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Climate Hustle

Why did sea level fall in 2010?

What the science says...

Select a level... Basic Intermediate

Sea level fluctuations during El Niño (rising) and La Niña (falling) are the result of large exchanges of water between land and ocean in the form of rain and snow. This averages out to zero over time. It does not affect long-term sea level rise, which comes from melting icesheets, glaciers, and thermal expansion.

Climate Myth...

Sea level fell in 2010
Large sea level fall in 2010 means IPCC sea level projections are wrong.

The last 18 months has seen some epic deluges throughout the world, countries such as Pakistan, Sri Lanka, Australia, the Philippines, Brazil, Colombia and the United States have been hammered with extreme flooding. It will take some time for studies about these episodes to appear in the scientific literature, so how the recent spate of massive floods stack up in a historical context is as yet unknown. 

A recent news release over at the NASA Jet Propulsion Lab, which was re-posted here at SkS, helps in putting the extreme flooding into perspective - so much rain and snow has fallen over land in the period from March 2010-March 2011 that it has contributed to a large fall in global sea level. But this is only a temporary effect, as water is swapped back-and-forth between the continents and ocean, and does not alter the long-term rise in sea level which results from warming oceans and the melting of the polar icesheets and glaciers worldwide.

         Color Bar

Figure 1 - sea level rise 1993-2011 from satellite altimetry. Image from NASA JPL

Short-term sea level fluctuations

Perhaps a poorly understood process, on climate blogs, is the large exchange of water between the ocean and land over short timescales (months/years), which is illustrated in the NASA JPL article. Tremendous volumes of water are evaporated from the world's oceans, and as the holding capacity of the atmosphere is small compared to the land and ocean, this evaporated water ultimately ends up back in the ocean, or on land in the form of either water, snow or ice.   

Each year there are seasonal fluctuations in global sea level which are caused by water swapping back and forth between the land, atmosphere and ocean, and asymmetric (unbalanced) heating of both hemispheres. Because most of the world's land mass is in the Northern Hemisphere, a large amount of water is stored there in the winter in the form of ice, snow and water, and this results in an increase in land-based water storage. At the same time, the Southern Hemisphere is angled closer to the sun, and because it is mainly ocean, the sun heats a huge pool of water, creating a rise in sea level through thermal expansion. The end result of these two out-of-sync processes is the variation shown below, and which results in seasonal sea level fluctuations of 6-9mms.

Figure 2 -Global and hemispheric-mean sea level from TOPEX/Poseidon and Jason-1. The two hemispheric signals partially cancel to produce a global signal with smaller amplitude. From www.cmar.csiro.au

The peak contribution of water mass to the global oceans generally occurs in September at the end of Northern Hemisphere summer as water, stored in snowpack, soils, lakes, rivers, soils and vegetation, is fed back into the sea. And the peak thermal component (ocean expansion from warming) occurs in April, at the end of the Southern Hemisphere summer - when the large expanse of Southern Ocean is exposed to greater solar heating. The annual global sea level peak matches the Northern Hemisphere run-off of water back into the oceans, which is the stronger of the two signals on a year-to-year basis. See Willis (2008) and Leuliette & Willis (2011) for an overview.

You will note the difference with figure 1, which has had the 'seasonal signal' removed to show the long-term trend. It should be obvious from both figure 1 & 2, that despite the large short-term fluctuations, these fluctuations are only temporary. Global sea levels continue to rise as the oceans warm and expand, and as more water mass is added to the oceans from the melting of land-based ice sheets and glaciers. 

ENSO and global sea level

Massive water volumes are also exchanged between the land and ocean connected with the ENSO phases, La Nina and El Nino. During La Nina there is typically an increase of rain and snow falling over land, which corresponds with a fall in global sea level. With El Nino, the atmosphere warms, drying out much of the global land surface and shifting rainfall over the ocean. This brings about a rise in global sea level. See image below: 

   

Figure 3 - annual precipitation anomalies (in mm) for typical El Nino. Note that yellow and light-green colors indicate statistically insignificant values. See Dai & Wigley (2000). Numbers represent the 8 largest tropical river basins 1) Niger, 2) Congo, 3) Okavango, 4) Indus, 5) Ganges, 6) Mekong, 7) Orinoco, 8) Amazon. See discussion below.

