Ocean acidification

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As well as playing the leading role in global warming, carbon dioxide (CO2) also dissolves in the ocean and affects its chemistry. This leads to a process known as ocean acidification

Overview

Unlike many other gases, carbon dioxide reacts with water when it dissolves to form a number of ionic and non-ionic species. These include: dissolved CO2(aq), carbonic acid (H2CO3), bicarbonate ions (HCO3-) and carbonate ions (CO32-). The ratio of these species varies with water properties such as temperature and salinity. Formation of these species also increases the hydrogen ion (H+) concentration of water. The concentration of this ion relates to the pH of a solution.

Since the beginning of the industrial revolution (in the mid-1700s), CO2 has increased in the atmosphere but not as much as it should have given human activities. A large part of the reason for this is the absorption of CO2 by the ocean. This absorption has changed the pH of the ocean, decreasing it from 8.179 to 8.104 (a change of -0.075). It is anticipated this this change may be as much as 0.3 to 0.5 by the year 2100.

Acidification is of interest because many marine organisms build their shells or skeletons out of calcium carbonate. This mineral is normally supersaturated in surface seawater, such that it cannot dissolve. However, as the ocean acidifies, the mineral gradually becomes less saturated, eventually allowing it to dissolve in seawater. Consequently, scientists are concerned that this will add an additional strain to ocean ecosystems already having to deal with global warming.

Studying ocean acidification in JModels

This is easy to examine using the carbon model. From the main control panel, select the option to ‘Add Fossil Fuels’. You can then decide whether to add the fossil fuels using a simplified sinewave function or else using historical emissions data and a particular future scenario. These scenarios, generated by the Intergovernmental Panel on Climate Change (IPCC), are derived from particular storylines with different interpretations as to how society will (or will not) curtail CO2 emissions in the future. The A1F1 scenario for instance is a pessimistic scenario assuming that emissions will continue rising steeply, whereas B1 is an optimistic scenario assuming a more ecologically-friendly future world that adopts non-fossil fuel sources. All of the scenarios only specify CO2 emissions between the years 2000 to 2100. A further text box allows you to specify the total overall emissions, which determine the model behaviour beyond the year 2100.

To put the numbers in context, cumulative global human emissions of CO2 to date are about 300 Gt C. If we eventually burn all recoverable fossil fuels then this will increase to an overall total of 4000 or 5000 Gt C. Larger total amounts could eventually ensue if, for instance, methane clathrates on the seafloor release methane into the atmosphere due to global warming.

Once you have specified the amount of fossil fuels, select ‘Run model’ and then examine the graphs of model results that appear on the screen. For more detailed analysis you can choose to ‘Save Simulation’ and then import the saved data file into a package such as Excel.

If you look at page 2 of the graphs you can examine the impacts that your chosen fossil fuel input has on surface and deep ocean pH (top-left panel). You can also see the impact on carbonate ion concentrations (top-right panel) and saturation states (middle-right panel) with respect to the two mineral forms of calcium carbonate (CaCO3): calcite (used by most planktonic calcifiers, such as coccolithophores and foraminifera) and aragonite (used by most coral reefs). There is concern that these CaCO3-using organisms will be detrimentally affected by ocean acidification, especially as saturation states decrease below present-day levels, and even more so when saturation states decrease below one (when seawater becomes undersaturated with respect to calcite and/or aragonite CaCO3). You can get an indication of the consequences of different CO2 emissions scenarios on seawater saturation states by inspecting the graphs after different model runs. If we emit large amounts of CO2, you can see from the model that we will be able to make the surface ocean undersaturated with respect to both mineral forms of CaCO3. We will also be able to make the surface ocean more acidic (lower pH) than the deep ocean for some centuries, a reversal of the normal (natural) situation. However, this model doesn’t tell you about geographical differences (for instance, that surface seawater under-saturation will occur first in polar latitudes).

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