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Corals under threat? Yes, but not much from either warming or acidification.

As part of an `interview’ with me, New Scientist published
critique by five scientists of two pages of my
book The Rational Optimist. Despite its tone, this critique only
confirms the accuracy of each of the statements in this section of
the book. After reading their critiques, I stand even more firmly
behind my conclusion that the threats to coral reefs from both
man-made warming and ocean acidification are unlikely to be severe,
rapid or urgent. In the case of acidification, this is underlined
by a recent paper, published since my book was written, summarising
the results of 372 papers and concluding that ocean acidification
`may not be the widespread problem conjured into the 21st century’.
The burden of proof is on those who see an urgent threat to corals
from warming and acidification. Here is what I wrote (in bold),
interspersed with summaries of the scientists’ comments and my

Take coral reefs, which are
suffering horribly from pollution, silt, nutrient run-off and
fishing – especially the harvesting of herbivorous fishes that
otherwise keep reefs clean of algae. Yet environmentalists commonly
talk as if climate change is a far greater threat than these, and
they are cranking up the apocalyptic statements just as they did
wrongly about forests and acid rain

Andy Ridgwell says `I agree that at least for some reef systems,
other, and more local human factors such as fishing and pollution
may be the greater danger’ and Jelle Bijma says `I do agree that,
for example, pollution and overfishing are also important problems,
some even more important than the current impact of ocean
acidification’. It was not therefore accurate of Liz Else to say
that the critics accuse me of failing `to recognize that there is
more to the health of corals than the amount of bicarbonate in the
sea’ They do not – she has misrepresented their views and mine.

Charlie Veron, an Australian
marine biologist: ‘There is no hope of reefs surviving to even
mid-century in any form that we now recognise.’ Alex Rogers of the
Zoological Society of London pledges an ‘absolute guarantee of
their annihilation’. No wriggle room there.

Chris Langdon agrees that such claims `may be extreme’. None of
the others provides any evidence to support such extreme claims.
Yet these remarks were widely reported in the media.

It is true that rapidly heating
the water by a few degrees can devastate reefs by ‘bleaching’ out
the corals’ symbiotic algae, as happened to many reefs in the
especially warm El Niño year of 1998. But bleaching depends more on
rate of change than absolute temperature. This must be true because
nowhere on the planet, not even in the Persian Gulf where water
temperatures reach 35°C, is there a sea too warm for coral

Ove Hoegh-Guldberg says that `the observation that corals grow
in the Persian Gulf today at temperatures of 35 °C does not mean
that coral reefs will be able to adapt rapidly to the current
upward shift in sea temperatures’ in other words, he concedes the
point I was actually making: bleaching is caused by rate of change
of temperature, not absolute level of warmth. This is not
understood by many commentators on the subject in both the
environmental movement and the media. I am glad to have it
confirmed, because it corrects a widespread misunderstanding.

Lots of places are too cold for
coral reefs – the Galapagos, for example.

Ridgwell says that `There are in fact several reef communities
in the Galapagos, so the inference that the Galapagos is “too cold”
is incorrect (or at best, mis-interpretable), although I agree that
colder temperatures are likely an important factor in the dominance
of non-reef coral communities in this location.’ Which is it?
`Incorrect’ or `an important factor’? He concedes my point in his
last phrase: `the dominance of non-reef coral communities in this
location.’ The very few reefs are in the warmer parts of the
Galapagos. Incidentally, Charles Darwin once wrote: `There are no
coral-reefs in the Galapagos Archipelago, as I know from personal

It is now clear that corals
rebound quickly from bleaching episodes, repopulating dead reefs in
just a few years,

None of the five challenge this statement. As an example, a
study of Fiji’s reefs following a bleaching episode (Lovell and
Sykes 2008. International Coral Reef Symposium) states: `Though
variable, substantial recovery to pre-bleaching levels was seen
within 5 years in many areas.’

which is presumably how they
survived the warming lurches at the end of the last ice

