Final thoughts

Fossil fuel combustion and land-use changes have released great amount of carbon to the atmosphere. In the 1990s, burning of fossil fuels added 6.2 gigatons of carbon (GtC) per year to the atmosphere. As a consequence, the greenhouse effect causes global warming temperatures. However, only a portion of CO2, i.e. 2.8 GtC increase per year, is being found in the atmosphere (Sabine et al. 2004). We partly owe this to oceans.

 

 

Fossil fuel combustion and land-use changes have released great amount of carbon to the atmosphere. As a consequence, the greenhouse effect causes global warming temperatures. However, only a portion of CO2 is being found in the atmosphere.

Ocean carbon uptake, both through physical (solubility pump) and biological (primary production) processes is acting as a net CO2 sink. Oceans release back to the atmosphere some of the carbon they absorbed through the natural carbon cycle. Some of the carbon nonetheless remains in the ocean. It gets trapped into sediments. Ultimately, it is this sequestrated carbon that is responsible for lowered atmospheric CO2 levels.

How much carbon the ocean pumps is under debate because measuring carbon balance in the sea is problematic. It is especially difficult to quantify sequestrated carbon.

Although uncertainty is associated with the extent of the ocean carbon uptake, we know it has been mitigating global warming.

However, the effects of carbon absorption are being felt and will be felt in the future. Feedback loops are alarming for climate change. Oceans are likely to loose some of their ability to pump CO2 because of the changes they have been mitigating. CO2 is less soluble in warmer water, so warming temperature negatively affect the solubility pump. And chemical changes are decreasing primary production. Under IS92a, dissolved inorganic carbon per unit change in atmospheric carbon will decreased by about 60 % in 2100 because of lowered levels in surface carbonate ions (Orr et al. 2005). Marine biology is also paying the price of carbon uptake.

The problem is that we know little about how much carbon is stored and transported in the oceans. So far, we know they are responding to climate change in unpredicted ways. We cannot count on oceans to absorb our emissions like they have done in the past. The scientific community is alarmed by the changes we are observing, both for carbon uptake and marine life.

Increased knowledge would help to a great extent. Precisely, projects are being funded. The ACE CRC is planning a large-scale research project in the Southern ocean. Uptake geochemical processes as well as transport to the deep oceans will be studied, through the use of moored observatories, profiling floats and opportunity ships. NASA and NOAA are preparing similar large-scale programs.

Hopefully these will help our understanding of the unknown ocean. It may continue to surprise us in unpredicted ways.

 

The vastness of the Ocean. Source: The National Geographic

Consequences of Ocean Acidification for Marine Life

 

Ocean acidification has dramatic effects for marine life.

Seawater calcium carbonate minerals concentration is supersaturated in some areas. However ocean acidification is making oceans undersaturated with respect to these minerals. In turn, calcifying organisms will be unable to construct and maintain their shells since they are built out of calcium carbonate minerals. Many marine organisms are calcifying species, such as oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton (NOAA).

Field observations as well as laboratories experiments show that shell pteropods start to dissolve as soon as they go under aragonite saturation horizon.

Thus ocean acidification means calcifying organisms would be negatively impacted. Warm, subtropical waters would be aragonite undersaturated from 1 700 to 2 800 ppmv (parts per million by volume). However, high latitudes, cold waters, which are abundant in planktonic shelled pteropods, have a lower undersaturation horizon. It is argued that subpolar waters will be undersaturated with respect to aragonite at a future 1 200 level representing 4 times preindustrial CO2 levels (Feely et al. 2004).

But this is being reconsidered. Orr et al. argue that even at twice preindustrial CO2 levels, undersaturation will occur. This is likely to happen in the next 50 years (2005).

A pteropod was put in water with the pH of predicted results in 2100. Here is what was observed.

Figure 1 Gradual decomposition of pteropod shell in acidic water. Source: Liittschwager 2007

Coral reef will be degraded. Under today's carbon dioxyde concentration, 60% of coral reefs are surrounded by undersaturated with respect to aragonite water. If CO2 levels keep rising up to 450ppm (which could happen in the next 20 years), this will be the case for more than 90% of coral reefs. Aragonite undersaturated waters make coral species bleach. There is serious danger that they may not survive the century (Pandolfi et al. 2011, Oceana 2014).

Many oher species will be negatively impacted by ocean acidification indirectly. Shell and coral organisms loss will have dramatic effects on the food chain since much of marine biology depend on these species. Humans will also suffer from these changes from shellfisheries.

Ocean acidification has mixed consequences for marine biodiversity. Seagrass and photosynthetic algae may profit from the increased acidification (Beaumont and Garrard 2014).

Thus ocean acidification bears consequences that go beyond alkalinity changes. Although some scientists remind this change is beneficial for a few species, there is general agreement that ocean acidification is overall negatively impacting marine life.
 

Oceans going acid

Ocean acidification occurs as a result of increased CO2 levels in seawater. CO2 and water molecules may lead to the formation of carbonic acid to maintain chemical equilibrium. The added carbonic acid molecules reacting with water molecules to give a bicarbonate ion and a hydronium ion.

Figure 1 Chemical process leading to ocean acidification. Source: NOAA

Figure 2 Historical pH level (1850) Broadgate et al., 2012

Since the outset of the Industrial Revolution, pH has decreased from 8.2 to 8.1, which represents a 30% increase in acidity (NOAA). And at the current environmental change rate, levels are going to keep rising. In 2100, modelled results for high CO2 emissions scenario predicted a decrease up to 7.1 pH units.

Figure 3 Projected pH levels (in 2100). Source: Broadgate et al., 2012

Ocean acidification is intensified by nitrogen and sulfur inputs from fossil-fuel combustion.

Human-induced alkalinity change is altering surface pH and atmosphere/ocean CO2 exchanges (Doney et al. 2007).

We can argue that the predictability of chemical reactions make future pH projections easier (Logan 2010). In comparison to climate change, ocean acidification would thus be easier to model. It is however not so straightforward. First, different CO2 release scenarios lead to different consequences. Even so, under one scenario (business as usual) we find a large range of prediction: 100% to 150% increase in hydrogen ion (IPCC). We can nonetheless agree that the increase is alarming.

Evidence proves that ocean acidification is likely to alter oceans ability to act as a net CO2 sink (Logan 2010). Lower surface carbonate ion levels will impede oceans' carbon uptake (Orr et al. 2005). However a decrease in calcium carbonate export from the high latitudes would increase carbonate ion levels at the surface. It would increase carbon uptake by 6 to 13 Pg (Petagrams) (Heinz 2004). This is built on a modelled laboratory extrapolation. As we have seen, simulated carbon uptakes by the oceans is easily inflated.