Land 'evapotranspiration' taking unexpected turn: Huge parts of world are drying up

Oct 10, 2010

The soils in large areas of the Southern Hemisphere, including major portions of Australia, Africa and South America, have been drying up in the past decade, a group of researchers conclude in the first major study to ever examine "evapotranspiration" on a global basis.

Most climate models have suggested that evapotranspiration, which is the movement of water from the land to the , would increase with global warming. The new research, published online this week in the journal Nature, found that's exactly what was happening from 1982 to the late 1990s.

But in 1998, this significant increase in evapotranspiration – which had been seven millimeters per year – slowed dramatically or stopped. In large portions of the world, soils are now becoming drier than they used to be, releasing less water and offsetting some moisture increases elsewhere.

Due to the limited number of decades for which data are available, scientists say they can't be sure whether this is a natural variability or part of a longer-lasting global change. But one possibility is that on a global level, a limit to the acceleration of the hydrological cycle on land has already been reached.

If that's the case, the consequences could be serious.

They could include reduced terrestrial vegetation growth, less carbon absorption, a loss of the natural cooling mechanism provided by evapotranspiration, more heating of the land surface, more intense heat waves and a "feedback loop" that could intensify global warming.

"This is the first time we've ever been able to compile observations such as this for a global analysis," said Beverly Law, a professor of global change forest science at Oregon State University. Law is co-author of the study and science director of the AmeriFlux network of 100 research sites, which is one major part of the FLUXNET synthesis that incorporates data from around the world.

"We didn't expect to see this shift in evapotranspiration over such a large area of the Southern Hemisphere," Law said. "It is critical to continue such long-term observations, because until we monitor this for a longer period of time, we can't be sure why this is occurring."

Some of the areas with the most severe drying include southeast Africa, much of Australia, central India, large parts of South America, and some of Indonesia. Most of these regions are historically dry, but some are actually tropical rain forests.

The rather abrupt change from increased global evapotranspiration to a near halt in this process coincided with a major El Nino event in 1998, the researchers note in their report, but they are not suggesting that is a causative mechanism for a phenomenon that has been going on for more than a decade now.

Greater evapotranspiration was expected with , because of increased evaporation of water from the ocean and more precipitation overall. And data indeed show that some areas are wetter than they used to be.

However, other huge areas are now drying out, the study showed. This could lead to increased drought stress on vegetation and less overall productivity, Law said, and as a result less carbon absorbed, less cooling through evapotranspiration, and more frequent or extreme heat waves.

Some of the sites used in this study are operated by Law's research group in the central Oregon Cascade Range in the Metolius River watershed, and they are consistent with some of these concerns. In the last decade there have been multiple years of drought, vegetative stress, and some significant forest fires in that area.

Evapotranspiration returns about 60 percent of annual precipitation back to the atmosphere, in the process using more than half of the solar energy absorbed by land surfaces. This is a key component of the global climate system, linking the cycling of water with energy and carbon cycles.

Longer term observations will be needed to determine if these changes are part of decadal-scale variability or a longer-term shift in global climate, the researchers said.

Explore further: Hurricane Edouard right environment for drone test (Update)

More information: Jung, M., M. Reichstein, et al. 2010. A recent decline in the global land evapotranspiration trend due to limited moisture supply. Nature , DOI:10.1038/nature09396

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User comments : 30

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Sean_W
1.6 / 5 (21) Oct 10, 2010
Please God, let somebody press the big red "no pressure" button in my presence. It will save me from having to listen to this complete garbage anymore. The reversal of a trend that was not well established to begin with is proof of the factor that was supposedly driving the trend... and has started driving it's reversal.

Just stop making cr4p up. Is that too much to ask? Just STOP it!!!
Pratyeka
4.5 / 5 (17) Oct 10, 2010
to Sean_W: Just stop whining and stop reading that which you call cr4p.
scenage
4.3 / 5 (3) Oct 10, 2010
Living in Australia, I can tell you that the last ten years, we were in drought. Now, it's precipitating at roughly the same levels as pre-1998. I'm sure on BOM (Bureau of Meteorology) you'd find pre-1998 rain fall levels. Thinking it's natural variability. Also surprised it's hard to find data on evotranspiration in Australia...?
stealthc
1.4 / 5 (21) Oct 10, 2010
yes lets take facts and make them say global warming, so the poor suckers buy this crap, nevermind if these facts are part of a natural cycle and we are proven wrong later on. Open your wallets and give us money to save the environment! Friggin parasites.
gjbloom
1.8 / 5 (9) Oct 10, 2010
Water vapor accounts for the largest portion of the greenhouse effect. If we're seeing a decrease in evapotranspiration, does this translate into lower water vapor and consequent greenhouse effect?
mertzj
4.2 / 5 (13) Oct 10, 2010
I cant believe how many people on here think global climate change is blowing smoke. I am not saying it is all true but I wont be that guy saying well maybe they were right in 40 years. Not to mention the climate changes... hopefully everyone here knows that right? Well dont you think it would be slightly beneficial if we could keep those natural changes from happening even if their not human induced? Which I am pretty sure we have a little something to do with it. Anyway guess im just open minded.
looseyarn
4.3 / 5 (6) Oct 10, 2010
this is a conspiracy of soils against the subtropical freshwater fish.
Caliban
4.4 / 5 (15) Oct 11, 2010
yes lets take facts and make them say global warming, so the poor suckers buy this crap, nevermind if these facts are part of a natural cycle and we are proven wrong later on. Open your wallets and give us money to save the environment! Friggin parasites.


