Ocean acidification: cooler or not, reason to take CO2 seriously

There is a rumour afoot among the global warming sceptics: a cooler global temperature trend since 1998 means that the IPCC is losing face, so the IPCC is moving ground from using temperature increase as justification for CO2 thriftiness to another threat – ocean acidification.

Bob Carter has argued this here at On Line Opinion. At Jennifer Marohasy’s sceptically inclined blog, contributor Paul Biggs has twice this year also suggested as much.

There may be a movement in the debate, but in my view, ocean acidification may genuinely be the biggest danger facing humankind.

Wikipedia has a useful primer on the topic. (It is also surprisingly short, probably due to the very intermittent publicity the topic receives compared to global warming.) The Royal Society study of 2005 gave the topic some momentum, and its lengthy report is not too difficult to read. Shorter summaries have been published by the Australian Department of Environment’s Antarctic Division, the Australian Academy of Science, and various other international bodies. Here’s my quick review of the chemistry, as culled from those links:

  • the average pH of surface ocean water has dropped over the last two centuries by about 0.1. Being a logarithmic scale, this equates to increased acidity of about 30 per cent. (Pure water is neutral with a pH of 7; oceans on average used to have a pH of 8.2.);
  • estimates based on “business as usual” CO2 emissions are that by 2100 there could be a further drop in average ocean pH of 0.3 to 0.5, which at the top of that range equates to a 300 per cent increase in acidity. (Although even then the ocean is actually still alkaline.);
  • with any substantial drop in pH, it will take thousands of years for natural ocean chemistry to get it back to pre-industrial levels;
  • ocean pH is believed not to have been as low as even current levels for a very long time (hundreds of thousands, if not millions, of years.);
  • crucially, changes of as little as 0.2-0.3 units can hamper the ability of key marine organisms such as corals and some plankton to calcify their skeletons, which are built from pH-sensitive carbonate minerals. (Calcium carbonate shells can be either in the form of calcite or aragonite, with the latter being more sensitive to lower pH.)

This chemistry seems well understood. It’s the effect on individual species of algae, other plankton, coral, molluscs and crustaceans, as well as the overall ecological consequences, that is trickier to quantify. No one is saying the oceans will become utterly sterile: sea life of some kind has clearly survived much higher levels of atmospheric CO2 since the earth began. But ocean chemistry has been stable for a very long time, while recent ice ages have come and gone; now it is undergoing major change.

A major concern is the effect on coral reefs. Laboratory tests indicate that coral polyps might not actually die, but simply stop producing their hard shell and change in appearance to something resembling sea anemones. The problem is, it’s the hard shell that creates and maintains a reef. Algae that are important for coral may not do well either. Just this week, there’s a report that corals indicate the ocean may be acidifying faster than expected. Australians who value the Great Barrier Reef have good reason to worry: cold water takes in CO2 faster, and the Southern Ocean already has what scientists describe as “low saturation” with respect to aragonite. (Basically, the lower the saturation, the bigger the problem for calcifying creatures). Increasingly undersaturated waters are expected to spread far north during this century (click on the map at the top of this page, and scroll down).

Even if you are prepared to live with the possible loss of coral reefs, another major concern is the effect on the food chain. It has been said that only ten or so of the thousands of calcifying organisms have been tested for sensitivity to increased acidity, and some clearly do worse than others. Pteropods (small swimming molluscs eaten by many types of fish) appear to be particularly sensitive, and scientists worry that they will be in real trouble in only 50 year’s time.

As this article says:

The demise of polar pteropods could provoke a chain reaction of events through complex ocean ecosystems. It is known for instance that pteropods are eaten by organisms ranging in size from zooplankton to whales and including fish. For instance, North Pacific salmon include pteropods as part of their diet.

Indeed, one researcher is quoted in Wikipedia as saying the loss of them in the food chain would be “catastrophic”. Other creatures appear especially sensitive to a combination of higher acidity and warmer water.

Sea urchins are in this category. Thus if the climate does become warmer, regardless of the cause, this may well increase the problem of acidification for some species.

