Arguments
Past 150,000 Years of Sea Level History Suggests High Rates of Future Sea Level Rise
Posted on 7 December 2012 by Rob Painting
- An accurately dated, near-continuous, history of sea level variations for the last 150,000 years has been compiled.
- Comparison with ice core data reveals that major global ice volume loss, as implied by sea level rise, has followed relatively quickly after polar warming. The Greenland ice sheet responding virtually straight away (0-100 years lag time), and a 400-700 lag for the Antarctic ice sheet.
- These response times are much faster than was previously commonly suspected, and implies that once sufficient polar warming is underway, future ice sheet collapse may be unavoidable.
- During all episodes of major global ice loss, sea level rise has reached rates of at least 1.2 metres per century (equivalent to 12 mm per year). This is 4 times the current rate of sea level rise.
Figure 1 - Sea level reconstruction from 150,000 years ago to the present. Relative sea level (RSL) in grey-shaded area, with RSL data in blue crosses. The downward-pointing red arrows indicates peaks in sea level rise exceeding 1.2 metres per century (12mm per year). The break in the record is due to the absence of foraminifera (upon which the reconstruction is based) as a result of excessively salty seawater during the last ice age. Adapted from Grant (2012).
Relevant Sea Level Background
The last few million years of Earth's climate has been dominated by the ice age cycles. These consisted of long cool periods (glacials) where giant icesheets have grown on the continental land masses at, and near, the poles. With the water evaporated off the oceans being locked up as ice on land, this ice sheet build-up substantially lowered global sea level. During the shorter, warmer, intervals (interglacials) the ice sheets have disintegrated, and with their glacial meltwater draining back into the oceans, sea level has risen. From the coldest part of the last ice age (roughly 20,000 years ago) to present, global sea level has risen an astounding 120 metres.
Although all the details are not well understood, the driving force behind these glacial/interglacial cycles are slow variations in Earth's orbit as it circles the sun, which slightly decreased/increased the amount of sunlight reaching the planet's surface. For the current interglacial, the orbitally-driven warming eventually came to an end after the Holocene Climatic Optimum (HCO), and by 4-5000 years ago all the vulnerable land-based ice had disappeared. The volume of the global ocean was static until the arrival of the Industrial Revolution, and by the 19th Century global sea level had begun to rise again. Despite undergoing short-term accelerations, and decelerations, globally-averaged sea level has undergone long-term acceleration up to the present day (Church & White [2006] , Merrifield [2009]).
Figure 2 - Global mean sea level from 1870 to 2006 with one standard deviation error estimates (Church 2008).
With some 60-70 metres worth of global sea level equivalent locked up in the vast ice sheets of Greenland and Antarctica, and with global warming well underway, it raises the question of how much sea level rise we are likely to see this century (and beyond), and just how fast this might happen. Because the dynamics of ice sheet disintegration are only very crudely known, and ice sheet modelling is in its infancy, there is a large range of estimates of future sea level rise. Many now seem to converge on 1-2 metres of sea level rise by 2100 - much higher than current rates. But is this realistic? A recent paper, examining past ice sheet disintegrations, lends credence to these estimates.
Rapid Coupling Between Ice Volume and Polar Temperature
A peer-reviewed paper, Grant (2012), outlines how the authors created a well- dated, and near-continuous, record of sea level over the last 150,000 years, a period which spans the last interglacial (the Eemian), and the last glacial maximum. Of particular interest is the finding that, during all periods of major global ice volume loss, rates of sea level rise reached at least 1.2 metres per century. An arguably more important finding that the more finely resolved dating uncovered, was that major ice sheet reductions (as implied by sea level rise) followed polar warming much quicker than had previously been suspected.
The backbone of the sea level reconstruction are Foraminifera (forams), tiny shelled marine creatures which float in the water column (planktic), or live on the seafloor (benthic). Because they utilize minerals dissolved in the surrounding seawater to build their shells, forams incorporate elements into their shells which can provide information about the climate at the time in which they lived. Examination of oxygen-18 isotope ratios in the shells of forams, retrieved from Red Sea sediment cores, has revealed that they serve as a useful proxy for relative (i.e local) sea level in the Red Sea (see Siddall [2003] & Siddall [2004]). Although a near-continuous record of relative sea level for the Red Sea has been constructed (Rohling [2009]), accurate, and independent, dating for comparison with ice-core data has proven problematic. Grant (2012), however, came up with a clever way around this roadblock.
Constructing a Well-Resolved Chronological Record of Sea Level
Much like the Red Sea, the Eastern Mediterranean Sea is a basin with only one narrow natural opening (The Strait of Gibraltar) which connects it to the rest of the oceans. This "basin effect" was exploited to build a sea level history in the Red Sea, because the extremely slow exchange of seawater within the basin means long local seawater residence times. The raising or lowering of sea level therefore acts to either shorten, or prolong the residence time of local seawater, and also diminish or enhance the powerful rate of evaporation in the basin. In other words, changes in the oxygen-18 isotopes ratios, found in Red Sea foram fossils, are extremely sensitive to sea level variations. So the isotopes are, in effect, recorders of local sea level.
