Oceans and CO2 Emissions

An important milestone has been reached – atmospheric CO2 concentrations have are now at 400 parts per million (ppm) - see the news story here. This is big news. And it is scary. This is brand new territory for humans – there hasn’t been this much CO2 in the atmosphere for as long as any anatomically modern humans have walked the planet. The difference between 399 and 400ppm isn’t all that much, in the bigger picture it’s a minute change, but the rate at which we have reached this milestone is truly frightening. CO2 concentrations are rising faster than any other time in history, and is caused by humans. The debate is long over, it is now scientific consensus. The major evidence we have for the human induced nature of this rise due to the burning of fossil fuels is the decrease in oxygen levels – not enough for you to panic about though; we’re not running out of oxygen. In the past atmospheric CO2 concentrations have fluctuated, both increasing and decreasing. However, this time the increase has been accompanied by a decline in atmospheric O2 concentrations as the combustion (burning) of organic things like fossil fuels uses up O2 as it releases CO2 (see Shaffer et al, 2009). There are many other well cited papers where evidence for this is verified.

The burning of fossil fuels is changing the planet in ways humans have never had to deal with before (image from moinlucky11 on Deviant Art)

This news is of concern to all who call Earth home. But it is not simply changes to our climate we should be worried about, although these are important. What on earth is going to happen to our oceans? Zeebe et al (2008) published an article in the journal Science asking just that – as CO2 concentrations continue to rise in the atmosphere, what effect will that have on the oceans and the life within them?

Transportation's reliance on oil is a huge source of carbon emissions (image courtesy of e8resources)

Simple chemistry states that as the concentration of CO2 increases in the atmosphere, there will be an overall “drawdown” of CO2 into the ocean to maintain pressure equilibriums. Therefore, it makes sense that the ocean is considered a major “carbon sink”, that has taken up about 40% of all man-made (or anthropogenic) CO2 emissions over the last 200 years. This has slowed the rise in atmospheric CO2 concentrations, and so has somewhat alleviated the changes in climate associated with this rise. However, when that much CO2 is taken up by the ocean, large scale changes in the chemistry of the waters occur. These can have potentially serious consequences for marine life, especially in terms of a change in global ocean pH. This drop in pH (making the waters more acidic) and the lowering of the saturation state for carbonate minerals (the amount of carbonate available for biological processes such as the formation of shells) has been called “Ocean Acidification”

Graph from the IPCC (2007) showing how, over time, the increase in atmospheric CO2 (orange) is closely followed by a decrease in gobal ocean pH (blue). The fluctuations in the curve are due to the seasons - the Northern Hemisphere spring sees the massive northern forsts taking up huge amonts of CO2, making the levels decline, while winter means the trees top photosynthesising and the CO2 levels creep up again.

 Many organisms in the ocean need calcium carbonate to build their shells and skeletons – things like abalone, crusting algae, mussels, oysters and corals. If there is less carbonate in the water, these organisms will be unable to build their shells and the dissolution of calcium carbonate under acidification conditions means that the organisms will literally disappear. In 2012, I conducted a project to test the effects of ocean acidification on baby abalone by looking at the shells with a scanning electron microscope at very high magnification levels. The results were startling, with the erosion of the shells happening right before my eyes (see the pictures below - click on the picture to view a larger version). Although the pH is not going to drop to 7.8 units until 2100, this is still worrying - the shell damage under these conditions and the less calcium carbonate avaliable in the water will mean that the organisms will have to spend more energy forming and replacing their shells than investing in growth.

High magnification images of baby abalone shells under different pH treatments. Far left is the control pH of 8 units; centre is the results of placing the shell in a pHof 7.8 units, and far right is the shell in a treatment on 7.5 pH units. The projected pH for the oceans for 2100 is 7.8 pH units.

 Calcium carbonate is the building block for coral reefs. These ecosystems are already under huge pressure from direct human impacts like overfishing, physical damage and pollution, and ocean acidification may become a more devastating threat in the future as CO2 levels continue to rise. More CO2 dissolving into the water means that the rate of erosion of reefs in this manner could become greater than the rate these reefs are built. Coral reefs are diversity hotspots, supporting a huge range of species and providing food and income through tourism for many people around the world. The loss of these systems will have devastating ecological and humanitarian consequences.

However, most corals occur in warmer waters, and since gases like CO2 dissolve more in cold water than warm, the threat to corals from ocean acidification may not be its most threatening impact. Many other calcifying species are of either direct or indirect importance to humans, and will be under threat due to ocean acidification. Abalone are of economic importance here in South Africa - a huge market exists in the Far East which allows for profitable farming of the species, and economic upliftment and job creation this ensures. Important fish stocks the world over depend on cphytoplankton as food, and the loss of these primary produces at the bottom of the food chain because of ocean acidification will have dramatic ecological, economic and humanitarian issues. For many people, especially the poor, fish is their most important source of protein. Therefore, Zeebe and his fellow authors advocate that carbon emissions must be reduced with immediate effect to avoid these consequences, regardless of climatic considerations.

From left: coral reef (image from NRDC); highly magnified image of coccolithophores (image from the University of Bremen); Atlantic caught herring (image from NOAA)

The scary thing about ocean acidification is that even if we stop all emissions immediately, the ocean will still continue to take up CO2 and the pH will continue to decline. Already the global ocean pH has declined by 0.1 units since the start of preindustrial times. Despite this, the legislation that governs what is deemed an acceptable change in oceanic pH has not been changed –1976 environmental standards of the USA still state that, for marine life, pH should not be changed by 0.2 units outside of the normal range. And since it has been shown in other studies that dropping pH has different effects on different groups of organisms, these standards need to be re-evaluated and CO2 emission limits need to be established in order to prevent future changes in pH that will be beyond those ranges.

Every little bit helps – what are you doing to safe guard the future of our oceans?