coral reefs doomed for sure
kleypas at cgd.ucar.edu
Tue Oct 2 11:23:09 EDT 2001
Thanks to Mike Risk for bringing up some misunderstood issues
regarding ocean chemistry changes in response to increased
atmospheric CO2 and how coral reefs might respond. Some of his
comments are good (e.g. that bioerosion is too often overlooked)
but some were broad misrepresentations of science (e.g. his comments
about ocean modelers and about the Kleypas et al. paper in Science).
So I am compelled to address several of his points:
> Any "reef model" that does not include it [bioerosion]...it's hard
> to be polite, here. These models would better be termed
> "Less-than-half-of-the-reef models."
I agree that any modeling effort needs to take bioerosion into account.
(and contrary the claim that the word was not mentioned in the Amer.
Zool. special issue, Kleypas et al. in the Am. Zool. issue DO mention
bioerosion several times as an important control on coral reef development).
We have also discussed bioerosion prominently in a follow-up paper in
Int. J. Earth Sci. (Kleypas et al. 2001).
Our paper in Science did not model reefs - nor were we trying to model
reefs. The thermodynamic calculations and modeling effort concentrated
on simply determining carbonate ion concentrations as a function of
temperature and pCO2. It is a simple calculation yes, but measured data
obtained through the JGOFS, WOCE and other programs illustrate that
ocean chemistry is indeed behaving as predicted. So I don't think
the challenge to predicted ocean chemistry changes is valid. The
chemistry will indeed be complicated in shelf environments by other
processes, but the buffering on most reefs, e.g. those which receive
significant exchange with open ocean water, will be minimal.
> 3. Oceanic/Climate Models. Notwithstanding their protestations to the
> contrary, I have found modellers to be resistant to data that upset their
> models, with that resistance being directly proportional to the amount of
> federal money invested to date. "One major problem with the current
> generation of GCM's is that the treatment of ocean circulation is still very
> crude." (Ruddiman, 2001: Earth's Climate).
> The implications of Smith et al, 1997, are that a meltwater pulse can divert
> or shut down the Gulf Stream in less than 5 years. To all of you out there:
> when the oceanic part of GCM's can model this, then start believing them-not
> before. The strong compartmentalisation of the mixed layer to which Bob
> refers is metastable, and temporary.
Prof. Risk misrepresents the science presented in the Kleypas et
al. paper. The HAMMOC model results were added to illustrate that
the time-scale to bolster alkalinity (via dissolution of reactive
sediments in response to increased atmospheric CO2, which depends
on deep ocean circulation) was too long to show an appreciable buffering
of the system over the next 200 years or so. At least in terms of
open ocean geochemistry, there is no source of alkalinity which can
adequately buffer the increased atmospheric CO2 for a few centuries, at
least. There have been many papers on this and a good place to start
is with David Archer's.
And in defense of modelers! (I myself am not a modeler, but the
coral-list should hear their side):
The Smith, Risk, Schwarcz and McConnaughey paper above (Nature 1997) is
a nice presentation of isotopic changes in deep-water coral skeletons
during the Younger Dryas event. These data undoubtedly record a change
in the water mass overlying Orphan knoll (50 26'N 46 22'W and 1600 m depth
- note that this location is not really the Gulf Stream, but the North
Atlantic Deep Water). However, these data do not *necessarily* record
a change in the western boundary current. Western boundary currents
can remain unchanged while water masses change (in fact, the Gulf
Stream tends to maintain its track under a wide range of conditions).
So this challenge (with insult) to modelers to duplicate implied
boundary current changes, based on corals from a single location,
does not provide adequate evidence that "a meltwater pulse can divert
or shut down the Gulf Stream in less than 5 years". Now that being said,
in terms of modeling changes in the Gulf Stream (and North Atlantic
circulation in general) in response to surface buoyancy changes
(i.e., changes in temperature and/or freshwater input), there ARE
models that do capture such changes, and they show that the response
CAN be rapid (5-10 years). Two examples of such papers:
Gerdes and Koberle, 1995. J. Phys. Oceanography 25: 2624-2642.
Lohmann and Gerdes. 1998. J. Climate 11: 2789-2803.
> So, in short, Kleypas et al:
> 1. depends on reef models that ignore >50% of the process
> 2. depends on outmoded oceanic circulation models
> 3. ignores some fundamental chemical questions.
Regarding 3 - Bob Buddemeier has already provided enough answers.
Certainly there are complications in carbonate chemistry near
continental margins, which will result in a range of reef response
to changes in carbonate chemistry. But given the volume of the
oceans versus that of river and reef sediments, isn't it likely
that coral reefs will be bathed in waters overwhelmed by the
increasing pCO2? I personally would like for Mike's #3 to be
true, but none of the chemical oceanographers that I have spoken
with (Takahashi, Broecker, Archer, Tans, etc.) have pointed to
any ignored fundamental chemical question in this hypothesis. My
fear is that Mike's statements like those above will convince many
to dismiss the carbonate chemistry issue based on hunches rather
than adequate scientific justification.
> My main concern with that paper is that it may have diverted intellectual
> and financial resources from more pressing problems. Sure, changes in
> saturation state will eventually affect....what? What will be left, in say
> 100 years? pH changes in the ocean, in my opinion, don't make the Top Twenty
> Reef Threats. The rate of present destruction from land-based sources and
> overfishing simply dwarfs everything else.
I agree that reefs sadly face many threats. We anticipated the
that some scientists would feel that their own "reef issue" would
be overshadowed by this problem. Because the calcification question
is global in nature, and because it is a direct and predictable consequence
of CO2 (even predictions of bleaching involve questions about just how much
the oceans will warm), I and others consider this a serious chronic and
increasing threat to reefs (and perhaps to other calcifiers such as
coccolithophorids - see Riebesell et al. 2000). But politically, the
issue is powerful, and any solution which would mitigate increases
in CO2 would certainly mitigate many of the other threats to reefs
as well. And honestly, this issue has gotten so minimal attention
and funding since the paper was published that I can only conclude
that most people don't fully understand its scope. I take some of
the blame for not pushing it hard enough, but there is also a
significant amount of misinformation that is going around.
Thanks again to Mike for bringing up these issues.
cheerio, J Kleypas
Climate & Global Dynamics
National Center for Atmospheric Research
PO Box 3000
Boulder, CO 80307-3000
(For FedEx use: 1850 Table Mesa Drive
with zip code: 80305)
PH: (303) 497-1316
FAX: (303) 497-1700
kleypas at ncar.ucar.edu
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