Grant (2012) likewise created a sea level history for the eastern Mediterranean Sea, with one distinct improvement; they were able to independently date the sea level variations by taking advantage of oxygen-18 isotopes stored in cave mineral deposits (speleothems) on land downwind of the eastern Mediterranean surface waters. With oxygen-18 isotopes in fossilized forams, and in the cave deposits (Soreq Cave), linked via the hydrological cycle, Uranium-Thorium dating of the cave deposits therefore gave an accurate date for both, and consequently the timing of sea level variations.
Because of the more complicated weather patterns in the Mediterranean however, the Mediterranean sea level history cannot be used to determine sea level variations with sufficient precision (Rohling 1999). To do that, the authors transferred their new Mediterranean chronology to the Red Sea sea level history. The basin isolation effect of both sea level records, gave sufficient oxygen isotope signal similarity for accurate transfer. The validity of this newly-dated sea level reconstruction was confirmed by comparison to other dated sea level benchmarks.
Figure 3a - Correlation of Soreq Cave (red line) and eastern Mediterranean (black line) oxygen-18 isotope signals. 3b - Eastern Mediterranean Sea oxygen-18 isotope record from another foram species (green line) and the highest probability sea level curve (blue line). The coloured dots and grey-shaded columns denote other paleodata used to validate and synchronize the reconstructions. From Grant (2012).
Sea Level Rise Closely Follows Polar Warming
With an accurately dated sea level reconstruction now available, the authors were able to compare these sea level variations in time with that of polar temperature, as ascertained by ice cores extracted from the Greenland and Antarctic ice sheets. Within the sea level reconstruction there are 6 periods where sea level rose rapidly, reaching rates of at least 1.2 metres per century - around 4 times the current rate of sea level rise (see Figure 1).
Considering that humans have been warming the climate for several centuries, a more significant finding was the short time lag between warming at the poles (as shown in the ice cores), and the response of sea level rise - which implies the disintegration of the ice sheets. In the case of Antarctica, large ice reductions occur within 400-700 years, and for Greenland, ice reductions occur very quickly - within 100 years.
Learning From (Sea Level) History
Despite glacial periods having much more vulnerable ice at lower elevation, and closer to the equator than interglacials, the orbitally-driven warming which eventually disintegrated the ice sheets was a leisurely affair. Ice sheet collapse came quickly due to the greater proportion of vulnerable ice. By comparison; today there is far less vulnerable ice, but the warming has been virtually instantaneous, in geological terms. In fact, in the last 300 million years, the Earth has not experienced (as far as we know) such a rapid rise in atmospheric carbon dioxide (Hönisch [2012]).
The altered characteristics of the background climate state, from glacial to interglacial, makes a direct comparison for modern-day difficult. But current sea level rise estimates, and the rates of rise shown in the reconstruction, are in the same ballpark. With global warming having been underway for several centuries now, and with the Antarctic and Greenland ice sheets undergoing accelerated ice mass loss due to polar warming, the past 150,000 years of sea level history suggests we should expect much higher rates of sea level rise in the future.
"So, we can see that [in this model] the Earth often pops rather suddenly into a warm interglacial state and cools a bit more slowly into a glacial state."
Basically, when ice sheets grow, they appear to be limited by precipitation. Even if you have 2 meters of precipitation a year that all freezes, it would take 1000-2000 years to build a continental ice sheet.
On the other hand, a back of the envelope calculation (direct sunlight, heavily absorbed) suggests that ice can melt at 10cm/day, or 36 meters per year.
(36MJ/m2/day = 100kg ice/m2/day = 10cm)
And ice dynamics/flow act to slow the growth of ice sheets and speed the melt.
So a crude approximation suggests that meltdown should be much faster than freezeup..
"Considering that humans have been warming the climate for several centuries, a more significant finding was the short time lag between warming at the poles (as shown in the ice cores), and the response of sea level rise - which implies the disintegration of the ice sheets. In the case of Antarctica, large ice reductions occur within 400-700 years, and for Greenland, ice reductions occur very quickly - within 100 years."
sea level history suggests we should expect much higher rates of sea level rise in the future."
That last sentence should read near future. ie next 10 - 50 yrs. ie the rate is around 1.5m per century, but we have be heating up relatively rapidly for the last 50yrs or so.
So the majority of the 1.5m rise is going to happen in the trailing part of the 100yrs.
So the rate is exponential as Jim Hansen has frequently pointed out and the data is indicating. ie a doubling every 7yrs at the moment.
