[Coral-List] HadISST 1.1 data: Bleaching is not due to Warming

Ove Hoegh-Guldberg oveh at uq.edu.au
Tue Jun 1 23:10:56 EDT 2004

Dear Martin,

The only problem is that one can use, very successfully, satellite
measurements of SST anomalies and predict mass coral bleaching by as much as
days and even weeks in advance.  Also - how many major bleaching events have
occurred without an associated SST anomaly?  Not many, it seems.  These
things need explanation.  

"Facts are facts" except yours are unreviewed and unsubstantiated. Why not
use the age-old method and submit your manuscript to proper peer-review?   



-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Martin Pêcheux
Sent: Wednesday, 2 June 2004 8:56 AM
To: coral-list at coral.aoml.noaa.gov
Subject: [Coral-List] HadISST 1.1 data: Bleaching is not due to Warming

Dear Coral-listers,

I know that I will shock the common belief on this very important topic. But
the facts are the facts. I was myself surprised. So I decided to send this
draft to the list before submission to a publication, hoping for comments or
critics. Thanks a lot. A long version is available at request.

HadISST 1.1 data : Coral Reef Mass Bleaching is not due to Global Warming.

Martin Pêcheux, June 2004
Nice University, 94260 Fresnes, France
Email : martin-pecheux at wanadoo.fr

Coral reef mass bleaching is the main threat on an Earth ecosystem. There is
a consensus among reef researchers that Global Warming is the cause. Here I
analyse HadISST 1.1 data, a new release of Sea Surface Temperature set, from
1950 to 2000, with 1° degree resolution, monthly averaged, at 50 reef sites
with known bleaching record. First, the Global Warming is all but global.
Whereas in Indian ocean the warming of yearly maximum SST is significative,
as well as in the Pacific, the Caribbean has seen a general cooling from
1950 to about 1986, then a warming. Secondly, I determine for the studied
sites the relevant parameter for bleaching ÆT, i.e. the minimum temperature
at which bleaching has occurred, minus the maximum temperature at which
bleaching was absent during the 1950-1978¹s. In fact, only 16% of the sites
have positive ÆT : there was generally higher temperature in 1950-1978
without bleaching than now with bleaching eliciting. Bleaching threshold is
also analysed. I must conclude that, although temperature is an obvious
parameter (as irradiance), Global Warming is not the primary cause of
bleaching. This lets a direct bleaching action of CO2 rise and seawater
acidification as the only possible other global cause of coral reef mass

Coral reef mass bleaching begin in 1979 (reviews in Williams and
Bunkley-Williams, 1990, Smith and Buddemeier, 1992, Glynn, 1996, Brown,
1997, Pêcheux, 1997 - unpublished/online, Hoegh-Guldberg, 1999, Coles and
Brown, 2003). Reef researchers were at first destabilised about its cause.
But with repetitive reports of bleaching with "above average" temperatures
in summer, and the following of "hot spot" formation from satellite (Goreau
and Hayes, 1994) in particular in real time (NOAA indices, Strong et al.,
1997, Lui and Strong, 2003,
orbit-net.nesdis.noaa.gov/orad/coral_bleaching_index), an overwhelming
consensus emerges as to pinpoint Global Warming as the cause. But, by
definition, "above average" temperatures and "hot spots" are a normal
phenomenon, which have occurred also before mass bleaching began. My goal
was to examine such phenomenon, i. e. what is the difference ÆT between
present temperatures with bleaching and past maximum without bleaching.
Mainly due to lack of historical data, there have been only one serious
attempt to check this feature. Coffroth et al. (1989) examine the 1982 event
of the (well studied) Great Barrier Reef. They conclude unambiguously that
the temperature was elevated but not exceptional. They also discard any
conjunction with other physical factors, such as luminosity, water
transparency, winds, rainfall or tide. Elms (1992, discussed in Atwood et
al., 1992) analyses the GOSTA data from 1950 to 1989 over fifteen 5°x5°
zones in Caribbean and found cooling of the yearly mean in fourteen of them,
with a mean of ­1.00°C/century. Some other less relevant works are analysed
in Pêcheux (1997). Lough (2000) studies the GISST data (1903-1994) and IGOSS
data (1982-1999) at 47 world-wide coral reef sites, in an approach rather
similar to mine. She focuses on global trends and on the unprecedented
1997-1998 event. Nonetheless, she pinpoints that 1958 was more extreme for
pooled Caribbean sites. Last but not least, it is known that bleaching of
large foraminifers peaks at summer solstice and not during the maximum
temperature a few months later (Williams et al., 1997).

