Last week the science community was shocked by the claim that 42% of the sea-level rise of the past decades is due to groundwater pumping for irrigation purposes. What could this mean for the future – and is it true?
The causes of global sea level rise can be roughly split into three categories: (1) thermal expansion of sea water as it warms up, (2) melting of land ice and (3) changes in the amount of water stored on land. There are independent estimates for these contributions, and obviously an important question is whether their sum is consistent with the total sea level rise actually observed.
foto (c) Stefan Rahmstorf 2012
In the last IPCC report (2007), the time period 1961-2003 was analysed in some detail, and a problem was found: the individual contributions summed up to less than the observed rise – albeit with rather large uncertainties in the estimates. In the years since then, much research effort has been devoted to better quantify all contributions. For the last decade there is also improved observation systems, e.g. the GRACE satellite mission and thousands of autonomous ARGO floats monitoring globally the warming ocean.
Last year Church et al. (2011) provided a new sea-level budget analysis (see Fig. 1). For the period 1972-2008 the budget is closed, with a total rise of about 7 cm. A bit over half of that is due to melting land ice, and a bit less than half due to thermal expansion. Land water storage makes a small negative contribution, because the water stored in artificial reservoirs (which lowers sea level) is estimated to be larger than the amount of fossil groundwater pumped up for irrigation (which mostly ends up in the sea). Also for the shorter recent period 1993-2008 (for which we have satellite measurements of global sea level rise, found to be about 3 mm per year) John Church and colleagues successfully closed the sea level budget. Granted, the uncertainties in the estimates are still significant so the issue cannot be considered completely resolved. Nevertheless, the Church et al. paper defines the current state of the art against which all further studies need to measure up.
Fig. 1. Sea level rise for 1961-2008. On the left the individual contributions are shown, on the right the sum of these contributions (red) is compared to the measured rise (black). Graph from Church et al. (2011)
The groundwater shock
On May 20, Nature Geoscience published a Japanese model simulation of global land water storage (Pokhrel et al. 2012), which surprised the expert community with the conclusion that 42% of sea level rise (about 3 out of 8 cm) over the period 1961-2003 is due to reduced land water storage. In contrast to earlier studies, reservoir storage was assumed to be smaller, but mainly groundwater pumping was calculated to be several times larger.
Are the new numbers realistic? I and many colleagues I spoke to have serious doubts. It is a model result which is in stark contradiction to data-based estimates. The simulation is based on a simple assumption: first the total water demand was estimated, second the availability of near-surface water, and then the shortfall was assumed to be completely supplied by unlimited use of fossil water. The realism of this assumption is debatable – to me it seems to run a risk of greatly overestimating the withdrawal of fossil water.
The uncertainties also need to be discussed: the fossil water withdrawal is estimated by subtracting two large, uncertain numbers. Yet there is no proper uncertainty analysis. Instead, a single number with three significant digits is presented (359 km3 per year for 1950-2000). That is almost five times the rate of 82 ± 22 km3 per year computed by Konikow (2011) for 1961-2008, based on data for groundwater usage and actual observations of water-level declines in aquifers being depleted. Leonard Konikow, a hydrologist with the US Geological Survey, says about the huge amount of groundwater depletion simulated by Pokhrel: “Groundwater hydrologists would have noticed if such a large volume of water were ‘missing'”.
A bit dubious is also the fact that for the largely overlapping period 1950-2000 Pokhrel et al. find that less than 20% of sea level rise is due to land water storage, not 42% as for 1961-2003. Yadu Pokhrel responded to my query that this is due to a large short-term increase in the landwater contribution to sea level between 2000 and 2003, combined with the fact that their rates are computed simply from the difference between the end points (2003 minus 1961). 2003 happened to be a drought year with little water stored on land. Church et al. compute their budgets based on linear trends, which is more robust by using all data points and not just the end points.
Pokhrel et al. don’t even mention the Church et al. paper (although that was published before their paper was submitted). They relate their discussion to the old IPCC finding of “missing sea level rise”, claiming to now have found the source of this missing water. The media largely followed this story line.
Impact on future projections
If the Pokhrel numbers were right, what would this mean for the future? There are two methods to estimate future sea level rise: complex process-based models, which try to compute all individual contributions (e.g. glacier melt) under changing climate conditions, and semi-empirical models, which exploit the observed relationship between global temperature and sea level and are calibrated with past data (see my article Modeling sea level rise at Nature Education). Both have their problems and limitations, and currently I don’t think anyone can seriously claim to know which will turn out to be closer to the truth.
