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

As part of an `interview' with me, New Scientist published a 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 replies.

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 reefs.

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 inspection'.

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 age.

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:http://news.bbc.co.uk/1/hi/sci/tech/8511670.stm). 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 assumed.'

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 temperatures.

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 buffered.

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 future.'

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 so.

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 pCO2'.

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 exaggerated.