As mentioned in the Amazon drought posts, the Walker Circulation is shifted over the tropical Pacific during El Nino, causing the moisture evaporated from land and sea to fall back over the ocean, rather than over the Amazon. El Nino also causes serious drought over Australia, and drying of Southern Africa, India and South East Asia, so much of that land-based moisture ends up back in the ocean too.

Recent research, Llovel (2010), has found that tropical river basins, such as the Amazon, are the main contributor to the EL Nino/La Nina exchange of water mass with the ocean, and the Amazon is a major contributor to the seasonal variation too. For clarity, I've labelled figure 3 with the location of the 8 main tropical river basins which contribute to ENSO-based fluctuations in sea level. Note how all the tropical river basins tend to dry out during the El Nino phase. Now compare with the period March 2010- March 2011, which was in the grip of a particularly powerful La Nina:

Figure 4 - change in land-based global water storage in the period March 2010-March 2011, as observed by GRACE gravity satellites. Image from NASA JPL.

All tropical river basins, apart from the Congo, have gained extra water throughout the latest La Nina. The huge increase in water mass over the Australian continent is rather hard to miss too. Not surprising given the enormous flooding there in the last year.

This regular fluctuation of sea level (falling during La Nina, and rising during El Nino) has been observed throughout the period of satellite-based sea level monitoring (1993 onwards). This can be seen in the de-trended (long-term trend removed to enable comparison) data in figure 5 below. Note that both the Multivariate ENSO Index (MEI, a measure of ENSO), and the related sea level change, fluctuate about zero. In other words ENSO does not contribute to long-term sea level.  

Figure 5 -  To compare the global mean sea level to the MEI time series (a measure of ENSO), the mean, linear trend, and seasonal signals from the 60-day smoothed global mean sea level estimates have been removed, and each times series normalized by its standard deviation. The normalized values plotted above show a strong correlation between the global mean sea level and the MEI, with the global mean sea level often lagging changes in the MEI. Image from the University of Colorado sea level research group page.

Joining the dots

We regularly get "skeptics" posting here, pointing out the large drop in sea level during 2010, however it never seems to occur to them why sea level dropped. That water hasn't just magically disappeared, it's simply found a new temporary home on land, as the residents of Vermont in the USA have sadly just experienced. But it is only temporary, eventually all that water held in lakes, wetlands, rivers, soils and vegetation will find its way back into the ocean, and sea level will rise again. Long-term, expect the sea to continue rising as the oceans warm and melting glaciers and ice sheets constantly add more water to the oceans, but don't be surprised if there's a large pothole, or speed bump, along the way.  

Intermediate rebuttal written by Rob Painting


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 11 July 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Comments

Comments 1 to 19:

  1. To a layman, the Grace plot appears to contradict the explanation that La Nina has transferred around 6mm of seawater onto the land, and to do so on two counts.

    First, assuming that both thermal expansion and cryosphere decline remained ongoing from 3/10 to 3/11, then ~3.2mm of the trend annual sea level addition is missing as well as the noted 6mm - i.e. around 9.2mm is missing in total.

    Second, the ratio of the area of land to sea being around 1:2.43, that 9.2mm of seawater should pile up substantially on land - i.e. to around 22.3mm on average. Even the 6mm volume would pile up to an average of 14.6mm on land, which Grace doesn't appear to show. Given the cartography of the Grace plot, it is hard to estimate areas of inundation in ratio to areas of drought, alongside the area of no net change, but studying it with the best impartiality I can apply, it doesn't appear to show more than an average of perhaps 4.5mm being retained on land - which, divided by 2.43, would represent only around 1.85mm of sea level.

