Ocean Acidification

Coral slab growthrings

The density and width of the annual growth bands in coral skeletons decline in response to ocean acidification, resulting in slower growth.

Rising levels of atmospheric carbon dioxide from fossil fuel combustion and deforestation is gradually altering the chemistry of the oceans, making seawater more acidic. The increased acidity has profound implications for all life on Earth, through its likely impacts on plankton, which is the basis of almost all marine food webs. It may also significantly affect the future of the Great Barrier Reef and other coral reefs. Ocean acidification and global warming are two very different, but equally important, spin-off effects of increasing atmospheric carbon dioxide concentrations.

Mean concentration of atmospheric carbon dioxide has ranged between 200 and 300 parts per million over the past 650,000 years (and possibly over as much as 20 million years; Raven et al. 2005). However, it has now risen to 387 ppm within about 100 years, predominantly due to the burning of fossil fuels and land clearing (Orr et al., 2005; Harley et al. 2006; Solomon et al. 2007). Depending on the rate of future emissions, carbon dioxide concentrations could rise to 540-970 ppm by 2100 (Raven et al. 2005).

Because the oceans readily exchange carbon dioxide with the atmosphere, carbon dioxide released into the atmosphere by human activity dissolves into the upper oceans. This changes ocean chemistry, resulting in increased concentrations of carbonic acid and a reduced concentration of carbonate ions.

The direct effect of these carbon dioxide emissions is that, averaged across tropical waters, temperature is estimated to have increased by about 0.7 degrees Celsius. Models also show that seawater pH has declined by 0.1 units to 8.1. As pH is on a log -10 scale, this change in pH is equivalent to a 30% increase in the acidity of seawater. This is important because the oceans are naturally alkaline, and many organisms such as crabs, corals, mussels and sea urchins rely on this alkalinity to form their calcium carbonate-based skeletons. So-called calcifying organisms make up more than a third of all sea life. Experiments have shown that a doubling of carbon dioxide concentration reduces calcification rates in some corals and coralline red algae by up to 40% (Langdon et al. 2000; Langdon and Atkinson 2005). At pH 7.9, corals and other organisms cease to calcify. Yet the current rate at which humans are releasing carbon dioxide will cause ocean pH to drop to 7.7-7.8 in the second half of the 21st century. The consequences for marine life would be catastrophic.

Some researchers now believe that the implications of ocean acidification for many marine species may ultimately be even greater than those of warming temperatures (Harley et al. 2006; Veron 2008). However, there are relatively few field-based studies investigating the consequences of ocean acidification. An exception is the study by Cooper et al. (2008) that documented a 21% decline in calcification (defined as the product of skeletal density and linear extension) of a species of coral (Porites) in two regions of the Great Barrier Reef over the last 20 years (Fig. 1). This was the first study to suggest that changes in ocean pH may already be having negative effects on coral calcification in the field.

Coral slab growthrings

Fig. 1: The density and width of the annual growth bands in coral skeletons decline in response to ocean acidification, resulting in slower growth.

The rate at which the acidity of the oceans is changing is also considered an important factor. The current rate of carbon dioxide increase is about 100 times faster than that known to have occurred over the past 650,000 years. Human-generated carbon dioxide emissions are rapidly altering ocean chemistry in ways that have not been experienced by coral reefs and other calcifying marine species throughout the past 55 million or possibly hundreds of millions of years (Sabine et al. 2004; Raven et al. 2005; Harley et al. 2006).

The potential consequences of these dramatic and ongoing changes in ocean chemistry remain poorly understood, and key research questions remain unanswered.

For example, how does ocean acidification interact with warming temperatures, and how will it alter the speciation of trace metals, nutrient cycles, food webs and key ecological processes in complex systems such as coral reefs? Will some calcifying organisms be able to adapt even with such a rapid rate of change? Might some organisms even be insensitive to changes in carbonate saturation and pH? Are there any groups of organisms that might benefit?

Many researchers suggest a critical threshold for the future existence of coral reefs around 480 ppm atmospheric carbon dioxide, which will change seawater pH, carbonate saturation, and temperatures to levels at which reefs will not be able to grow fast enough to compensate for erosion (e.g., Kleypas et al., 1999; Hoegh-Guldberg et al. 2007). In any case, profound changes in ecosystems are likely to occur long before this threshold is reached. While more research is needed (reviewed in Kleypas et al., 2006), the existing science is clear: urgent policy action is required if severe reef degradation and the collapse of marine food webs is to be averted as a result of rapid ocean acidification and climate change.

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Ocean Acidification

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Ocean Acidification

Further Reading