One of the premier future victims of climate change, besides, ya know, OURSELVES, has been corals, those bizarre marine invertebrates which collectively form reefs by secreting calcium carbonate. Most live in symbiosis with dinoflagellates, exchanging safety for energy derived from photosynthesis. Coral bleaching, according to NOAA, occurs when
Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centered around the northern Antilles near the Virgin Islands and Puerto Rico expanded southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined.
Acidification is important as it affects the ability of coral to build a form of calcium carbonate called aragonite.
Back in 2000 NewScientist (paywall) summarized coral reef research in connection with climate change:
The team manipulated carbonate concentrations by regularly adding acid to the water to recreate the effect of increased CO2. For the first two years of the project, the team held the carbonate concentration near the low level predicted for 2050, which is 30 per cent lower than today’s ocean concentrations. The following year they increased it to the current level, and the next year raised it beyond pre-industrial levels.
To determine calcification rates, they measured how much carbonate and calcium were removed from the water between acid additions. The reef showed no sign of acclimatising to lower carbonate concentrations, says team leader Chris Langdon of Columbia University, and their results suggest that the decrease in coral growth between 1880 and 2065 will be about 40 per cent. “That’s an enormous hit on shallow marine ecosystems,” says Buddemeier.
“It was quite surprising to see that it was so strong,” says Langdon. Previous projections were lower. But the idea that rising levels of CO2 would have such a heavy impact on corals is well supported in the geological record, he says. At least twice in Earth’s history, levels are thought to have risen even higher than those predicted for the end of this century. During those times large reefs were nonexistent and whole coral families became extinct.
Just this April came this note from Ove Hoegh-Guldberg in NewScientist:
Once, it might have taken the extra ocean warming of an El Niño to cause bleaching, but we’re now getting to the point where even regular temperatures are getting high enough. When we predicted this in 1999, I became a pariah. People were saying “No, that’s not possible.” But it’s coming true – no one’s been able to knock that idea off. And I think that 20 years from now, every summer will be too hot for corals: they will disappear as dominant members of tropical reef systems by 2040-2050. It’s hard to argue it any other way.
But now NewScientist‘s Michael Slezak (20 June 2015) reports on a new discovery – some corals survive these newly stressful situations. Some of the surprise is due to using poorly chosen proxies for reality:
As carbon dioxide is pushed into the oceans, it forms an acid. This causes a subtle change in chemistry that lowers the water’s saturation in aragonite, a form of calcium carbonate that corals use to grow and build reefs. Above a saturation state of 1, aragonite begins to precipitate out of the water and can be used to form shells. For years, coral biologists have used this measure as a proxy for estimating reef growth rates. Oceans are currently at an aragonite saturation state of around 3.8. Early experiments suggested corals would stop building reefs when the saturation state dropped below 2.5.
Recently, that crucial level has been put in doubt by people looking more closely at how corals build reefs. “All those alarming predictions were based on just the chemistry,” says Adina Paytan at the University of California, Santa Cruz.
So after some research, there’s a tentative push to update the predictions:
Overall, the team found that acidification had about half the impact on coral reef building than previously thought. They calculate that under the worst climate change scenario, with a rampant rise of greenhouse gas emissions, reef-building will slow by between 15 and 35 per cent by 2100, depending on the coral species. Studies by Alexander Venn at the Scientific Centre of Monaco came to similar conclusions. “Ocean acidification doesn’t help, but it’s not by itself a major problem,” says McCulloch.
In an ironic twist, when McCulloch plugged in the added effect of ocean warming, the story got even better. “Corals in warmer temperatures tend to calcify faster,” he says. “Warming helps the process if it’s not stressful.” His conclusion: coral reef building rates won’t change as CO2 emissions rise this century. Not one bit.
But don’t sit back comfortably in your chair:
And there’s still the other impact of climate change. When things get too hot, the algae that live in symbiosis with corals – lending them their vibrant colours and a ready supply of energy-rich sugars – move out, leaving the reefs looking pale and ghostly. Low-level coral bleaching isn’t uncommon or irreversible. But when temperatures rise rapidly, these algal battery packs move out for too long and the corals die.
And these results are not yet fully accepted by the marine biology community – such as Hoegh-Guldberg. However, keep in mind that corals are living organisms, with DNA that mutates – meaning genes coding for phenotype features useful to survival in the new environment can be passed on. So if corals die, it doesn’t mean they all die – or can’t come back:
Of all the world’s reefs, those in the Seychelles were worst affected by the 1998 bleaching, says Nicholas Graham at James Cook University in Townsville, Australia. He and his colleagues gathered data from 21 coral reefs around the Seychelles during the 17 years following the El Niño. At first it was near-total destruction. More than 90 per cent of the coral was gone, a state of affairs that was largely unchanged for a full decade. Nine of the 21 reefs were taken over by seaweed and, in Graham’s words, are as good as lost. In 2006, his team published a gloomy report on the reef’s outlook whose pessimistic conclusions have often been cited in reports about corals and climate change.
But between 2005 and 2011, something remarkable happened: coral cover returned almost completely on every other reef. Hard coral had covered 28 per cent of the area before the bleaching; by 2011 it was back up to 23 per cent. Graham says the reefs are on a clear path to a full recovery. Similar findings have been coming in from reefs all over the world.
A useful reminder that if climate change overwhelms humanity, it doesn’t mean a dead world – just a world without humanity. Other organisms will adapt and continue.