Fig. 2. Change in sea level in mm per year due to the contribution of groundwater pumping (black curves – estimated based on data by Konikow 2011 and Wada et al. 2010) and water storage in artificial reservoirs (blue – this contribution is negative, i.e. lowers sea level). From Rahmstorf et al. (2011).
For the process-based models, the high fossil water pumping rates according to Pokhrel would simply have to be added to the projections (artificial reservoirs are generally thought to not offset much of this in future, because reservoir construction is well past its peak and there is not much scope for a large expansion). Last year we published simple projections of the groundwater pumping contribution (Rahmstorf et al. 2011, see Fig. 2), based on the data by Konikow (2011) and an earlier study by Wada et al. (2010) together with the medium UN global population projection. In the upper of the two curves, groundwater pumping raises sea level by 10 cm by 2100. If, based on Pokhrel, we assume groundwater pumping rates that are roughly twice as high, this could add 20 cm to sea level. Very recently, a new study by Wada et al. (2012) gave a more detailed projection up to 2050 which lies in between our two curves. By 2050 they find 2-4 cm sea level rise due to groundwater pumping. If the rate did not increase any further after 2050, this would add up to 5-8 cm by 2100. Whether 5, 10 or 20 cm – it is clear that groundwater pumping is a factor that must be accounted for in future sea level projections.
The impact of groundwater pumping on semi-empirical projections is smaller, because here we have two partly compensating effects. On one hand there is the added water as just discussed, on the other hand the climate-related part of the projection gets smaller, since the climatic effect on past sea level rise is also smaller, which affects the calibration of the model. In our paper we found that accounting for groundwater depletion according to Wada (i.e. upper curve of Fig. 2) lowers the projections for a moderate global warming scenario (RCP4.5) by 6 cm. If we assume again that Pokhrel’s numbers are roughly twice as high as this, also for the future, then our best estimate for this scenario would come down to 91 cm sea level rise, as compared to 98 cm in our ‘default case’ (for which we used the lower curve of Fig. 2, based on the Konikow data).
Overall, accounting for the Pokhrel landwater estimates would thus tend to increase the process-based sea level projections and lower the semi-empirical projections, thereby reducing the discrepancy between the two – in my view a very welcome feature. But do I believe it?
Weblink
PIK sea level pages (publications, data, graphs, animations and more)
References
Church, J.A. et al (2011) Revisiting the Earth’s sea-level and energy budgets from 1961 to 2008, Geophys Res Lett 38, L18601, doi:10.1029/2011GL048794
Konikow LF (2011) Contribution of global groundwater depletion since 1900 to sea-level rise. Geophys Res Lett 38:5. doi:10.1029/2011gl048604
Pokhrel, Y.A. et al (2012) Model estimates of sea-level change due to anthropogenic impacts on terrestrial water storage. Nature Geoscience, doi:10.1038/NGEO1476
Rahmstorf, S, Perrette, M & Vermeer, M (2011) Testing the robustness of semi-empirical sea level projections. Clim. Dynam. 97, 1-15, http://dx.doi.org/10.1007/s00382-011-1226-7
Wada Y, van Beek LPH, van Kempen CM, Reckman J, Vasak S, Bierkens MFP (2010) Global depletion of groundwater resources. Geophys Res Lett 37:L20402. doi:10.1029/2010gl044571
Wada, Y et al (2012) Past and future contribution of global groundwater depletion to sea-level rise. Geophys Res Lett 39, L09402, doi:10.1029/2012GL051230
Eh, language nit:
“What makes sea level rise” vs. “What causes sea-level rise”
;-)
[Response:What makes you think this? Is the same grammar, I use rise as a verb here just like think. Anyway, here’s a Dutch guy living in Finland discussing the English language with a German guy … We need natives. -stefan]
Yes, but how sensible is this assumption? Isn’t the shape of their curve very different?
Article here. According to their Figure 1, the depletion of groundwater (GWD) is an almost linear function of time, meaning that the extraction rate would be constant… if this continues into the future, the “calibration” and “projection” effects for the semi-empirical approach would precisely cancel :-)
(Such precise cancellation doesn’t happen in our paper with either Wada et al. or Konikow.)