    No doubt you will have accurate data from the Grace plot and can thus provide the proper figure for additional water on land. In surmising that it may be of a different scale to what should result from the missing 9.2mm of seawater being stored on land, if this were correct, I'd be glad to read your views on the percentage increase in airborne water vapour that the missing water now represents, given a normal mean equivalent of just 25mm of global rain being held as vapour.

    It would also be helpful to learn just what would be the CO2e of that increase in water vapour, and just how La Nina, and maybe other phenomena, could have evaporated that volume to remain as water vapour without an observed commensurate spike in global temperature - assuming that a global 1.0C rise is actually required to raise global water vapour by just 7%.

    I feel I must be missing something(s), as it's all rather puzzling.

    Regards,

    Lewis
    Response:

    [DB] "I'd be glad to read your views on the percentage increase in airborne water vapour that the missing water now represents"

    Lewis, remember that atmospheric water vapor excesses have a residence time of about 9 days.  Thus that water piled up onto land masses has a slow trek through cachments, impoundments and reservoirs on its way back to the sea.  And some will make its way into water tables as well.

  2. Lewis. I'm not so sure about all that water piling up on land. Quite a lot of it is 'in' the land.

    Australia's vast, drought-parched landscapes have soaked up a lot of water. Dry and cracked wetlands have filled (not all of them, unfortunately), most rivers, lakes, dams, reservoirs and rainwater tanks are brimful and many millions of thirsty trees and other plants have replenished their starving cells. And on top of that, I expect that we'll soon be seeing some figures about how much of the flooding and soaking rains have replenished groundwaters and deeper aquifers.

    And we're not the only ones. I'm just not familiar enough with other geographies to venture any opinions.

    More numbers from more observations and analyses are needed before we can get too detailed on this one.
  3. DB - thanks for your response - which I'm afraid still leaves me puzzled.

    As I understand it, Grace is the most sensitive instrument yet built for observing gravitational anomallies, and has been calibrated to identify changes in the presence of water both on and within the land. I'd agree that extreme rainfall will, on some adverse terrain, be very slow in its return to the ocean, and some will be subsumed into aquifers, but this doesn't explain why Grace doesn't show the gravitational reponse on land of the volumes of seawater that went 'missing'. What it appears to show on and in land - as best I can judge - is around 20% of that volume, and while I'd doubt it is accurate to say 0.5%, the idea of it being wrong by 80% seems implausible.

    NASA data on the actual net volume of anomalous water Grace recorded on the land would clarify the issue, but as it stands, it appears that ~9.2mm is 'missing' from the sea, of which Grace can sense only around 1.85mm on land. Short of a surprising leak in the seabed somewhere, this implies that around 7.3mm is now somehow being retained as airborne water vapour, being, as you say, recycled about every 9 days. Quite why this should be happening seems unclear.

    Regards,

    Lewis
  4. Adelady - thanks too for your response -

    It's good to hear of the land around you getting the chance to recover somewhat from the awful drought.

    As far as I know, Grace is able to sense the gravitational signature of additional water, whether it's on the surface or deep underground. Thus it shows the net change at year's end for each area of land, and it is the sum of these changes across all lands that seems to me far short of what has gone missing from the sea.

    As you say more numbers and analysis are needed to clarify the issue.

    Regards,

    Lewis
  5. LewisC - trying to estimate numbers from that graphic is not a reliable method. I assume the data will find its way into the scientific literature - but I will check up on that.

    The idea that the atmosphere is holding the equivalent of an extra 7.3mm of sea level, during 2010-2011, is a tad absurd. The exchange of water between the ocean and land surface can lead to mean sea level fluctuations up 8mm during ENSO events. See Llovel (2010) cited in the post. During the extreme El Nino of 1997/1998 sea level rose a whopping 20mm over the short-term, so we know that large fluctuations are possible.

    More telling, perhaps, is that La Nina is when we typically see cooler surface temperatures, and therefore a corresponding decrease in atmospheric water vapor. El Nino, on the other hand, is when heat is given up by the ocean surface to the atmosphere, and this warming increases the water vapor content of the atmosphere on a global scale. See Trenberth & Smith (2005).