Here I used the HadISST 1.1 data, newly released by the British Atmospheric
Data Centre, Met Office Hadley Centre, UK, an improved reconstructed Sea
Surface Temperature record (Rayner et al., 2003;
badc.nerc.ac.uk/home/index.html) and descendant of GOSTA-GISST data. I
restricted my analysis to the years 1950-2000. SST are averaged on a 1°
longitude x 1° latitude grid for each month. Bleaching records at 50 world
wide sites are taken from the literature or from the Reef Base
(www.reefbase.org) and the GCRMN reports (www.aims.gov.au). The bleaching
temperature was taken as the temperature of the month at which the bleaching
began if it was known, or (rarely) of the month before if the temperature
was superior, otherwise it was used the maximum temperature of the year
(Table 1).

RESULTS (Table 2)
Variance of yearly maximum SST.
At each site, yearly maxima appear as normal distributed (stochastic) (Fig.
2). There is a slight excess of "abnormal" standard score greater than 3
standard deviations (return time >741 years - Gauss integral). They
correspond to the El Niño 1983 (Panama, Galapagos) and the 1998 cycle
(Bahamas, Galapagos, Phuket, Seychelles, Maldives and Kenya), and also
Lizard Island, GBR, January 1970. For the 50 sites, the standard deviation
of the yearly maximum is Poisson-distributed, being 0.384°C±0.137 (range
0.240 to 0.966). A geographical differentiation exists: Caribbean is less
variable, 0.298°C±0.030, than Pacific, 0.461°C±0.173, and Indian ocean,
0.350°C±0.040. Warming of the yearly maximum SST. A simple analysis of
warming, not of the year data, but only of the maximum each year, is
informative. In Indian ocean (9 sites, without Persian Gulf), the warming is
regular, being +1.055°C/century, p<0.0001. In Pacific (20 sites), the
warming is not so great, being +0.707°C/century, p=0.008. At contrast, in
Caribbean (18 sites), there is a cooling during a first period, 1950-1986
(determined by maximising slope) of -1.075°C/century, p<0.0001, then a
strong recent warming, 1987-2000, +1.803°C/century, p=0.008. ÆT, minimum SST
with bleaching minus maximum SST 1950-1978 without. ÆT is generally
negative. Only 9 sites have a positive value. The mean value is
-0.668°C±0.685 (-2.25 to 1.01), normally distributed. Values are all
negative for Caribbean (-0.821°C±0.466, -1.68 to ­0.02), negative for Indian
ocean (-0.331°C±0.493, -1.15 to 0.41). It is flat distributed for Pacific
(-0.672°C±0.883, -2.25 to 1.01), where positive value are encountered mainly
in the East Pacific with the El Niño 1983 (Coco Islands and Galapagos), and
in the West Pacific. The 1950-1978 maxima are rather evenly distributed,
apart 13 world wide occurrences of the year 1969, as already noted by Elms
(1992) in Caribbean.
Bleaching threshold from maximum mean.
I search for a bleaching threshold by subtracting the mean of yearly maximum
SST 1950-2000 from the minimum bleaching SST. The value is, without high
values of the El Niño 1983 at Galapagos and Coco Island, -0.0057°C±0.540
(-1.184 to 1.157). The threshold is well correlated with ÆT (r2=O.785,
p<0.0001). The discrepancy with NOAA indices (bleaching at 1°C above maximum
mean) does not come from different maximum means.