One thing that does appear likely to do well with increased CO2 is sea grass. While dugong will presumably welcome the news, it’s hard to see how substituting reefs with beds of sea grass is going to be a good deal for ocean productivity overall.

As to the effect on plankton, the picture is admittedly unclear. Coccolithophores, important single celled algae that build a shell of calcite plates, were believed from initial lab studies to suffer stress and deformity under increased acidity. (This is mentioned in some of the Australian articles listed above.) But, early in 2008, a surprising new study with a different experimental set-up indicated that a few species actually did better under increased CO2. Before “anti-alarmists” claim this as a victory, there seems to be some scepticism amongst scientists about the results and their implications, and further studies are bound to be under way.

If coccolithophores really do respond by continuing to get heavier, this could work as an increased CO2 sink, at least if their population does not drop. However, there are also the uncertainties as to whether it might displace other useful phytoplankton, and how its predators will respond. (Do they like or dislike eating heavier coccolithophores?) It would seem that one would have to be a strong optimist to believe that all future research will be positive, and that heavier coccolithophores alone will unexpectedly save the planet. It is also likely to take many years to be certain as to what is going on.

Which brings us to the real disaster scenarios. The earth has undergone intermittent major extinctions on both land and the oceans, and the length of time over which they have taken place indicate that you can’t blame them solely on asteroid hits or extreme vulcanism. Here are two suggested contributory mechanisms related to CO2 that should give concern:

  • if the oceans lack enough oxygen, anoxic bacteria may make huge quantities of poisonous hydrogen sulphide gas, which could kill on both land and in the sea. According to this article, the estimated quantity of atmospheric CO2 at the time of the last two big extinctions was just below 1,000ppm, a level which the earth may need only a century or so to reach. (If the ocean’s carbon sinks collapses significantly due to increased acidity, it may be reached even faster than the current IPPC projections expect.);
  • very recently, Scott Wooldridge, a researcher from the Australian Institute of Marine Sciences, has put forward a “unifying hypothesis for mass extinctions” that increased ocean acidity may, at specific levels, interfere with the operation of an important enzyme called urease. The effect of increasing acidification may therefore not be uniform, but have particular “dead zones” which interfere with life. The first such zone is reached when ocean pH reaches 7.9, which can be expected with atmospheric CO2 of 560ppm: a level easily reached within 50 years. This theory is novel and (according to its author) useful in explaining some unusual features of past mass extinctions. It certainly warrants serious investigation, as it suggests that something akin to another extinction event is within ready reach unless urgent action is taken on CO2 emissions.

It is clear from reading the material that there are many, many scientists who are very concerned about the fate of the oceans. It seems a common feeling that the IPCC process has badly underestimated the importance of the fate of the oceans when considering climate change.

What’s more, there seems to be very little in the way of convincing reasons being offered against taking it seriously. Bob Carter is a marine geologist and enviromental scientist, and has certainly been published in these fields. I stand to be corrected, but so far I have not found anything in which he has set out any detailed grounds to dismiss concerns about ocean acidification. He did claim at On Line Opinion that the Royal Society’s estimate of future pH decline by 0.5 units is “known to be exaggerated by a factor of about 3,” but did not cite the basis for that claim.

It seems that the most sceptics can credibly argue is that we should wait until the consequences of acidification are fully understood. There are, it is true, possible “winners and losers,” but the likely losers appear to be things we should really value. With CO2 currently at 380ppm and rising at a couple of percent per year, just how helpful will it be to wait 20 years if it proves that reefs are indeed about to collapse, there is nothing much to replace the pteropods in the food chain, not all plankton like increased acidity, and even the humble mussel and oyster will not be easy to grow? All of these dangers exist even if current global temperatures completely flat-line.

Furthermore, if the serious danger level for mass extinction-like events in the oceans is around 560 to 900ppm of CO2, there is absolutely no time to waste.

In the absence of better arguments from the sceptics, or an implicit optimism that Nature will look after us (an attitude usually more attributed to Greenies rather than warming sceptics), it’s very difficult to avoid the conclusion that ocean acidification is indeed reason enough for the world to just get on with working out effective ways to dramatically reduce future CO2 emissions.

Steven Watkinson, On Line Opinion, 11 July 2008. Article.

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