[Response:Indeed – I noticed as well that there is very little increase in the rate of groundwater extraction in Pokhrel et al, in contrast to other studies. In my view that is another feature that does not sound all that credible. -stefan]
A little light relief http://denialdepot.blogspot.co.uk/2012/05/summer-shattered-no-warming-since.html#comment-form
The statistical minutia notwithstanding, the Saudi experiment in pumping their aquifers for irrigation proves the theory is right. Their aquifers are depleted and most of the water has presumably ended up in the ocean.
Had they, or anyone else, irrigated instead with desalinated water or water that was captured before it ended up in the oceans then their aquifers would remain, sea levels would be less and the irrigated desert would be a Natural carbon sink.
The area of the Earth’s surface covered by the oceans is 361 million square kilometres. A few years ago the mean estimate for sea level rise was 480 mm, which is the amount used in this example.
To maintain current levels it would be necessary to sequester 173,280 km3 (361,000,000 km2 X .00048km) of desalinated water in the world’s hot desert – if this was the only approach to the problem.
Warm deserts cover an area of 15,559,000 km2, therefore .0111km (173,280 km3/15,559,000 km2) of water will have to be taken up in the deserts over the next hundred years or 1.11 m every year.
According to the U.S. Geological Survey, for the year 2000, the rate of application of water for irrigation purposes in the U.S. was 2.48 acre-feet, which is near enough the desert take-up required to prevent sea level rise.
The lowest hanging fruit for the Middle East and North Africa, it seems to me, are their oil tankers deadheading with salt water as ballast rather than fresh water in segregated collapsible bladders that would keep the water clear of oil residue.
According to the 2006 Review of Maritime Transport by the United Nations Conference of Trade and Development, Geneva, in 2005 total world shipments of tanker cargoes reached 2.42 billion tons of which 76.7 per cent was in crude oil for a total of 1.85 billion tons.
The specific gravity of Texas crude oil at 15.5°C is 873 kg/m3 whereas purewater at 4°C = 1000kg/m3, thus the world tankers transported roughly the equivalent of 1.62 billion tons of pure water which is 1.62 BCM.
Saudi Arabia is the largest producer of desalinated water in the world. In 2004 the volume of water supplied by Saudi Arabia’s government-operated desalination plants reached 1.1 BCM and by 2009, new plants were expected to add an additional 0.58 BCM of water per year.
Deadheading tankers could therefore match the output of Saudi desalination plants and could presumably double the 32,000 km² the country has currently under irrigation.
If all of the world’s hot deserts were irrigated, they would sequester 6.8 gigatons of carbon a year whereas according to the Kansas State Soil Carbon Center, the atmospheric carbon pool is currently expanding by about 6.1 gigatons.
Converting ocean heat to energy with OTEC is another viable means of addressing the sea level problem but it seems the easiest approaches should be tried first.
Jim Baird, that looks worthy of thought. I looked at your website:
http://www3.telus.net/gwmitigationmethod/Summary.htm
I don’t know enough to identify whether it could work, and it appears you are involved in the idea as business – money with mouth, one might say.
A little aesthetic vs. readability quibble: You might make the type a color that contrasts in value with the glittering sea – I had to “select” to create contrast and make it readable.
> readability
This tools removes all the aesthetic, leaving readable text.
Bookmarklets for Zapping Annoyances – Jesse Ruderman
Thanks Susan Anderson, I’ve received that complaint before – the aesthetic as opposed to money with mouth.
Will have a look at revision.
Susan:
I see what you mean; the white text blends into the sunlit water on the right-hand side of the column.
A quick-and-easy fix is to simply highlight all the text. In this case it stays white, but the background turns blue. You can scroll it up and down without losing this effect.
Chris Winter, thanks for the great tip and easy fix.
Jim Baird #4:
If you’re right, the other implication is that this is a short-term effect because there;s a limit to the amount of fossil water that can usefully be used. While there are some huge aquifers out there, it’s hard to imagine they are on the same scale for example as ice sheets that are up to kilometres thick on a continental scale.
In any case I don’t think you can dismiss the data as “statistical minutiae”. Either it’s reasonable to add up the numbers in a certain way or it’s not.
This article is another good example of how science is done. A paper is published with cause to doubt the result. The follow up is discussion of whether the result is reasonable followed up I would hope by a paper correcting any errors found in the paper. Even the Nature family can sometimes let through bad science, and this can and should be corrected. Those wanting to make a political case will of course focus on the results that please them so this paper will get wide circulation whether shown to be flawed or not.