    Of course, the oceans are still warming and the land ice is still melting, so long-term sea level will rise. This might be a rather large "pothole" on the road to higher seas though - given that La Nina looks set for a double-dip.
  6. I didn't see this graph in the discussion, so I submit it because the correlation between Multivariate ENSO Index and detrended-corrected global sea mean level seems eloquent :



    Source
  7. Thanks Papy, demonstrates the relationship nicely.
  8. This also comes from the same source. Putting the recent drop in the longer term context makes the "pothole" look relatively smaller, and well within the longer trend.

    http://sealevel.colorado.edu/content/revisiting-earths-sea-level-and-energy-budgets-1961-2008
  9. Post updated to include figure 5. Some extraneous text snipped.
  10. Lewis C and Rob:

    I computed these values from GRACE data back in September, commented here. My computations indicate that about 80% of the sea level decline between March 2010 and March 2011 was due to increased storage on land.
  11. Thanks Keith. We''ll see how well that stands up against the peer-reviewed literature, when the paper by Carmen Boening is published.
  12. 6mm drop in sea level may sound like a lot when you consider all of the world's oceans, but Australia was bone dry prior to the recent floods, so rather than the rain run off the land in to the rivers/oceans as you'd expect, it soaked it up like a sponge.
  13. mace@12 Please try to use some self-skepticism when putting forward a new hypothesis and at least apply a sanity check before posting. The surface area of the worlds oceans is ~3.6×10^8 km2, the surface area of Australia is only 7,617,930 km2, 1/47th of the surface area of the oceans. So for Australia as a sponge absorbing a 6mm rise in seal levels would be equivalent to absorbing 282mm of rainfall over its entire surface (most of which doesn't get much rainfall). Do you think that is at all plausible? If mean sea level were that sensitive to local flooding, it would bounce up and down like a yo-yo.

    Also I would suggest that you should confine yourself to discussion on a smaller number of threads. Your posts rather suggest a lack of basic knowledge on a number of basic topics, and posting wild theories like this gives the impression of trolling/spamming, especially when posted to multiple threads. This is intended as friendly advice, there is plenty of time to discuss these topics, and science is better served by depth of discussion rather than breadth.
  14. Sorry about that Dikran. I kind of see your point now about Australia but I read the article and previous posts so just wanted to launch a hypothesis out there to see if any fellows felt it was plausible. The article also identifies Columbia, the US, Brazil and Pakistan as having some heavy flooding. I don't think it's quite so dry in those places, though, so I agree my hypothesis is probably falling down.

    I think I've only posted to 2 threads, so far but apologies again. I will confine my posts to just 1 thread in the future.
  15. mace, you are welcome to post comments to as many threads as you feel capable of carrying on a dialogue on with any who participate with you, provided you are on-topic for that particular thread and that your comments comply with the Comments Policy (here). That being said (per Dikran above), for better internalization of things learned, fewer is probably best.

    Also note that this is a science-based website, so any hypothesis one wishes to float would need be accompanied by supportive references to the peer-reviewed literature.
  16. Hi Daniel Bailey, thanks for letting me post on more threads but I think I'll stick to just this one as I need to internalize my thoughts.

    The topic's about sea level falling, and the articles saying that this is because of more rain falling on the land than is normal. Obviously, rain would normally run off the land in to the rivers and oceans pretty quick, so I'm trying to think up why this hasn't happened in 2010. It dawned no me that it might be that it's being sucked in to the land, so I looked at the countries in figure 2 of the GRACE diagram, and Australia looked a likely candidate for this sponge effect. There definitely seems to be slightly more dark blue than dark orange in that picture, and the two can't be convoluted because the dark blue indicates higher quantities of surface lying water but it doesn't factor in how much has been absorbed in to the earth. The direct link is here:- http://grace.jpl.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=53

    A Nasa climate scientist, Josh Willis, has put it more eloquently than I can, but I reckon he's saying the same thing. I guess we'll have to wait until next year for an update of the sea level data. Even if it doesn't show a bounceback, I think this could be due to a lagging effect as the water has to penetrate through the rock to get back to the sea.
  17. mace @16, the inland area of Eastern Australia, approximately the entire area of Queenland, New South Wales and Victoria inland of the Great Dividing Range, consists of three very large flood plains. The largest is the Murray Darling Basin, with an area of just over a million square kilometers. The Darling and tributaries drains nearly all of inland NSW, and a large section of southern Queensland, with a river system that drains into the Murray, and then into the sea in South Australia.

