about reached that now, and no signs of the rate of rising temps slowing down, nor any reason to think it will anytime soon.

As for solid science, that comes in the form of evidence and well established physics, models and observations. Or have you not seen the floods and droughts around the world in the last decade? As for predictions, these are climate predictions, not weather. Long term, on the scale of decades and centuries, not hours or days.

The models seem to be holding up quite well. Unfortunately for all of us.

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By Eric Peterson (Front Royal, VA 22630)
on November 6th, 2013

Neil, despite some ice sheets breaking free and a warming (expanding) ocean, sea level rise is a minimal 1 inch per decade.  That is not alarming nor will it be for decades.  The problem with making multiple claims of “alarming” changes is that they are mutually exclusive.  There cannot be more storminess on average without negative feedback since an increase in the water cycle causes cooling.  If the article above is true, then there will be a decrease in the water cycle and therefore more warming.  But it can’t be both.

As for ocean acidification, there are natural fluctuations (mainly daily and annual) that exceed the man-made acidification.  The same deep ocean sequestering of heat that is claimed to be the current manifestation of global warming (since the atmosphere is warming very little), also sequesters carbonic acid.  Essentially the deep ocean is our current dumping ground for heat and acid.  In the long run that’s not a good thing, but for many decades we will get away with it.

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By Eric Peterson (Front Royal, VA 22630)
on November 6th, 2013

Kermit, you said it yourself: more water evaporates.  Therefore there will be more rain.  The only real question is where it will fall and climate models are not very good at predicting patterns (basically all over the map).  The answer is still mainly more rain.

You ask “Why is more water vapor necessary for temps over 1°C? ”  Because the CO2 rise alone will produce about 1C total from preindustrial CO2 levels.  The rest of past and projected warming is due to water vapor feedback.  The basic problem is that water vapor feedback is not a nice constant implied by a “sensitivity” number, but a variable dependent on short and long term trends in weather.  So not only more water vapor as I claimed, but more evenly distributed water vapor (which I left out).  If water vapor is bunched up then that means less positive feedback.  If it is spread out, that means more positive feedback to base CO2 warming.

You ask “have you not seen the floods and droughts around the world in the last decade? ”  Is that a fact, or just anecdotal?  “A new paper out in the current issue of Nature finds little evidence to support claims that drought has increased globally over the past 60 years. ”  From http://rogerpielkejr.blogspot.com/2012/11/little-change-in-drought-over-60-years.html   Floods are almost completely anecdotal.  Far too short a return period to make any valid statistical claims about the past decade.  When such claims are made, they fall apart easily (e.g. Brisbane).

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