Emissions from International Shipping Vessels Contribute to Ocean Acidification

Hassellöv, I.M.; Turner, D.R.;Lauer, A.;Corbett, J.J.; Shipping contributes to ocean acidification. 2013. Geophysical Research Letters. doi:10.1002/grl.50521

The laptop you are presently using to read this article, and most of the things you own, were produced and assembled first in a different country, then brought to the US on cargo ships. Thousands of cargo carriers cross the world’s seas and oceans each year; they handle the bulk of international trade.  Until recently, the effects ship emissions have on the ocean was difficult to quantify because of the distance and frequency of their shipping routes. Hassellöv and colleagues used a model to estimate the contribution of shipping-derived ocean acidification on a global scale.

What is ocean acidification?

Ocean acidification, commonly known as “the other” CO2 problem, describes the decrease in pH and saturation state of biologically important calcium carbonate minerals with the ocean’s uptake of anthropogenic CO2 emissions. pH is a measure of the activity of free hydrogen ions present in a solution. A solution that contains more free hydrogen ions is more acidic, and has a lower pH. Changes in pH are known to adversely affect marine fauna and may lead to changes in biological communities. Pteropods, tiny organisms known as “sea butterflies”, have become the poster child of ocean acidification.

The change in pH due to the addition of an acid depends on the total alkalinity of seawater, which is a measure of all significant compounds that react with and consume free hydrogen and “buffer” changes in pH. In seawater, total alkalinity (TA) can be defined as the net sum of anions (negatively charged species such as carbonate and borate) and cations (positively charged species such as free hydrogen). The presence of carbonate and borate species in large quantities allows seawater to buffer pH changes. Ocean acidification is usually attributed to CO2 emissions; however, any substance that decreases the ocean’s pH can contribute to its acidification.

Sulfur oxides (SOx) are produced during combustion of sulfur-containing fuels. Nitrogen oxides (NOx) are produced primarily from nitrogen in the air during high-temperature and high-pressure combustion. While NOx formation depends mainly on the combustion temperature, the amount of SOx produced is directly related to the sulfur content of the fuel. An example of a fuel with high sulfur content is diesel. The sulfur content of the fuel depends on the limits established by Emission Control Areas (ECAs). ECAs are areas subject to international environmental regulation.
Hassellöv and her colleagues used a model to estimate the deposition of sulfuric and nitric acid over surface water from ship emissions of sulfur and nitrogen oxides (SOx, NOx). They then quantified the contribution of these emissions to ocean acidification and compared it total anthropogenically-driven ocean acidification. Their motivation was to see if heavily-crossed trade routes  were also “hotspots” of ocean acidification.

You can think of a model as a simplified representation of a system. The system can represent a physical system, for example the mixed layer found at the surface of the ocean. When you are interested in looking at the effect of something, such as the effect of wind speed on the mixed layer depth, you can turn this effect on or off, or dial it up. By tweaking a model, scientists can understand how the system they are modeling works.


Because their goal was to estimate the effect emissions may have on ocean pH, Hassellöv and her colleagues used data from the years before ECAs was established:2000 and 2002. Using various resources they obtained salinity, temperature, mixed layer depth, alkalinity, and carbon dioxide partial pressure at a resolution of 1° latitude and longitude. Monthly SOx deposition resulting from shipping emissions were obtained from model simulations with the global aerosol-climate model EMAC/MADE. The ship emission data provided monthly mean emissions representative for the year 2002. The annual emission totals from shipping were 9.2 Tg (a teragram is 1012 grams) for SO2, 0.35 Tg for primary SO4 for SOx, and 16.4 Tg (NO2) for NOx. The total mass of deposition from shipping emissions is approximately equal to the mass of 4.3 Great Pyramid of Gizas.

A really cool aspect of the model was that it was a coupled physical-chemical model. This means that ocean circulation patterns, such as the deepening of the mixed ocean depth, play an important role in the model’s results.

The results of the model from one month depended on the results of the model on previous month because Hassellöv and her colleagues were interested in the cumulative effects of acid deposition.

Hassellöv and her colleagues used one model to estimate the pH of the ocean’s surface using salinity, temperature, mixed layer depth, alkalinity, and CO2 partial pressure. Then they used the results from the EMAC model, a model that estimates sulfuric and nitric acid contribution from ship emissions, and put it into the previously mentioned model to estimate the effect these acids had on the ocean’s pH. Quite simply by comparing a theoretical output with one that includes the stressor, one can estimate the influence of the stressor. In this study the stressor is the acid that forms from the shipping emission deposits (ΔpH = pH calculated with the stressor – pH calculated without the stressor).

What did they find?

The largest effects of SOx and NOx input from shipping are seen in parts of the northern hemisphere, where ~85% of all shipping emissions coincide with seasonal stratification. As a result, this stratification concentrates the acid emissions within a relatively shallow surface mixed layer. The negative values mean that the ocean was acidified. ΔpH values generally range from -.001 to -.005. pH is at a log scale, so this pH decrease translates to about a ~1/4th of a percent increase in [H+].

Figure1.Calculated surface water pH changes arising from shipping-derived inputs of SOx and NOx. Calculations begin with the month of January.
Figure1.Calculated surface water pH changes arising from shipping-derived inputs of SOx and NOx. Calculations begin with the month of January.

Significant coastal acidification  is shown for August then decreases during the autumn as a result of the surface and deep waters mixing. The lack of data close to the coasts is due to the limitations of the global oceanographic atlases used (represented in white boundaries near continents). Hassellöv and her colleagues expect that pH changes close to the coasts may be larger than those calculated in more open waters because of heavy shipping traffic in the vicinity of major ports.

Figure2. Calculated gross annual acidification from SOx, NOx as a function of latitude (30,675 data points).
Figure2. Calculated gross annual acidification from SOx, NOx as a function of latitude (30,675 data points).

The calculated near-coastal seasonal acidification of 0.0015–0.002 pH is without a doubt significant. The deposition of shipping emissions not only matches the CO2-driven acidification but also reduces the alkalinity of the water. Reducing the alkalinity of seawater affects it ability to buffer pH changes.


Hassellöv and her colleagues show that ship emissions have an acidifying influence on the ocean’s pH. Like the vapor contrails left behind by plane, these images show how heavy ship traffic leaves trails in the ocean.

It is important to note, however, that the pH effects found in this study are of the hundredth order. The pH decrease must be higher for significant changes in the saturation state of biologically important calcium carbonate minerals. Anthropogenic CO2 emissions are projected to increase, and their projected impact on the world’s oceans is expected to decrease by a tenth of a pH unit. CO2 emissions are projected to have a significant influence on pH in the future. What should be noted however is that the addition of CO2 and carbonic acid do not affect alkalinity. The SOx and NOx emissions have had a negative impact on alkalinity, meaning that they weaken the ocean’s ability to resist pH change.

Should models address the double-whammy effect NOx and SOx have on our world’s oceans?


One thought on “Emissions from International Shipping Vessels Contribute to Ocean Acidification

Leave a Reply

Your email address will not be published.