Both Ridgwell and Hoegh-Guldberg claim that current rates of
temperature change are unprecedented. Ridgwell says that the
deglacial transition `was a few degrees centigrade in about 4000 to
5000 years. In the future, we are looking at a few degrees in a
hundred years – perhaps 50 times faster (certainly, one to two
orders of magnitude higher).’ Hoegh-Guldberg refers to a rate of
change `that is many times higher than even the most rapid shifts
in conditions seen over the past million years or more.’ These are
astonishing statements to anybody with even a cursory knowledge of
the scientific literature on the ending of the last ice age. The
current rate of temperature change since 1975 is estimated at about
0.161 degC per decade (and is incidentally not statistically
distinguishable from that in the 1860-1880 or 1910-1940 periods –
see Roger Harrabin’s interview with Phil Jones here:
By contrast the deglacial transition was characterized by `local,
regional, and more-widespread climate conditions [which]
demonstrate that much of the Earth experienced abrupt climate
changes synchronous with Greenland within thirty years or less’
(Alley 2000. Quaternary Science Reviews 213-226), including `a
warming of 7 °C in South Greenland [that] was completed in about 50
years’ (Dansgaard, White and Johnsen 1989, Nature 339: 532). That
is a change roughly nine times as fast as has happened since 1980 –
in Greenland or anywhere else. Another study gives even bigger
numbers, saying that the `abrupt warming
(10 ± 4 °C)’ at the end of the Younger Dryas and the
warming at the end of a short lived cooler interval known as the
Preboreal Oscillation `may have occurred within a few years’
(Kobashi et al 2008 Earth and Planetary Sciences 268:397). Nor was
this rate of change confined to Greenland. As one article
summarises, `temperatures  from the end of the Younger Dryas
Period to the beginning of the Holocene some 12,500 years ago rose
about 20 degrees Fahrenheit in a 50-year period in Antarctica, much
of it in several major leaps lasting less than a decade.’ (Science
Daily, Oct 2 1998). It is remarkable how few scientists working on
other aspects of planetary ecology seem to know about these recent
conclusions of much faster changes in the past. No climatologist
would these days claim that current rates of change are
unprecedented in `the past million years or more’.

It is also apparent from recent
research that corals become more resilient the more they experience
sudden warmings.

None of the five challenges this statement, which is based on a
paper by Oliver and Palumbi 2009 (MEPS 378:93), which concluded
that corals are `tougher than we thought’ (interview with Science
News May 22, 2009) and on Baker et al 2004 (Nature 430:741), who
say: ‘The adaptive shift in symbiont communities indicates that
these devastated reefs could be more resistant to future thermal
stress, resulting in significantly longer extinction times for
surviving corals than had been previously

Some reefs may yet die if the
world warms rapidly in the twenty-first century, but others in
cooler regions may expand.

Ridgwell agrees `that eventual colonisation and expansion of
corals into regions previously too cold will, in theory, be
possible at some point in the future’ so there is no inaccuracy in
my statement. He merely says that it is `unclear’ whether dispersal
and colonisation can occur fast enough to keep up with increasing

Local threats are far more
immediate than climate change.

Ridgwell agrees `that at least for some reef systems, other, and
more local human factors such as fishing and pollution may be the
greater danger’ but says this may not be true for those in
protected areas – because the local threats there have been
reduced. That is merely a statement of the obvious. But the
greatest threats to coral reefs come outside protected areas.

Ocean acidification looks
suspiciously like a back-up plan by the environmental pressure
groups in case the climate fails to warm: another try at condemning
fossil fuels.

A statement of my opinion based on what follows.

The oceans are alkaline, with an
average pH of about 8.1, well above neutral (7).

Langdon confirms this: `Yes, it is true that the surface oceans
are slightly alkaline at a pH of 8.1′ but then says that `the
declining pH of the surface ocean is one of the most firmly
established facts in climate change science.’ Is he implying that I
dispute this? I do not. Incidentally, the pH of the ocean varies
hugely, being below neutral in some inshore areas influenced by run
off from the land. On some coral reefs it goes as low as 7.5 at
night and as high as 9.4 in the day (Revelle and Fairbridge 1957).
Remarkably there are parts of the sea with pH already far lower
than it can possibly go as a result of carbon emissions. In one
hydrothermal spot off Iceland, it is 5.36-7.29.Yet four-decade-old
mussels have learned to cope with even this acidity, though growing
half as fast as in normal waters (Tunnicliffe et al 2009, Nature
Geoscience 10.1038).

They are also extremely well

Langdon agrees: `And yes, the oceans are well buffered’.

Very high carbon dioxide levels
could push that number down, perhaps to about 7.95 by 2050 – still
highly alkaline

Presumably it is here that Bijma thinks I `introduce confusion
about the term “acidification”‘ merely because by saying that 7.95
is still highly alkaline, I am accurately reminding the reader that
there is no prediction of the oceans becoming technically `acid’ –
ie having a pH lower than 7. Far from introducing confusion, I was
attempting to reduce the very confusion so often encountered by
readers who think that acidification will lead to oceans that are
actually acid. In any case, my statement is accurate.

and still much higher than it was
for most of the last 100 million years

Ridgwell agrees: `Ocean pH in the past (at least, according to
published reconstructions) was indeed lower than now during the
Cretaceous, and probably lower than anything we will manage in the

Some argue that this tiny
downward shift in average alkalinity could make it harder for
animals and plants that deposit calcium carbonate in their
skeletons to do so. But this flies in the face of chemistry: the
reason the acidity is increasing is that the dissolved bicarbonate
is increasing too –

Langdon agrees: `Matt is correct that bicarbonate concentrations
are increasing’.

and increasing the bicarbonate
concentration increases the ease with which carbonate can be
precipitated out with calcium by creatures that seek to do