So collapse of our society as we know it is inevitable and just around the corner, even from Only a SLR perspective!
This with the increase in the rate of rise just reported, the known uneveness of sea level, particularly affecting the Eastern USA, NASA's recent sea level rise report (expect 2m by 2100 globally, that is a little more New York of course), increased Hurricane storm surges, and it seems inevitable that New York does need relocating and Florida in the next 50-100years and with the rate warming accelerating and this being at the fastest ever noted in geological time along with the record impulse of CO2 and SO2 cooling about to be lost, and it does beg the question what on earth are we doing?
Its not like there is a spaceship that can take us away anywhere sensible and if divine intervention is coming they are cutting it fine!
Yet all that is discussed is reducing emissions slowly, and spending billions of extra tonnes of carbon to keep the lights on by putting up so called low carbon renewables and their infra-structures (also totally dismissing the other environemntal impacts associated with them in clinging onto the power green bullet), to provide fresh water security with de-salination plants, to build huge sea defences that will be futile in less than 50years, fighting senseless wars, to grow addictive cash and fuel crops, to pull down forests, to, to ,to, to, to, to charging billion of mobile phones toxic waste batteries.
Where do you put Floridians and New Yorkers? West Scotland isn't that populated, Greenland? There will be new land here as well, that will emerge from the localised falling of sea levels there as the ice sheet shrinks.
Well if we can put a man on the moon surely we work that out?
Maybe the military arround would do a better service for their nations by working on these sorts of problems rather than killing people pointlessly???
Like how to abruptly stop using fossil fuels??
Isn't humankind mature enough yet to just stop fighting and realise than more is acheived by working together rather than in violent opposition to each other, well apart from progressing weapons development that is.
How much carbon does the arms industry use? And how environmentally toxic is it?
Anyway back to reality, worsening weather, middle east turmoil, rapid ice melt, CO2 emissions growing, more oil being mined, arms industry growth, economic fraility, biodiversity devastation, willingly making toxic materials, extreme inequalities, politicians happily gambling on 50:50 odds of an end of the world situation, highest emissions ever, permafrost tipped already, arctic amplification in severe feedback mode, and all with a dose of a total apparent lack of realisation of the actual scale of this problem.
How many people in Bangladesh are needing to move?
Mid / South USA dessert refugees?
And so on and so forth.
How much more carbon can be afforded?
Peak CO2...
450ppm, scarey!
425ppm.....too worrying
400ppm...still need to move New York and Florida.
350ppm...the carbon debt, still give greater than 5% of 1.5C-2C, so New York and Florida still need to move.
Therefore how much more carbon are people theoretically prepared risk?
How tight is the carbon budget?
????
Can you give the source of your claim (if Hansen, then full text is available in which I'm very interested).
IMO, the dSLR acceleration cannot be that fast: we are already melting very fast: 35my-1, which is just 4-6 times slower than Heinrich events as seen on figure 1. So we've just 2 of those "doubling every 7yrs" before we reach a desintegration of Heinrich proportions.
I'm not saying Heinrich on top of Holocene optimum won't happen but that it cannot happen so fast. Perhaps we will see a doubling of dSLR in this century, perhaps the second dubling later at the end of this century (most of us will not live up to this, only our children). IS response is a slow but sure process in today's state of climate. No one was able to quantify it to date let alone predict "doubling of dSLR every 7yrs"... The section title "Sea Level Rise Closely Follows Polar Warming" is very intriguing but left unquantified: did anyone heard "how closely"? 100y or 500y?
I find the practice of such curve fitting questionable. Especially, if it arives at the improbable conclusion of dSLR 50mky-1 (5mcentury-1), not seen in last 1500ky, far in excess of Heinrich events. On top of that, it must be remembered that strong negative feedbacks on melting rates do exists, like ice flow rates and cooling of the upper ocean. And there is simply not enough ice (far less than the emount of Laurentide IS than triggered Heinrich events) to sustain the exponential rate. So Rahmstorf 2007, being a conservative, semi-empirical linear model, concluding 1m SLR in 2100 is more realistic.
Plainly the doubling time (if it exists at all) must terminate rather quickly, and is very likely to do so at, or less than, the Heinrich event rate.
Being fair to Hansen, his point appears to be, not that sea levels will rise by five meters in this century, but that far more rapid rises than is allowed for by linear increases are possible, so that the likely rise is significantly above linear projections. He may be right in that. I would not exclude a two meter rise, although I believe the 1 to 1.2 meter range to be far more plausible.
Oh, and there's the other small matter that if it gets to the stage of a 10 metre increase in sea level, it's not likely going to be stopping there. And if we've pushed the system that far, the final equilibrium point described by those aforementioned other, extremely serious impacts isn't one in which there will exist on Earth anything resembling a Western-style human society. Indeed, if it reached that point there'd likely not exist any society where even printed words and reticulated water supplies were in use.