This article does nothing of the sort. It states plainly that GW is a _possible_ culprit, but that it could just as well be natural variability. That this could be a system of negative feedback loops or part of a natural cycle.

The only thing stated as a certainty is that this picture has become apparent after analysis of about 3 decades of data. And that further research would be necessary to reach a greater level of certainty. Conclusions whose validity is unimpeachable.

You can stop with the knee-jerk denialist noisemaking. Next time read the f***ing article.

scidog
2.5 / 5 (2) Oct 11, 2010
natural cycles? whats natural anymore?.left alone the "cycle"-whatever that is works it's self out.toss in a monkey wrench in the form of the unnatural industrial world and i guess it won't cycle anymore,it will just burn out.
pauljpease
5 / 5 (12) Oct 11, 2010
Good discussion here. I think the bigger picture is that we "armchair" scientists find it very easy to use information like this to promote our personal point of view. But where were all of you yesterday, telling us about how evaportranspiration accounts for 50% of solar energy input on land? That sounds like kind of a big deal. Did any of you even know this? I'm just curious if everyone else here is actually an expert on the unimaginably complex science of Earth's climate and it only appears as if you just read headlines and then spout off the same tired opinions. Seriously, how many actually pondered the effect of evapotranspiration on local climate? I know I didn't, but of course I'm no expert. This stuff is complicated, and it's just possible that you haven't thought of everything that's going on here. Just my two cents.
daen
5 / 5 (14) Oct 11, 2010
@Sean_W: The Nature article clearly describes the observational and modelling techniques, discusses the findings, points out that a trend has been found, and suggests reasons for that trend. The analysis of the article here at PhysOrg summarizes those findings and offers additional commentary from one of the co-authors. Now, I'm assuming you either haven't bothered to read the original article itself, or can't because you don't have a Nature subscription (hint: if you're going to comment on Nature papers here, it behooves you to subscribe), and yet you somehow feel that your ignorant and politically motivated opinion is sufficiently important to share with us. Here's a tip, and possibly the best one you'll get all year: I, and others, will care a lot more what your opinion is when you show that you haven't simply reacted to the words 'global warming' but have actually done some informed crtitical thinking. Good luck with that.
Shootist
1.4 / 5 (11) Oct 11, 2010
Thinking it's natural variability.


When it comes to the global climate everything is Natural Variability.

The document http://www.ipcc.c...nts.html is very complex, very technical, and there is no obvious way to find out how measurements are taken or why weights are assigned; at least not obvious to me. I am not ignorant of mathematics but I confess to having tried to find the parts of the document that describe the actual operations performed to get 1/10 degree accuracies in air and sea temperatures, and not being able to find them.