Bleaching record
.It is certainly incomplete. But a more precise record could only lower the
minimum temperatures at which bleaching begin. There is some inverse
correlation between ÆT and the number of event per site (r2=0.274,
p=0.0001), but whatever the sites are well monitored or not. The ÆT is based
on the assumption that no "unexplained" mass bleaching took place before
1979 (in Bonaire, Hot in Williams and Bunkley-Williams, 1990). This
certainly holds since 1950. There was enough coverage of reefs by
scientists, at least in enough places. There are several reports of mass
bleaching before 1979, always with obvious local causes (heated reef flat,
thermal pollution, fresh or turbid water, cold event, sedimentation (see
ref. in Pêcheux, 1997). And, at contrast to corals which stop calcification,
foraminifers show frequent spectacular shell abnormalities in association
with bleaching, which were unknown in previous time (Talge et al., 1997,
Pêcheux, 1997) apart at the Cretaceous/Tertiary boundary (Pêcheux, 1999).
Relevance of HadISST data for coral reef mass bleaching. Its accuracy might
not be perfect, but I used only the most recent data. In any case, the
magnitude of the mean ÆT, 0.68°C, is far above any inaccuracy. The average
over one month may not capture the exact value of SST, but the time dynamics
are slow and regular. The 1°x1° grid (~110 km square) may obscure slightly
the formation of relevant "hot spots", and be less significative for shore
reefs. But this surely does not hold for fore reefs well bathed by oceanic
waters. And I am confident that the physical relationship between mesoscale
open ocean temperatures and local shore ones could not have changed so
dramatically since 1979. HadISST data can be well compared with the NOAA
satellite bleaching-warning indices in real time, which are quite
successful. They have a slight higher spatial resolution (50 km) but
bleaching events show extended zone of warm waters. Time resolution is twice
a week, but the most precise index, the Degree Heating Weeks, must reach 4
on its scale before bleaching, corresponding to 1 degree above maximum mean
during 4 weeks. Bleaching threshold. It is null because I used the minimum
bleaching SST, at contrast to Goreau and Hayes (1994) and NOAA indices,
which established empirically a +1°C threshold from all records. The value
of the mean standard deviation, 0.384°C, means that a SST with "1°C above
mean maximum", i.e. 2.6 time the standard deviation, occurs each 217 years
(range 63500 to 6.65 years), which is obviously false. The standard
deviation has not increase recently (mean SD=0.362°C for 1990-2000). I guess
that NOAA indices capture important events but not all, in particular in
Caribbean with low variance. A null minimum bleaching threshold means that
bleaching is in the yearly maximum mean, said, it has a 50% probability, or
can occurs every two years, which is also obviously false. This does not
mean that the data are false, of course. Temperature is a factor but not the
only one. There are also other triggering factors, the firsts in mind being
clear sky (full light and
UV) and dolldrum time (reduced cooling evaporation, more water transparency,
less circulation, and, more important in my opinion, impaired exchanges at
organism interface, i.e. the "unstirred layer effect"). Whatever the case,
the small variances mean a high sensitivity to a few tenth of °C. A very
crude calculation of the true bleaching threshold can be done using the
frequency of the bleaching events and the variance of the maximum
temperature. The mean frequency of bleaching is 14.2% (mean of event number
2.98/21 years) (return time 7.05 years), corresponding to 1.076 standard
deviation of the maximum mean (0.384°C), thus giving a bleaching threshold
of 0.413°C.

The cooling of the maximum SST over the Caribbean till 1986 and, more
definitive, the negative values of most ÆT, almost twice the standard
deviation, conduce to the disturbing but inescapable conclusion that the
increasing temperature is not the primary cause of coral reef mass
bleaching. It appears when maximum summer stress, of course, but my result
call for other synergetic factor(s). And this is not contradictory with the
fact that mass bleaching may occur also more and more frequently with future
Global Warming. Ultra-violets, at first often considered, are not in cause,
as they have not increase in tropics, and there is no relationship between
ozone drawdowns and bleaching events (Pêcheux, 1996a). Moisture deficit
increases over warm pools in tropics (Flohn and Kapala, 1989, Graham, 1995),
but it is of low probability that it induces in the 80¹s an hydrological
pattern change great enough to be responsible of local new maxima 0.7°C
above previous one. It would also rather increase nebulosity over warm
waters ("cirrus anvil thermostat"). And of course there were summer dolldrum
times with clear sky before 1979. This lets a direct effect of CO2 rise as
the last possible global cause. There are many arguments in favor of this
hypothesis (Pêcheux, 1997). CO2 is, like many other parameters, a bleaching
factor (Pêcheux, 1994, 1996b). CO2 rise from 280ppm to 360ppm CO2 is already
equivalent to a warming of 0.4°C, more probably 1.2°C (Pêcheux, 2002), which
fits well with ÆT. Often forgotten, the fact that bleaching affects not only
corals but also all other cnidarians, molluscs, sponges, ascidians, and
large foraminifers in symbioses with either dinoflagelates, diatoms or
cyanobacteria implies a fundamental limitation of their photosynthesis, the
two main of which being photoinhibition and photorespiration, both under
strong influence of symbiose-limited CO2. Given future expected CO2 level,
and with the addition of Global Warming, coral reefs must be considered in
great danger. The belief in adaptation to CO2 and temperature rises 100
faster than at the interglacial termination is rather a matter of opinion,
with very few facts (discussion in Coles and Brown, 2003, Hughes et al.,
2003). I think that coral reefs will disappear in the next decades if CO2
rise is unabated. If so, fight for strong mitigation of anthropogenic CO2
rise must be our top priority.

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