Transforming water into acid… and back

The carbon cycle

Earth is a dynamic system that exchanges energy and materials between different spheres and outer space. One of the important circulation systems is the carbon cycle.

Figure 1: The annual flux of CO2 in GigaTons (Gt) or billions of tons between each of the Earth’s reservoirs. Each reservoir serves as both a source of and a sink for carbon, as indicated by opposing arrows. The carbon released by burning fossil fuels is an unbalanced contribution to the global carbon budget. The total carbon from burning of fossil fuels has increased from 5.5 Gt to 7-8 Gt between 2003 and 2007 (NASA/AIRS, https://www.flickr.com/photos/atmospheric-infrared-sounder/8265010034, https://creativecommons.org/licenses/by/2.0/legalcode)

Carbon is altered chemically and its compounds attain different physical states. Usually, the exchange of carbon between the lithosphere, the hydrosphere, the biosphere and the atmosphere is maintained in a delicate and naturally balanced equilibrium, with carbon sources and carbon sinks being in constant interaction. Sinks and sources are defined as subsystems that capture carbon or release it into the atmosphere where they act as greenhouse gases like carbon dioxide or methane.

Figure 2: Evolution of the budget of carbon sinks and sources (climatesafety, https://www.flickr.com/photos/
climatesafety/4745854611, https://creativecommons.org/licenses/by-nc/2.0/legalcode).

However, human activities constantly increase the imbalance in carbon sources, leading to a growing concentration of carbon-based greenhouse gases. As Figure 3 illustrates, the amount of atmospheric CO2 has increased dramatically since the beginning of the 20th century. The growth rate is unprecedented for the recent several hundred thousand years. There is a broad consensus among climatologists that this contributes significantly to the global warming seen today. Carbon dioxide concentrations can be measured both by sensors on ground and with dedicated Earth observation probes from space by remote sensing. Successful space programmes for monitoring greenhouse gases globally are Europe’s Envisat, Japan’s GoSat as well as NASA’s OCO-2 satellite. Europe’s Copernicus programme with its Sentinel satellites will also help understand the effects of increasing levels of greenhouse gases released into the atmosphere.

Figure 3: This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution until February 2016. (Vostok ice core data/J.R. Petit et al.; NOAA Mauna Loa CO2 record/NASA/JPL, http://climate.nasa.gov/evidence/, public domain).

The pH value

The pH value is a measure of the strength of acids. Its value represents the concentration of free hydron (“H” ^+) or hydronium (“H” _3 “O” ^+) ions. The value is defined as:

The concentration of hydronium ions c(H3O+ is given in units of moles per litre. The mole is a standard unit for the amount of a given substance. pH indicators change their colour depending on the pH value of the solution. This helps measure the pH value.

Oceans as a carbon sink

Up to 30-40% of manmade carbon dioxide is captured in oceans, rivers and lakes. The gas efficiently dissolves in water. Therefore, oceans are a very powerful and significant carbon sink.

Figure 4: Air-sea exchange of carbon dioxide (McSush (modified), Hannes Grobe (original), https://commons.wikimedia.org/wiki/File:CO2_pump_hg.svg, https://creativecommons.org/licenses/by-sa/2.5/legalcode).

Although the ability of water to capture and store CO2 helps reduce greenhouse gases, it comes at a high price. The dissolution of CO2 in water changes its chemistry. As a result, the water becomes more acidic. The acidification and its consequences can be split up into three chemical reactions. First, carbon dioxide and water form carbonic acid.

CO2 + H2O → H2CO3

The acid is immediately split up into its ions, one of which is the hydron ion that reacts to form the hydronium ion H3O+. The free hydron or hydronium ions are characteristic of an acid. This is reflected in the definition of the pH value (see above).

H2CO3 → H+ + HCO3-

The acidic solution reacts with carbonate ions that are abundant in ocean water. They are the building blocks e.g. for the exoskeletons of shellfish like snails, mussels as well as corals.