You could make a career of reading that document; and since its authors have expressed a lack of confidence in the document's editors, it seems to me that we taxpayers, who are expected to pay for the billions that the policy recommendations would cost, are entitled to a simpler explanation." quote Jerry Pournelle, Ph.D. http://jerrypourn...l#dialog
winthrom
5 / 5 (8) Oct 11, 2010
I 1971 I wrote a paper for my climatology class at U of nebraska, at Omaha, on evapotranspiration and tornado tracks. I used data from the second half of the 19th century and tornado tracks from the 1st half of the 20th century. (The data tracked well and showed that areas with high evapotranspiration had higher predictions of tornadoes). Evapotranspiration data is available back as far as the Civil War. I think that this article is bringing out a real threat. Despite the political folks hacking about imaginary threats, there is real science here. I hope the more level headed folks can make a case that sticks.
StillWind
2 / 5 (11) Oct 11, 2010
As usual, no one notices the elephant in the room. Almost daily, "Scientists" are discovering something that they didn't know, or contradicts what they "thought" that they knew.
Of course the Earths climate is changing. It has been for 5 billion years.
I challenge anyone to provide any proof to the contrary.
On the other hand, if we can find a way to make a positive change, then by all means, let's do so.
However, I have yet to see any real evidence that we even understand the basics, let alone have the wisdom or ability to actually do anything (positive or negative) in a system that so dwarfs our capabilities, that continued argument is nothing short of ludicrous.
daen
5 / 5 (10) Oct 11, 2010
@StillWind: I'm not sure what you're saying. If on the one hand you are saying that our ignorance is greater than our knowledge, but that we must try to understand, then I am 100% with you. But if on the other hand you are saying that our ignorance is so great compared to any knowledge we could glean from studying our planet's climate, and the reasons why it is changing, and any that attempt to do so is worthless, then I absolutely disagree. Papers like this broaden the clearing of scientific understanding in the great dark forest of ignorance: bramble by bramble, maybe; nevertheless, broaden it they do.
GSwift7
1.2 / 5 (5) Oct 12, 2010
Studies like this one don't surprise me. There have been far too few studies in regard to the water cycle. Far too little is known about long term trends of the water cycle, and if mankind has had an affect on it. Therefore it isn't surprising to me when studies like this one uncover something unexpected. The models are full of false assumptions because of gaps in our knowledge of the system. What does surprise me is that there isn't more interest from our scientific community. Everyone wants to study carbon and methane, while ignoring the big elephant in the room. Someone above suggested that the big elephant is our lack of knowledge. I'm not sure I agree with that very much. I think we are doing well in regard to our progress and knowledge. The past century has been amazing. The problem as I see it, is that we are very selective about which parts of our knowledge we employ. Every climate scientist knows the potency of H2O as a greenhouse gas, but it's hardly mentioned anywhere.
GSwift7
1.8 / 5 (5) Oct 12, 2010
If you google greenhouse gas, you'll find a mountain of web pages that talk about CO2. Sites like NOAA, NASA, USGS, and CRU have pages and pages of 'information' about CO2. Some of them briefly mention water vapor, and some of them don't mention it at all. They don't like to present the percentages in a way that directly compares water to other factors. They don't show graphs of water vapor trends. They don't have nice little illustrations showing how water traps heat in our atmosphere so effectively because it's not only a super absorber but also a perfect heat storage medium. Am I the only person here who thinks it is strange that nobody wants to talk about water?
thermodynamics
4.5 / 5 (8) Oct 12, 2010
GSwift7: I will try this one more time. Every model in existence focuses on water. The entire energy transport basis for every model is water vapor because it evaporates at the equator and moves energy through latent heat toward the poles. Every model has complex routines that handle the evaporation and condensation of water. Water is talked about constantly in the form of solid, liquid, and vapor. The motion of the oceans and rivers are parts of the models. Evaporation off of bodies of water are parts of the models. Precipitation is part of every model. Clouds are being modeled (not well but they are working on that). So, what is being missed in your mind? What is that "Elephant in the room?" I really am having a hard time understanding what you think is being missed. If you actually read the papers you will find that the models all cover water as the engine moving energy from the equator to the poles.
GSwift7
1 / 5 (3) Oct 12, 2010
From IPCC working group 1 summary papers: "Therefore flux adjustment terms were often added to the surface fluxes of heat, water and (sometimes) momentum which were passed from the atmosphere to the ocean model. Flux adjustments are non-physical in that they cannot be related to any physical process in the climate system and do not a priori conserve heat and water across the atmosphere-ocean interface. The flux adjustments were specifically chosen to give a stable and realistic simulation of present surface climate (especially the sea surface temperature and sea-ice cover), and were often as large as the annual mean model fluxes themselves"
GSwift7
1 / 5 (4) Oct 12, 2010
This is what I am talking about. They actually kind of 'work around' the water problem, rather than actually simulate it. They are forced to basically enter arbitrary adjustments because the real process is far too complex to model. So, the most important part of the whole thing is just plugged in rather than calculated because the calculated values end up being way off. That's why I laugh when I read predictions of global warming leading to floods, drought, heavy storms, etc. If they keep forcasting a heavy hurricain season every year, then I'm sure they will eventually be right one of these years.

So, yes they account for it in models, but the size of the gaps in the models because of water are usually minimalized or not even mentioned most of the time. I wasn't just talking about models though. I'm not sure what got you talking about models again. I was mostly talking about discussions like Kyoto and Copenhagen, where water is NEVER discussed.
thermodynamics
4.6 / 5 (9) Oct 12, 2010
GSwift7: Do you have any idea what you just wrote? Your original comment was:

"Am I the only person here who thinks it is strange that nobody wants to talk about water?"

So, after I pointed out that the models are built around water vapor transport you wrote:

"So, yes they account for it in models, but the size of the gaps in the models because of water are usually minimalized or not even mentioned most of the time."

Then you said: "I was mostly talking about discussions like Kyoto and Copenhagen, where water is NEVER discussed."