    Next largest is the Cooper Creek Catchment, with an area of 297,000 km^2. The Cooper Creek Catchement reaches as far north as my birth place, Mount Isa and drains into Lake Eyre, a normally dry salt pan below sea level. The area in Queensland drained by Cooper's Creek and the Diamantina (a tribuatary) is called the channel country because of the very large number of normally dry river beds that cross it.


    (Click on picture for full sized photo, which is well worth the look.)

    North of the Cooper Creek Catchment is the Gulf Country, a wide area drained by a number of intermittently flowing rivers into the Gulf of Carpentaria. The area of the gulf country is about 186,000 km^2.

    Combined, all three flood plains have an area approximately half of the Mississipi Basin, but unlike the Missisipi basin, most of the area is arid with only intermittently flowing rivers. It is also exceptionally flat. Floods in the Cooper Creek in Queensland take 9-10 months to travel its 1,300 km length to Lake Eyre. The land is so flat that raging floods travel at the glacial pace of 0.2 km/hour. Water traveling to the Murray down the Darling takes a similarly long time. Consequently much of the 2010 Queensland flood is either just now reaching the mouth of the Murray, or reached Lake Eyre a month or so ago, where it will now sit until it evaporates away. The land was so wet that the rivers in the channel country still have water in them.

    In addition to this natural storage, many of Australia's dams where at very low capacity before the floods, but are now very full. Wivenhoe Dam near Brisbane, for example, would have captured a volume of water close to that of Sydney Harbour (mostly during 2010). Combined that means a truly staggering quantity of water is being stored in Australia's river systems and dams which was not there 2 years ago.

    Dikran Marsupial is correct. The amount of water involved is not enough to account for the dip in sea level in 2010 by itself (and Australia was certainly not the only area flooded in 2010). Never-the-less, that water which is stored in Australia's rivers will not return to the sea as quickly as it was taken from it. It will be five or more years before Australia dries out (assuming we do not have ongoing rainfall, which we currently have). I suspect similar stories can be told in many other regions of the world, so while I expect sea levels to resume their inexorable rise, it will not be an immediate turn around.
  18. In addition to Tom's comment above, mace, please note that it can take several months to years for rainwater deposited into catchments and watersheds to return to the sea. A compounding factor is the replenishment of depleted aquifers.

    A nice, open-access, recent review is by Church & White 2011:

    Sea-Level Rise from the Late 19th to the Early 21st Century
    John A. Church • Neil J. White

    Surv Geophys (2011) 32:585–602
    DOI 10.1007/s10712-011-9119-1


    [Source]
  19. mace@14 Scientific discussion is best served by having a high signal to noise ratio, so sanity checking theories before putting them forward (or better still performing some background research) is vital in making sure that ones contributions are signal rather than noise (which is why I only contribute to discussions where I have some background and merely lurk on discussions where I don't). It takes a long time to understand even the basics, so reading the articles on SkS before posting is a good idea. If in doubt, posing the theory as a question is probably a better approach, e.g. "what effect has the recent floods in Australia had on global mean sea level?". There is nothing wrong with not knowing the answers to such questions (I would deinitely defer to Tom on this one!).

    It is true that other countries have had floods this year, however I suspect that is true of most years (the places in question may vary from year to year), and that the volume of the oceans means this variability has relatively little effect on sea levels. In guaging the effect of these floods on mean sea level, we ought to extend the discussion back in time to get an idea of what effect we should expect to see (I don't know the answer to that).

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