[Response: Good point: before we published our projections last year, I asked a groundwater hydrologist whether this was reasonable or whether we’d run out of fossil water before. He said there was plenty enough for those projections (i.e. up to raising sea level by 10 cm). Would be great if some knowledgeable hydrologists would join the discussion here! -stefan]
Do they give region by region, or aquifer by aquifer estimates? If so, couldn’t the models be compared with the observations here? If they agree then maybe the models are right. If they don’t, then the observations are more likely to be right.
#1 They’re both sound OK.
for similar reasons
“what makes hair colour change” and
“what causes hair colour change” are both OK.
(verb applied to a compound noun)
It doesn’t work with adverbs though
e.g.
“What causes global warming?” is OK
“What makes global warming?” isn’t.
similarly
“What makes dogs bark” OK
“What causes dog-barking?” OK if strange.
“What makes dog-barking?” Borat
#12 oops!
editing typo line 1
“They both sound OK” or
“They’re both OK”
Hi Guys
Here in NZ we use a lot of groundwater – however most or nearly all is very young with the aquifers being re-charged annually
I know there are some large fossil aquifers – but I would have expected most countries to be more like NZ
Jim Baird, Even it all tankers traded with Saudi Arabia, that amount of fresh water is 1 cu km – of no consequence. However, if we ratchet bask the vision to simply warehousing water in any form (forget the greening of the deserts), there are plenty of areas where seawater could be pumped into geologic basins that have no role in irrigation or use as potable water. A good place to start would be uninhabited depressions that are below sea level – e.g. the Qattara Depression (http://en.wikipedia.org/wiki/Qattara_Depression#cite_note-19) were 1,213 cubic kilometers could be poured to bring 19,605 square-kilometers up to sea level, producing a transformative inland sea.
Last week the science community was shocked by the claim that 42% of the sea-level rise of the past decades is due to groundwater pumping for irrigation purposes.
I think the 42% figure relates to total net terrestrial storage contribution, inclusive of negative reservoir storage changes. Their groundwater depletion figure on its own is ~1.1mm/yr, over 60% of observed sea level change.
Philip Machanick (10), my interest is in resolving the problem regardless of its source. I agree with you the threat from ice sheets would seem exponentially greater than our ability to pump aquifers dry. The Saudi experiment however demonstrates that if you try hard enough you can impact sea levels through this mechanism.
How then would you address the ice sheet problem?
A recent study by researchers from the Canadian Centre for Climate Modelling and Analysis at the University of Victoria and the University of Calgary concluded that even if we stopped putting CO2 into the atmosphere today the impact from the greenhouse gases already in the atmosphere will cause unstoppable effects, including sea level rise of at least four metres, over the next 1,000 years.
The reason is the ocean’s thermal inertia.
The science would indicate then, it seems to me, we should convert as much of the accumulating ocean heat – 330 TW a year over the period 1993 to 2008 according to the NOAA study published in Nature 05/20/2010 – to energy in accordance with the First Law of Thermodynamics.
OTEC is the technique to accomplish this and it would be a triple threat against sea level rise. You would cool the ocean limiting thermal expansion, limit the erosion of icecaps for the same reason and cooler oceans dissolve more CO2.
Irrigating deserts is another tool which addresses both the cause and effect.
It seems to me totally irrational we are doing neither.
I am accused above of having an economic interest. To date the capital flow has been all outward over a period of 25 years. I make no apology for wishing this circumstance would reverse and as quickly as possible.
It seems to me there is far too little Constructive Capitalism being practiced at the moment. And perhaps far too much study as well, when what is required is action.
Could salinity tests or other be used to estimate fresh water flows into the oceans near land?
Clifford Goudey (15), no objection to anything that works.
By my calculation above, 173,280 cubic kilometers of water would have to be taken up. The Qattara Depression would only make a dent and seems to be one of the larger areas where this approach is possible.
It is also likely going to be a problem convincing people of these regions their land should be a sacrifice zone.
Those whose land would be turned into productive fields are likely to be more responsive.
Didn’t the Romans salt Carthage so that nothing would grow there?
One square kilometer is a pittance but it is a start and it seems like an opportunity being squandered.
Do any of the studies/models/projections take into account changes in the water holding capacity of soils? Just curious as i know a few percentage points increase in soil organic matter translates into substantially higher water holding capacity. Obviously current global trend in agricultural land is very much in the other direction – ie decreasing organic matter therefore decreasing water retention
to Jim Baird: I’m afraid, You can’t get the heat out of the ocean as it is heat of quite a low temperature level and You end up shoveling the lion’s share of it to somewhere else with even a lower temperature level which makes in sum no difference concerning thermal expansion.