These systems are driven in the main by sea surface heat which can be sapped to produce energy with OTEC. The more energy produced the more the ocean surface is cooled.
Part of this heat is converted to mechanical energy in the process but the bulk, 20 times the energy produce, is moved to the deep water heat sink that has a lower coefficient of expansion.
Kevin Trenberth, points out in a paper, "An imperative for climate change: tracking Earth's global energy", "The warming required to produce 1 mm SLR if the heat is deposited in the top 700 m of the ocean can take from 50 to 75 1020 J, or 110 1020 J if deposited below 700 m depth.
In other words you can move heat from the surface to the depths with OTEC to counter 50 percent of current sea level rise due to thermal expansion.
By sapping the heat of hurricanes you also diminish the amount of heat that would be moved from the tropics towards the poles and thus forestall melting.
This is the lesson of Hurricane Sandy, no one apparently cares to learn.
Disclosure - patents pending
Down the road I suppose it's conceivable that overturning might become so sluggish as to need help but as it stands now the volume in play is quite astounding, about 575,000km3/year or ~1km3/minute for the Atlantic. Not sure if the Pacific is larger or smaller but let's assume bringing the Pacific into play doubles that number. A lot of infrastructure would be necessary to obtain a useful boost on those numbers.
Sea surface temperatures running at over 3C above normal along the Atlantic coast from Florida to Canada where the driver for hurricane Sandy. Global warming is estimated to have contributed about 20 percent of this heat which would not have been available had we been producing 2.5 terrawatts of OTEC power the past 10 years.
Kevin Trenberth points out, "With every degree C, the water holding of the atmosphere goes up 7%, and the moisture provides fuel for tropical storms.
Five terawatts of OTEC power each decade would negate this increase.
Five terawatts requires a lot of infrastructure but then so did the Manhattan Project and war efforts in general. I prefer to think that we should be on a war footing where global warming is concerned and so did over 70 percent of other Canadians in a poll a while back.
That would essentially bury warm water, reduce surface temperature and infrared radiation to space with surfaced cold water, and overall increase the radiative imbalance and the accumulation of energy in the climate. It would in fact increase the warming of the deep ocean while simultaneously increasing the rate of energy accumulation in the oceans as a whole.
In other words, I suspect OTEC would (to a small amount - current energy use is 1% of the greenhouse imbalance) worsen global warming.
OTEC would not necessarily worsen GW. You considered just the TOA energy imbalance. Let's consider carbon cycle feedbacks. Perhaps speeding the deep ocean circulation would increase the rate of CO2 intake, even speedup silicate rock weathering by 2 orders of magnitude to 100y timescale:)
But I'm skeptical about OTEC feasibility. We know so little about the effects of such invasion into the ocean circulation, that we must learn alot before attempting such geo-engineering.
KR, I advise using a heat pipe rather than a massive cold water pipe which moves cold water to the surface. Hurricanes use phase changes to move heat rapidly as does a heat pipe. The difference being the hurricane has the stratosphere as the heat sink as opposed to the abyss.
As to radiative forcing the sea surface is subject to a diurnal heat cycle. It warms by day and cools by night.
OTEC operates around the clock and removes heat from the surface by night as well as by day. Each morning therefore, IMHO, the surface would begin to warm from a lower base that without OTEC and since the same volume of water is being warmed by the same heat source, it seems to me, the diurnal high temperature should be less as well?
Chriskoz, IMHO using a heat pipe rather than cold water pipe overcomes many of the environmental problems associated with OTEC and reduces the cost as well because for a 100MW plant you need a 1 meter diameter pipe rather than one of 14.5 meters. I have a patent pending for a system that uses a counter-current heat flow and metering system for the capture and governing of the return of the latent heat of condensation back to the surface both to limit the impact on the Thermohaline as well as maximize the amount of energy the ocean can generate. My thinking is in the long run it is best to convert as much ocean heat to work as possible, which requires the capture and return of the latent heat, but during hurricane seasons it is best to move as much surface heat to the depths as possible.
OTEC can not be implemented on the scale I would like to see overnight. In the course of building it out we would have plenty of time to learn of the ramifications before doing any significant damage.
I believe though it has potential for geo-engineering because it the process we would be producing all the carbon free energy we need.
The input of those on this sight far more technically skilled than myself is appreciated.
As to the diurnal cycle, heat loss and gain are dependent on the gradient - more energy will be absorbed by a cooler ocean surface during the day, less emitted at night, and again an amplification of the radiative imbalance. OTEC may not be able to provide the warming mitigation you claim.
This is, however, completely off topic for historic sea level rises. I would suggest a thread actually on geoengineering, such as this one.