Bleaching corals and global warming.

[Rick Grigg]

John,

	Not to downplay the potential significance of bleaching events on coral
reefs, it should also be understood that reefs are capable of rapid
recovery after bleaching events have occurred.  Sometimes this can take
place in as short a period as 3-5 years.  In other places, recovery may
require 10 years or several decades depending on intervening disturbance,
growth rates, etc.  Now, if severe bleaching events are episodic in nature,
any model developed to predict long-term outcomes must include intermittant
recoveries.  To site one example, the reefs in Palau are described as
undergoing significant recovery from the '98 bleaching event.  I mention
this not to lessen our concern or attention to the problem, but to suggest
that recovery stages be built into the models.
						Rick Grigg

At 10:44 AM 11/13/00 -0500, John Ware wrote:
>Dear List,
>
>For an alternative view of the possible future of bleaching events, you
>may wish to consider the paper that I presented at the 8th ICRS in
>Panama in 1996 and which is in the proceedings.  My prediction method
>differed from Ove's is several ways:
>
>1- A non mechanistic global temperature model based on spectral analysis
>of long term temperature trends.  Such a model can have significant
>cooling periods (e.g., could have a 'Little Ice Age'), as well as
>significant warming periods due to natural climatic variations.  The
>mechanistic models used by Ove do not have such events.
>
>2- Because of the possible natural variations and the relative
>simplicity of the model, my probability calculations were based on an
>average of 1000 runs, not a single run.
>
>3- I considered both an IPCC 550 warming scenario and a 1/2 IPCC 550 to
>account for possible latitudinal temperature gradients as well as the
>potential (and controversial) cooling effects of aerosols and also a
>possible 'thermostat' effect.
>
>4- I considered possible acclimation of the coral/algal symbiosis. 
>While there does not appear to be any direct evidence for acclimation,
>my simulations showed that even acclimation lags of 25-50 years could
>substantially mitigate warming effects.  Acclimation lags of this
>duration would probably not be detectable in the relatively short time
>that bleaching events have been observed to be increasing in frequency.
>
>5- Not presented in the paper was the observation that bleaching events
>tended to be grouped in time and not evenly distributed.  This is, of
>course, due to low frequency shifts in global climate (*not* at
>Milankovitch time scales).  Alternatively, long periods without
>bleaching events cannot be taken as evidence of a lack of warming for
>the same reason.
>
>Having said this, the basic difference between the computations that I
>made and the (much more famous) results presented by Ove, is that
>disaster for reefs occurs a few decades later in time.  However, the
>reason that I bring this alternative view to your attention is that,
>based on my modeling, we could have significant periods with little or
>no bleaching even though global warming continues.  Should the next 10
>years or so show little bleaching, we should not consider this evidence
>that reefs are acclimating or that warming is not occurring or that
>there is some sort of failure in Ove's predictions.  
>
>While we should not over-react to a period of increased bleaching, we
>should likewise not become complacent if a period without bleaching
>occurs.
>
>John 
>
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>     *                                                           *
>     *                       John R. Ware, PhD                   *
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