Tim, Deserts can reach temperatures as high as 45 degrees C and will radiate this heat back to the atmosphere, exacerbating the warming problem.
Vegetation influences how hot the surface of the land can become. In areas where vegetation is dense, the land surface temperature very rarely exceed 35 degrees C. As you rightfully point out it also retains moisture.
> changes in the water holding capacity of soils
Good question, that. It could change reversibly or irreversibly — and the same question is worth asking about aquifers. Some collapse physically when the water’s drained out, and can’t refill (Kettleman in California is a famous example); in other materials the empty space can hold the void spaces open for a while so there’s a place water can fill again later.
http://www.scribd.com/mschoenberg/d/39335365-Groundwater
‘The term “base-level” is usually related to surface water drainage systems and erosion processes. Several definitions exist to this term in the literature, one of which is “The lowest level to which a land surface can be reduced by the action of running water”. In general, the groundwater base-level was described as “a drainage level for the aquifer that represents the lowest groundwater level that will occur from groundwater flow only” (Olin 1995). It is herein, in analogy, defined as the ultimate discharge zone down-gradient of groundwater basin….’
just had a very rough bash at some numbers for soil water retention.
1% increase in soil carbon results in additional 60000gallons of water held per acre
Approx 16.6billion acres farmland globally (7.6 arable, 9 grazing)
So 1% increase in soil carbon across the board would mean something like 4500cubic km of additional water stored on land.
Pre agriculture soil organic matter levels of 10-15% were not uncommon. Post agriculture levels of below 2% are the norm. Also read of a rancher who has increased his soil organic matter content by 8% in less than a decade.
In February I brought up this topic on Skeptical Science. It is interesting to see the same topic brought up here. Wonder if the Skeptical Science crew will go after if as much as they did when I brought it up.
http://www.skepticalscience.com/news.php?p=2&t=67&&n=1278
Dominik Lenné, if you are not converting heat to work by OTEC then you are producing the electricity mystically.
Granted conventional methods pump over 20 times more heat to the depths than energy produced.
Richard Smalley suggested 10 billion people would need 60TW by 2050 with a good deal of being required to produce water.
If you created this 60TW with conventional OTEC you would pump 1.2 peta watts to the depths, which is the same amount as heat driving the Thermohaline. Obviously not a good situation.
I suggest therefore a counter-current heat transfer system that captures the latent heat of condensation in a condensed working fluid and brings it back to the surface. Ideally you would produce Smalley’s 60 TW by extracting 120TW of the 330 currently being accumulated and dumping 60TW to the depths.
Major hurricanes can release between 50 and 200 Terawatts of heat energy and there are as many as 21 major storms a year and many smaller storms. Clearly therefore the oceans are capable of producing all of the renewable energy the world needs provided the bulk of the heat used vaporize the working fluid is recycled back to the oceans surface in the same matter as a hurricane returns heat to the earth’s surface as falling rain.
Produce 60 TW with fusion or fission on the other hand and you generate an additional 120 TW of entropy, which invariably ends up in the ocean to melt Antarctica all the quicker.
Subglacial erosion (Pine Island glacier):
http://www.agu.org/pubs/crossref/pip/2012GL051651.shtml
I remember only a few years ago, ‘moulins’ were unexplained but thought to be produced very slowly — then someone imaged one being produced rapidly under one of the Antarctic glaciers. This stuff all used to be thought to happen slowly.
Surprise …
@24 >Also read of a rancher who has increased his soil organic matter content by 8% in less than a decade.
Tim, do you have link for this one?
Thanks–
http://www.unl.edu/nac/atlas/Map_Html/Stable_and_Productive_Soils/National/Soil_Organic_Matter_Content/Soil_Organic_Matter_Content.htm
“This is the map of soil organic matter derived from the national STATSGO database which was developed by the Natural Resources Conservation Service. The color scale ranges from gray sandy soils to dark brown loamy organic peats. Again the midwest stands out, and so does the Okefenokee swamp in south Georgia and the Everglades of Florida.
Taken from the report:
A New High-Resolution National Map of Vegetation Ecoregions Produced Empirically Using Multivariate Spatial Clustering by William W. Hargrove and Robert J. Luxmoore “
There are certainly some fascinating ideas in the geo-engineering department, including desert greening. Other than the question of scale, and ‘likely’ rates of deployment and long-term C sequestration, a question that might need some firm answers is what side-effects (particularly regional) may arise from albedo changes and all that extra moisture being evaporated/transpired from those land masses. I suspect there are some interesting factors to study there.
