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.

Oceans in the carbon cycle

Source: NASA

Hey fellows!

To wrap up past weeks' discussions here is a video. It overviews components of the carbon cycle in relation to the oceans. Here are the major points that relate to this blog's issues.

According to NASA, oceans would contain 50 times the amount of atmospheric carbon, making their bed the largest carbon reservoir.
They play a key role in the carbon cycle, to the extent that they have been slowing down global warming. By absorbing carbon, they are mitigating the effects of fossil fuel combustion.

I used to ask myself why the intensive use of fossil fuel led to what seemed to me a gradual, slow change. The consequence seemed relatively small in comparison to the proportion of the underlying cause. I guess one reason to that would be the ocean's carbon uptake.

How then can one measure the effects of climate change on the oceans? To what extent have they alleviated rising CO2 levels? Monitoring the oceans is extremely difficult. Satellites help in this regard (NASA). However they are currently unable to monitor most deep oceans' properties. Currently, remote sensing is far more efficient at gathering terrestrial data than oceanic data.

The video also introduces what I will talk about next: the consequences of increasing levels of atmospheric carbon for ocean acidification and marine life.
 

Feeding back loops

We suspect that the biological pump is not working at its full capacity (De La Rocha 2007). Research is being conducted to find ways to improve its current capacity. These include carbon capture and storage methods whereby carbon is taken from point sources and transported to a storage 'facility'.

Numerical model simulations proved that CO2 could be directly injected to the deep ocean through the moving ships method (Se-min, Sato, and Baixin 2008). Although this may reduce atmospheric CO2 levels in the short term, sequestrated CO2 has negative impacts on marine biology (Orr et al. 2005).

We ought to take these CO2 sequestration forcings with caution. We can rarely get rid of something without consequences being felt.

We suspect increasing temperatures will negatively affect the carbon pump. Carbon storage will

decrease, ultimately inflating atmospheric CO2 levels (Henson et al. 2011). The oceanic carbon pump

is thereby linked to another positive feedback. Increased atmospheric CO2 is likely to raise

temperatures in turn decreasing ocean particles’ ability to pump carbon. And the loop goes on.

That is because oceans and atmosphere are two interwoven systems. A change in air temperature within the ocean/atmosphere system affects mechanisms at work in oceans. In turn oceanic responses affect atmospheric levels of CO2.

I guess that needs to be stressed. Positive feedbacks will increase climate change.

It seems that global warming is not dramatically alarming in itself. To us human beings, 2°C do not represent a huge difference to our body temperature.

Perhaps that’s a reason why a good number of people are not realising how much global warming will affect the environment.

But does anything function in isolation on earth?

We have come to think of systems as separate wholes. Conceptualisation makes our brain represent systems as being divided into independent parts. I can picture an ocean floating alone in emptiness.

Concepts are extremely useful to understand mechanisms in complicated, intricate systems. But let’s bear in mind that the environment does not function like that. There is no change that does not potentially bear consequences on its surroundings.

How much is marine biology pumping?

Last week I described how carbon gets pumped into the ocean by diatoms. It is of great interest then, to quantify the extent of remineralised carbon.

The extent of primary production varies geographically. In upwelling regions, primary production is greater. Satellite imagery shows that near the coast and in the Northern Hemisphere, primary production is larger (figure 1).

Figure 1 Oceans net primary production. Source: NASA

Large seasonal variation exist in oceanic primary production.

Figure 2 Seasonal variation in net primary production (index). Source: Lutz et al. 2007

This makes quantifying the biological pump difficult. How can we infer primary production from one measurement to another spatial and temporal condition if primary production is spatially and temporally distributed?
We can look into which methods are used and how reliable they may be.

One way of measuring the biological pump is called the f-ratio. Primary production can be divided into recycled production (RP) and new production (NP). The extent of NP roughly represents how much carbon gets pumped into the ocean each year, if we look at long time scales (Henson et al. 2011). The f-ratio is given by NP/ NP+RP and estimates the uptake to 12 GtC yr−1 (Laws et al., 2000).

However this result should be taken with caution. NP is fed by nitrate coming from deep water through vertical mixing. Results were derived from this assumption. But it appeared in more recent studies that nitrate can be fuelled from surface waters as well; representing up to half of NP’s supply (Yool et al. 2007). To tackle uncertainties, Henson offers an alternative method using thorium-234 (²³⁴Th). It traces particles exporting organic carbon. The ThE ratio then gives how much carbon is pumped with: ²³⁴Th- derived export/ Primary Production

Using the f and the ThE ratios, results can be generalized globally depending on sea surface temperature. A correlation exists between temperature and production. Indirectly, cold waters induce more production than in warmer, tropical waters. The thorium approach brings a result of 5 GtC yr−1. (Henson et al. 2011)

Henson cites other methods which range from ∼5.7 GtC yr−1 to ∼20 GtC yr−1 (2011). We can acknowledge that the difference is great. The largest result date back to the late 1970s, containing higher uncertainty due to reduced scientific knowledge. Nonetheless many contemporary papers point out to about 10 GtC yr^-1(Dunne et al. 2007; Laws et al. 2000). How can we interpret this wide range of results? In such a context, can we trust these values?

First, what leads to such discrepancy should be looked at. Researchers deal with high uncertainty. Little is known about oceanic data, and the ways they are measured are imperfect. What is more, data is gathered at specific time and location. It overlooks temporal and regional variability because conclusions are derived on the assumption that oceans are homogeneous (Henson et al. 2011). Then, it would be useful to use a consistent approach used at different places and time of year. We could choose the thorium method which appears to be more efficient.

The extent of sediment trapped carbon is hard to quantify. It is crucial however to measure the extent of this reservoir since it is this sequestrated carbon that is responsible for lowering CO2 levels. It is done by collecting particles in sediment traps. Controversy surrounds this method. Although there is some disagreement, we expect the sediment reservoir is about 150 GtC (see figure, IPCC 4th report 2007).


We suspect that the biological pump is not working at its full capacity (De La Rocha 2007).

How is the biological pump reacting to increasing atmospheric CO2 as a result of anthropogenic changes? Are there other current environmental changes likely to affect oceanic primary production?
 

To conclude we observe high uncertainty attached to measuring the biological pump. In the end, the largest carbon sink remains widely unknown (Henson et al. 2011). More research on methodology ought to be conducted as it is crucial to gain knowledge on oceanic carbon fluxes. For the simple reason that current and future anthropogenic changes may alter such fluxes. And we need to know what their past and present state is to see in which direction we are heading.

Lastly, we can see that the extent of the biological pump may have been inflated. This is alarming for future atmospheric carbon levels. In predicting these, we may have measured the carbon pump to absorb more carbon than in reality.