Here there seems superficially to be a disagreement, but in
reality there is none. Ridgwell, Langdon and Bijma say that
carbonate levels fall rather than rise as a result of increasing
dissolved carbon dioxide. But I don’t say that carbonate levels
rise. I say that the biological precipitation of carbonate by
organisms is easier at higher bicarbonate levels. And Langdon
confirms this: `Matt is correct that the skeleton and shell
building of some species is unaffected or even increases under
reduced pH’. My evidence? For example, Ries et al 2009
(Geology37:1131) found that in seven of the 18 species of
calcifiers they observed `net calcification increased under the
intermediateand/or highest levels of pCO2‘.
And that their results `suggestthat the impact of elevated
atmospheric pCO2 on marine calcificationis more varied
than previously thought, while Hendriks et al 2010
Estuarine, Coastal and Shelf Science 86:157) found that
 the ion chemistry inside the bodies of calcifiers is more
important than that outside them, and there is evidence that some
of them – eg coccolithophores – actually find it energetically
easier to deposit carbonate shells at slightly lower pH.

Even with tripled bicarbonate
concentrations, corals show a continuing increase in both
photosynthesis and calcification.

My source was the Herfort et al 2008 paper, which Ridgwell says
is irrelevant, because of its experimental design. That’s his
opinion, which others in the field do not share. In any case, my
statement was a correct and precise description of the result.

This is confirmed by a rash of
empirical studies showing that increased carbonic acid either has
no effect or actually increases the growth of calcareous plankton,
cuttlefish larvae and coccolithophores

Hoegh-Guldberg disagrees: `Call it inconvenient but the vast
bulk of scientific evidence shows that marine calcifiers such as
coccolithophores, corals and oysters are being heavily impacted
already by ocean acidification.’ He provides no reference. By
contrast, I cite Iglesias-Rodriguez et al 2008 (Science 320:336).
They state: `From the mid-Mesozoic, coccolithophores have beenmajor
calcium carbonate producers in the world’s oceans, todayaccounting
for about a third of the total marine
CaCO3production.Here, we present laboratory evidence
that calcification andnet primary production in the coccolithophore
species Emilianiahuxleyi are significantly increased by
high CO2 partial pressures.Field evidence from the
deep ocean is consistent with theselaboratory conclusions,
indicating that over the past 220 yearsthere has been a 40%
increase in average coccolith mass’.

As for oysters, Miller et al. 2009 (PLOS ONE 4:
10.1371) found that oyster larvae `appeared to grow, calcify and
develop normally with no obvious morphological deformities, despite
conditions of significant aragonite undersaturation,’ and that
these findings `run counter to expectations that aragonite shelled
larvae should be especially prone to dissolution at high

As for sea urchins, Lacoue-Labarthe et al. 2009
(Biogeosciences 6)  report that `decreasing pH resulted in
higher egg weight at the end of development at both temperatures (p
< 0.05), with maximal values at pH 7.85 (1.60 ± 0.21 g and 1.83
± 0.12 g at 16°C and 19°C, respectively).’.

As for corals, Suwa et al. 2010 (Fisheries science
76) report that `larval survival rate did not differ significantly
among pH treatments.’

Lest my critics still accuse me of cherry-picking studies, let
me refer them also to the results of Hendrikset al. (2010,
Estuarine, Coastal and Shelf Science 86:157). Far from being a
cherry-picked study, this is a massive meta-analysis. The authors
observed that `warnings that ocean acidification is a major threat
to marine biodiversity are largely based on the analysis of
predicted changes in ocean chemical fields’ rather than empirical
data. So they constructed a database of 372 studies in which the
responses of 44 different marine species to ocean acidification
induced by equilibrating seawater with CO2-enriched air had been
actually measured. They found that only a minority of studies
demonstrated `significant responses to acidification’ and there was
no significant mean effect even in these studies. They concluded
that the world’s marine biota are `more resistant to ocean
acidification than suggested by pessimistic predictions identifying
ocean acidification as a major threat to marine biodiversity’ and
that ocean acidification `may not be the widespread problem
conjured into the 21st century…Biological processes can provide
homeostasis against changes in pH in bulk waters of the range
predicted during the 21st century.’ This important paper alone
contradicts Hoegh-Gudlberg’s assertion that `the vast bulk of
scientific evidence shows that calcifiers… are being heavily
impacted already’.

In conclusion, I rest my case. My five critics have not only
failed to contradict, but have explicitly confirmed the truth of
every single one of my factual statements. We differ only in how we
interpret the facts. It is hardly surprising that my opinion
is not shared by five scientists whose research grants depend on
funding agencies being persuaded that there will be a severe and
rapid impact of carbon dioxide emissions on coral reefs in coming
decades. I merely report accurately that the latest empirical and
theoretical research suggests that the likely impact has been

By Matt Ridley | Tagged:  rational-optimist