When I saw the post title, I immediately assumed this was about land subsidence due to pumping aquifers (could be oil fields as well). But I guess subsidence would be too restricted or localized to have a global impact.
But I also wonder about the identification of the source of pumped water that is being considered, and its fate. Do the pumpage figures reflect ground water that is not circulating (in which case it will eventually discharge)? How do irrigation waters divide into wet biomass, runoff, and evapotranspiration? And what about pumpage that derives at least a component from induced recharge from streams?
Many thanks for this Stefan, really helps in understanding the issues
A.J. and any other interested parties.
Dr. James Lau and I have formed a loose organization we call the Ocean Energy Group to try and advance OTEC as an energy/environmental solution and a major solution to sea level rise. Our association evolved from a mutual commitment to a deep water condenser, which circulates a small volume of working fluid to the depths and pumps the condensed fluid back rather than circulating massive amounts of hot and cold water. This we believe overcomes most of the cost and environmental concerns inherent in the conventional OTEC approach.
Our technology is evolving and we still are strongly debating the merits of CO2 as a working fluid and the necessity for counter-current heat flow.
Dr. Lau has made a brilliant suggestion of scaling the approach from a lab scale consisting of two tanks representing the ocean’s hot and cold reservoirs. Conditions in any ocean could be replicated in these tanks and the effect of OTEC on the reservoir tested. It would also be the cheapest way of optimizing the approach before scaling.
Our efforts would benefit from expansion and the kind of debate exemplified on these pages.
Anyone interested can contact us at oceanenergy at telus dot net.
Re- Comment by Jim Baird — 2 Jun 2012 @ 10:57 AM currently at #22:
I am no expert, so please explain for me how higher ground temperature in the desert versus, for example, below a forest canopy is an important factor for increasing global warming. It is my understanding that desert soil has an albedo around 2.5 times that of forest foliage.
Steve
I agree with Steve Fish, the tradeoff greening and humidifying a high-albedo dry-air desert is going to lose as well as gain. The total volume of water can’t change sea level by much, the water will come back via the atmosphere.
The other tradeoffs involved with trying to change the Sahara or the Sonora or Australia’s deserts into irrigated sites have been assessed over and over for a long time. Nobody’s solved the problem of salinization of irrigated soils, nor of contaminating the groundwater with nitrates — anywhere, yet, have they?
Heck, you could just desalinate Mediterranean water and let it flow downhill to restore the diminishing Dead Sea, make hydropower, and — hmmm, what would you do with all the salt?
“Method” patents are pretty weird. Either the processes described are already in existence and proven (in which case you can do the business case numbers) or they’re not.
If OTEC becomes viable, the energy needed to pump all that water could be used to replace fossil fuel and the net gain would I think be greater, faster, with no downsides.
But this is already way off topic.
Steve Fish, by definition deserts lack water the evaporation of which has a significant cooling effect due to the absorption of the latent heat of evaporation.
Irrigated deserts would further cool the deserts through the process of evapotranspiration.
DESERTS ARE COLD AT NIGHT:Because of the lack of water in the ground, and little water vapor – a significant greenhouse gas.
If agricultural groundwater release figures significantly in sea level rise, the Coasean economic remedy would be to reduce evaporation in agriculture.
Hank hit the nail on the head.
I‘ve been silently following this side-thread with growing concerns. One of the biggest current environmental problems produced when extracting freshwater from seawater is what to do with all of the salt, which is usually collected in the form of brine. At the scale at which Jim Baird appears to be proposing this, there would be an awful lot of it (starting with 32 grams of various salts per liter of seawater). Since no desalination process is completely efficient, this can amount to up to a gallon of brine for each gallon of freshwater produced.
Desalinating ocean water also poses another environmental problem: it is virtually impossible to do efficiently without killing a whole lot of the plankton that exists in ocean water to begin with. In a warming world, who knows how this will affect the eggs and/or larvae of many ocean creatures already at risk.
I’m also wondering how ever-increasing amounts of degraded plastics in ocean water will affect this already problematic pollution stream…
I know this sounds a little obvious, but to address the problem of rising sea levels as the result of anthropogenic global climate change, why not focus on finding more and/or better ways to reduce the emission of CO2 gas into the atmosphere, which got us into this situation in the first place?
Dwindling freshwater supplies on a warming planet with a growing human population is a huge problem; so is climate change, and its associated rising sea levels. Even if one could solve the energy issues, I don’t think that water desalination is the best answer for any of these problems.