H+ + CO32- → HCO3-

These reactions occur at the surface of water bodies like the oceans. As a result, the formation of carbonate compounds like lime is hindered, or in extreme cases, existing exoskeletons can even get dissolved. The net equation of the reaction chain is shown in Figure 5.

Figure 5: Illustration of how CO2 dissolved in water consumes carbonate ions. It impedes calcification or may even lead to decalcification of sea shells (NOAA PMEL Carbon program, NAOO public domain).

Although the salinity of sea water mitigates the effect of acidification, the tendency remains. Apart from in-situ sample measurements, new technologies are available to determine ocean pH levels on a global scale using remote sensing from Earth observation satellites (Figure 6).

Figure 6: This map shows the first estimates of surface ocean pH using salinity data from ESA’s SMOS with satellite sea-surface temperature measurements and additional auxiliary data. There is a spatial variation of the pH across the globe. Cold waters near the poles tend to be more acidic because of the ability of cold water to better dissolve carbon dioxide than warm water (ESA/R. Sabia, http://www.esa.int/spaceinimages/Images/2015/01/Surface_ocean_pH, https://creativecommons.org/licenses/by-sa/3.0/igo/legalcode).

Such maps also indicate that polar regions are more strongly affected by acidification than others. This is because cold water can better dissolve CO2 than warm water. Wide range water currents are known to connect the oceans of the world. As a consequence, water is exchanged between latitudes. So acidic, i.e. CO2 rich water, is transported from the poles to the equator regions. The water gets heated on its way and releases part of the stored CO2. Therefore, oceans can also be regarded as a regionally confined carbon source.

This influence of water temperatures has also been confirmed by data models that capture the past and projected evolution of global pH levels, as shown in the climate reports of the IPCC (Intergovernmental Panel on Climate Change, see Figure 7). All projections show a stronger acidification of the polar regions than other regions on Earth.

Figure 7: Past and projected evolution of oceanic surface pH levels. The models were calculated for the most optimistic (RCP2.6, Representative Concentration Pathways) and the most pessimistic scenarios (RCP8.5) for the evolution of atmospheric CO2. (a) Time-series of surface pH shown as the mean (solid line) and range of models (filled), given as area-weighted averages over the Arctic Ocean (green), the tropical oceans (red) and the Southern Ocean (blue). (b) Map of the median model’s change in surface pH from 1990 to 2090 (IPCC Report, 2013, Working Group I, Chp. 6, p. 532, permission for reproduction granted).

The impact of acidification on marine life

Growing acidification of the oceans and coastal regions endangers the delicate equilibrium of marine life. Several species grow exoskeletons with carbonatic structures (corals, sea snails, mussels, etc.). These carbonates, mostly limestone, dissolve under the influence of carbonic acid. For example, sea snails, also known as sea butterflies, are one of the victims of acidification (Figure 8). Their shell becomes more fragile, which for them is a life-threatening situation. Experiments have even shown that such creatures lose most of their shells after exposure to acidification levels expected in the near future. Since they are the basis of an entire food chain, their extinction may have a tremendous impact on a large portion of marine life.

Figure 8: In laboratory experiments, the shell of this sea snail dissolved over the course of 45 days in seawater adjusted to an ocean chemistry projected for the year 2100 (Credit: NOAA Environmental Visualization Laboratory (EVL), https://commons.wikimedia.org/wiki/File:Pterapod_shell_dissolved_in_seawater_adjusted_to_an_ocean_chemistry_projected_for_the_year_2100.jpg, public domain).

Another example is microscopic, single-cell algae called coccolithophores (Figure 9). They form shells that consist of calcium carbonate scales. After they die, they sink to the sea floor. This process removes carbon from the global carbon cycle. If the formation of the carbonate shell is impeded, this carbon sink becomes less effective.

Figure 9: Image of a single coccolithophore cell produced with a high-resolution scanning electron microscope (Credit: Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility) (https://commons.wikimedia.org/wiki/ File:Emiliania_huxleyi_coccolithophore_(PLoS).png), https://creativecommons.org/licenses/by/2.5/legalcode).