[Coral-List] Comment on Atoll formation

Richard Dunne RichardPDunne at aol.com
Sat Oct 2 02:53:12 EDT 2010


  Atoll formation and Sea Level Rise

As Chip Fletcher infers - there are important regional differences in 
sea-level rise. In the Chagos region of the Indian Ocean for example, it 
has consistently been demonstrated that sea-level rise has been much 
less than the global mean of 3.4 mm per year. Church et al 2004 (used by 
the IPCC 2007 AR4 Report) and 2006 (Global Planetary Change) have 
demonstrated historical rises of only 0-1 mm per year. More recently Han 
et al 2010 (Nature Geoscience) have shown that there are human-caused 
atmospheric-oceanic circulation changes in the Indian Ocean which are 
likely to persist into the future, which are depressing sea-level rise 
in the Chagos area so that even with existing sea ice melt, the rate of 
change has been negative or minimal (-0.5 to + 3.9 mm per year).

The recent authoritative Royal Society review (2010) of the evidence in 
the IPCC AR4 Report has highlighted the considerable uncertainties in 
this area of global warming science (compared to estimates of 
temperature rise). It has only endorsed as "very likely" a figure of 
20cm per century for the global mean sea-level rise (rather than the 35 
cm AB1 scenario value in the IPCC) and added that additional rise due to 
ice sheets is at present insufficiently understood.

There is incontrovertible evidence for a global rise in sea-level. 
However, the magnitude is still subject to considerable uncertainty and 
controversy and there are significant regional effects which both 
magnify and depress the overall rate of change. We should be cautious to 
assume that all atoll islands will be uninhabitable by the end of the 
century. Like Micronesia, some may well be in imminent peril but others 
may survive for generations.

Richard P Dunne


On 30/09/2010 19:33, John McManus wrote:
> Yes. The fresh water lenses will disappear long before the land is
> inundated. Desalination is too expensive for most small island communities.
>
> John
>
> -----Original Message-----
> From: coral-list-bounces at coral.aoml.noaa.gov
> [mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Chip Fletcher
> Sent: Thursday, September 30, 2010 2:13 PM
> To: Coral-List at coral.aoml.noaa.gov
> Subject: Re: [Coral-List] Comment on Atoll formation
>
> Re: Atolls and rising seas
> Sea level inundation in Micronesia (western equatorial Pacific) is already
> threatening the livability of atoll islets. By the time Dickinson's atolls
> (a great paper, by the way) become "unpinned" in the second half of the
> century, islets will have likely been uninhabited for decades. The process
> is underway now. Over 2006-2009 inundation associated with extreme tides
> (known locally as "king tides") eroded shorelines and damaged soil,
> agro-forestry, and groundwater throughout Micronesia. Without emergency
> water and food many islet communities would have had to abandon their lands.
> These events were so alarming that the Federated States of Micronesia
> declared a national state of emergency on Dec 30, 2008. If inundation events
> persist with any regularity in the coming decade, we may see a significant
> emigration of atoll communities. Generally high water in the western Pacific
> since 1998 (satellite altimetry indicates SLR of 5-10 mm/yr in the region)
> may reflect decadal processes, and it may be too early to definitively say
> this the signature of global warming. Nonetheless, the regionally rapid SLR
> provides a glimpse of processes in a warming world; and it would be prudent
> to prepare for more of the same...Chip Fletcher, Hawaii
>
>
>
> -----Original Message-----
> From: coral-list-bounces at coral.aoml.noaa.gov
> [mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Ulf Erlingsson
> Sent: Thursday, September 30, 2010 3:26 AM
> To: Coral-List at coral.aoml.noaa.gov
> Subject: Re: [Coral-List] Comment on Atoll formation
>
> I took a look at that paper and note that he has a VERY generic sea
> level curve, referenced to Clark et al (2004) which I don't have
> access to. However, the 2004 article deals with the sudden sea level
> rise at 19000 before present, not a Holocene maximum. Dickinson
> SHOULD have made reference to an article that explicitly investigated
> the Holocene sea level ATA THAT SITE. As it stands, the conclusion
> rests on loose sand. It could equally well be wrong as right.
>
> On 2010-09-29, at 17:40, Lescinsky, Halard L wrote:
>
>> I'd suggest that Ulf, and others who think that most inhabited
>> coral cays are simply the result of incremental build-up of storm
>> debris, should take a look at the recent paper by William Dickinson
>> (2009, GSA Today 4-10; http://www.geosociety.org/gsatoday/archive/
>> 19/3/pdf/i1052-5173-19-3-4.pdf ).  A take home message of
>> Dickinson's work is that inhabited cays are generally stabilized or
>> "pinned" by reef deposits that formed during the Mid-Holocene
>> Highstand 4,000 years ago when sea level was a couple meters higher
>> than it is today.  The islands, therefore, owe their existence to
>> exposure and cementation of carbonate during sea-level fall, NOT
>> keep-up style carbonate accretion.  When rising sea-level floods
>> the islands in the near future they will again become
>> uninhabitable, as in fact they were until about 1500 years ago when
>> falling sea-level first exposed them and allowed for their
>> colonization.  ---  Hal Lescinsky
>>
>>
>>
>> -----Original Message-----
>> From: coral-list-bounces at coral.aoml.noaa.gov [mailto:coral-list-
>> bounces at coral.aoml.noaa.gov] On Behalf Of Ulf Erlingsson
>> Sent: Wednesday, September 29, 2010 10:13 AM
>> To: coral-list at coral.aoml.noaa.gov
>> Subject: Re: [Coral-List] Fw: Chagos MPA - Continuing international
>> dispute overboundaries
>>
>> Let me point out that the argument that atolls will sink due to sea
>> level rise due to global warming fails to take into account the
>> geomorphological mechanisms by which atolls are created and grow.
>> They grow, slowly, through coral reef growth (as should be no news to
>> anyone on this list), through littoral processes, and through
>> chemical precipitation, i.e., when the water gets supersaturated with
>> respect to CaCO2 the calcium is precipitated in layers on ooid sand
>> grains, eventually building up large land masses such as the Bahamas.
>>
>> The problem is only when the sea level rises quicker than these
>> processes can keep up with. However, the predicted rise in sea level
>> is much smaller than the quickest and largest sea level rise in the
>> recent past. See http://erlingsson.com/authorship/CIS2GOM.html
>>
>> Of course, that event did drown large areas that are now sea floor,
>> but others managed to remain above water. A question of interest is,
>> naturally, what lessons we can learn from that event.
>>
>> Ulf
>>
>> On 2010-09-28, at 22:41, Douglas Fenner wrote:
>>
>>> But those same atolls may not be
>>>   inhabited much longer, due to sea level rise.
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