Smartypants
Craig, the objective is to keep sea levels in equilibrium. They are rising due to thermal expansion, icecap melting and aquifer pumping. OTEC is a tool for addressing thermal expansion. Melting and aquifer pumping add fresh water and thus decrease salinity. Desalination would simply maintain the status quo. Another objective is to capture runoff before it enters the water or mixes with the water at the ocean/river interface. This would be the low hanging fruit as are tankers deadheading to the MENA to carry it the desert.
Martin, OTEC done properly can provide all of the energy we need carbon free. As an energy carrier it can produce hydrogen, ammonia or methanol to replace transportation fuels. Most believe there is already significant sea level rise built in due to the thermal inertia of the ocean. The only way you can eliminate that is by backing the heat out, converting it to work.
We are also well over the 350 ppm considered sustainable. Natural sinks, like irrigated deserts are the best and quickest way to get back down to that.
Craig, I missed your phytoplankton point and it is extremely important because they are the base of the ocean food chain and the lungs of the planet.
The study Global phytoplankton decline over the past century by Daniel Boyce postulates the volume of phytoplankton in the world’s oceans, which produce half of the oxygen in the atmosphere by consuming the equivalent amount of carbon dioxide, has been declining steadily for the past half century-down about 40 percent since 1950.
“What we think is happening is that the oceans are becoming more stratified as the water warms,” said Boyce. “The plants need sunlight from above and nutrients from below; and as it becomes more stratified, that limits the availability of nutrients.”
OTEC would cool the surface to alleviate this problem.
Some propose upwelling of cold water with conventional OTEC would promote phytoplankton growth and thus marine aquiculture. I think it more likely it would overstimulate this growth, as does agricultural runoff from the Mississippi and as in that case dead zones would result.
Entrainment and impingement is also a problem moving massive amounts of water with conventional OTEC. The proposal of Dr. Lau and I is to instead move the working fluid in a closed cycle to the depths for condensing and then pumping it back. The heat dumped to the depths in that case first reduces thermal stratification then induces gentle convection that would bring with it nutrients vital to phytoplankton and those on which they feed.
Craig re salt. Open cycle OTEC produces water by flash evaporation so the residue is slightly more salty but there is no solid to deal with. The problem is the best OTEC sites aren’t that close to where the water is needed so transportation is a big problem and the infrastructure is massive and costly. Tankers would be an economical solution to a small part of the transportation problem.
My preference however is to produce hydrogen by electrolysis. It is both an energy as well as a water carrier and is a fraction the weight of water. There are transportation issue if overcome I think this would be the way to go.
This is a great graphic: http://ga.water.usgs.gov/edu/earthhowmuch.html
All Earth’s water in a bubble
This drawing shows various blue spheres representing relative amounts of Earth’s water in comparison to the size of the Earth. Are you surprised that these water spheres look so small? They are only small in relation to the size of the Earth. This image attempts to show three dimensions, so each sphere represents “volume.” The volume of the largest sphere, representing all water on, in, and above the Earth, would be about 332,500,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)), and be about 860 miles (about 1,385 kilometers) in diameter.
The smaller sphere over Kentucky represents Earth’s liquid fresh water in groundwater, swamp water, rivers, and lakes. The volume of this sphere would be about 2,551,000 mi3 (10,633,450 km3) and form a sphere about 169.5 miles (272.8 kilometers) in diameter. Yes, all of this water is fresh water, which we all need every day, but much of it is deep in the ground, unavailable to humans.
Do you notice that “tiny” bubble over Atlanta, Georgia? That one represents fresh water in all the lakes and rivers on the planet, and most of the water people and life of earth need every day comes from these surface-water sources. The volume of this sphere is about 22,339 mi3 (93,113 km3). The diameter of this sphere is about 34.9 miles (56.2 kilometers).
One source of groundwater storage not apparently considered is the change caused by changing climate itself. There is evidence of long-term natural decline in some areas, simply due to natural discharge exceeding natural recharge. That difference would now end up in the ocean and contribute to sea level change. There are obvious changes in some basins in the Great Basin, but I’m less clear on whether this could be a global phenomenon or whether the magnitude could be sufficient to account for the “missing” source of water.
>> What we think is happening is that the oceans are becoming more stratifie
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> OTEC would cool the surface to alleviate this problem.
Oh please. OTEC would do how much to change stratification of the world’s oceans? Some fraction of one percent, but how small a fraction? And reducing stratification reduces thermodynamic efficiency of OTEC.