And again you are wrong. Everywhere they present papers on modeling water vapor is accounted for. They had information at both Copenhagen and Kyoto (I have to assume you were neither there nor read the reports).

Have you ever modeled anything? Do you know what the term "abstraction" means in modeling? You need to get some experience, do some reading and notice that the models have water, atmosphere, sun, earth, and biosphere working together.
GSwift7
1 / 5 (5) Oct 12, 2010
Gee thanks for telling me what I said Thermo. Golly, I guess I should have said that "we know everything there is to know about water vapor, clouds and aerosols. The models are perfect and the predictive accuracy of the models is so good that every prediction they make has come to pass." How silly of me to assume that water vapor is given proper consideration in policy meetings. I seem to have overlooked the water cap and trade meetings they are having all over the globe lately. Oh and 'My Bad' for claiming that 99% of greenhouse gas articles, books and web sites seem to skip over the fact that CO2 is nothing compared to water as a greenhouse gas.

Yes, I have made computer models. Back when I was doing earospace engineering work I put together a really neat airflow model of an aircraft carrier deck, for example. Funny thing about it is that if my model had been unable to make predictions, then I would have had to fix it before I used it for anything.
GSwift7
1 / 5 (4) Oct 13, 2010
I'll repeat this part in case you didn't get it the first time: "Flux adjustments are non-physical in that they cannot be related to any physical process in the climate system and do not a priori conserve heat and water across the atmosphere-ocean interface. The flux adjustments were specifically chosen to give a stable and realistic simulation of present surface climate (especially the sea surface temperature and sea-ice cover), and were often as large as the annual mean model fluxes themselves"

I'll try to type slowly so you can understand Thermo. That means that they have to cheat in order to make the simulations come close to reality. The cheat factor isn't derived from theory, it's just substituted in place of the values the simulation calculated. The cheat factor is decided on by the researchers. I'd like to think that they do the best they can, but suppose someone like Caliban is working on the team. I mean he rated my post where I quoted the WG-1 summary as 1/5?
Skeptic_Heretic
5 / 5 (7) Oct 13, 2010
Almost daily, "Scientists" are discovering something that they didn't know, or contradicts what they "thought" that they knew.
Of course they are. If science had discovered everything about reality already it would stop.
VOR
not rated yet Oct 14, 2010
guess we should get busy building the worldwide solar-powered irrigation system (lol?)
thermodynamics
5 / 5 (4) Oct 14, 2010
GS7:

You sound like you are adjusting your flux-capacitor on your time machine from Back-to-the-Future.

First of all, let me address your comment about the "cheat factor." Is that what it is called or is it called a parameter??? Have you heard of "dimensionless numbers?" Such as the Reynolds Number or the Prandtl number? Or the Mach number? How about other "cheat factors" that are strewn throughout any fluids or thermodynamic code. Plus, there are relationships for which there are no dimensionless numbers so, good engineers build them and use them. So, why don't you give me an example of a meaningful "cheat factor" (since you seem familiar with them) and let's see if we can figure out what they are. You are insulting some coder somewhere when you actually don't even know what they were coding. Let's look at an example? Show us what you think is ruining the world?
Justsayin
1 / 5 (1) Oct 17, 2010
appears to have a high correlation to the sun cycle or sun spot activity....justsayin
marjon
1 / 5 (3) Oct 17, 2010
Plus, there are relationships for which there are no dimensionless numbers so, good engineers build them and use them.

These are usually called 'constants' because they have been found to vary little over time, a very long time, based upon experimental data.
Thrasymachus
4.3 / 5 (6) Oct 17, 2010
Here's my problem with your harping on water vapor, G. If you take the entire atmosphere of the Earth, and you ask, "How much more water vapor could the atmosphere hold at its current temperature and pressure?" the answer you would get is, "Not much." The reason for this is elementary, both liquid and solid water exist freely and at some place in the world, it is raining. In order for water vapor to be driving an increase in global temperatures, either something else has increased the temperature, allowing the atmosphere to hold more water vapor where it can have it's greenhouse effect, but then it's not the driver, or the atmospheric pressure must have dropped, which we can be sure hasn't happened because our barometers would have noticed.
marjon
1 / 5 (5) Oct 17, 2010
both liquid and solid water exist freely and at some place in the world,

Hang out around the coast of Arabia. There is a region called the marine layer were the air is nearly saturated with water vapor.
That is where the Namibian desert life gets its water, from the air.
"Although annual rainfall is limited to only .2 to 3 inches (5 to 76 mm) per year, thick fog from the Atlantic often blankets the dunes to create enough moisture for many species to survive."
http://www.nation...315.html
Force that warm air up a mountain range and it precipitates filling rivers like the Amazon and creating rainforests in Washington state.