Sorry, I won’t follow this OTEC digression further. It ought to be on a blog somewhere it can be commented on, but it’s way off topic.
The news from Antarctica is relevant though; posted some in the open thread hoping the scientists will revive this topic.
Jim Baird #40
Sorry Jim, you’re mistaken about that.
The only potential way that OTEC can help is by replacing current greenhouse gas emitting ways of power generation, bringing down the existing — and growing — imbalance in the Earth’s heat budget due to those greenhouse gasses. Compared to that, anything the OTEC installations themselves do to the heat content of ocean water at various depths is very nearly negligible.
What OTEC does, as a minimum, is move heat from the surface to the deep; this could have a climatic effect, e.g., on the formation of hurricanes, but that would be local and unrelated to the big picture. You cannot take heat out of the ocean unless you have a place to put it; the only way to put it back into space is by radiating it out. You cannot “convert it to work”; that only works for temperature differences.
What Martin, and others, have said. This looks like an attempt to get a “method patent” on a bunch of different technologies combined, to do something that wouldn’t work.
A few of the many ideas that are wrong, and would be obviously wrong if the numbers were worked out:
— Add humidity to dry desert air — and the desert no longer radiates as much heat away to space during the night.
— Evaporative cooling is a local surface effect, it’s rearranging the heat by transporting it within the planet’s atmosphere, not getting heat off the planet.
— Shuffling heat around with OTEC is not cooling the planet, it’s rearranging heat within the ocean.
Yes, OTEC is a promising technology and a good idea to improve. Use it to replace fossil fuel and that will help reduce human-caused warming.
#10 Stefan asked for a hydrologist to comment.
What seems to be lacking from this discussion is the fact the little of the water abstracted for irrigation finds its way back to the sea; the basic aim of irrigation is to provide plants with water for transpiration not available from rainfall. In a well designed irrigation system in a region which does not have a wet season (i.e. a part of the year in which rainfall exceeds potential evaporation) some extra water is pumped to prevent a build up of salts in the soil but often this excess returns to an aquifer rather than a river (which big users of fossil water like Libya and Saudi Arabia do not have).
Martin, if I’m not mistaken OTEC operates like any other Rankine Cycle, heat vaporizes a fluid to drive a turbine and the vapor is then condensed and the cycle is repeated. It seems to me your argument is like saying a nuclear plant doesn’t consume uranium when it produces power.
Paul Curto states in his article, American Energy Policy V — Ocean Thermal Energy Conversion, with respect to producing 2.5 TW of power with OTEC “The amount of heat dumped by that much OTEC into the ocean’s heat sink at depth is therefore just over 50 TWth, and that is also equal to the heat removed from the surface plus the power output, about 53 TW.
My take from this is you have converted 2.5 TWh to 2.5TWe.
In another part of the article he comments on early OTEC investigation, “Oddly enough, the concern was that we might cause an Ice Age!”
It turns out we have the opposite problem which I believe OTEC can resolve.
The massive heat dump to produce 2.5 TW (which I admit is not much cooling) would limit the amount of power you could get from the ocean and thus the amount of heat you could covert to energy because you are sapping the hot reservoir and warming the cold thus reducing the delta T.
That is why I think you need a counter-current heat flow to limit the amount of heat you take out of the surface as well as the amount you dump. If you could convert 60TWh to 60TWe then I think you would be making a difference. Especially if you are replacing carbon sources and thus are chipping away at a problem already created rather than one that is accelerating away from you.
I stand to be corrected.
Jim Baird opines in #40:
Craig, the objective is to keep sea levels in equilibrium. They are rising due to thermal expansion, icecap melting and aquifer pumping. OTEC is a tool for addressing thermal expansion.
I’ve seen you say this several times now Jim, But I’m far from convinced.
As I understand it, the extra heat being absorbed because of increased GHG’s, which ends up mostly in the oceans, greatly exceeds that used by mankind. Taking out what we need isn’t going to make a dent in the oceans heat budget. Mixing the heat into the cold depths doesn’t make it go away, it just warms the depths. The depths will expand when they warm.
So, please, show me where I’m wrong.
The other thing that concerns me is the effect of the mixing. Would cold waters would make their way toward the surface due to convective currents? Nutrients being brought up? CO2 being brought back from millenia ago?
Cooling the sea surface *sounds* like a good thing given our rising temperatures, but can we, for example, reasonably model how localized changes will affect regional weather?