EU Science Hub

Ocean acidification exacerbated by emissions from ships on major shipping routes

Ship at sea
Jun 08 2016

The theme of the 2016 World Oceans Day celebrated on 8 June is “Healthy Oceans, Healthy Planet". The day coincides with the official publication of a new JRC study simulating the impact of SO2 emissions from ships on ocean acidification.

The modelling study found that along major shipping lanes, sulphur dioxide (SO2) emissions from ships can further ocean acidification with a rate that is twofold with respect to that caused by carbon dioxide (CO2) emissions.

Some ships use exhaust gas cleaning systems, or "scrubbers", to wash their exhaust gases in order to meet the current EU regulations on air quality that restrict the release of SO2 emissions. The resulting acid wash water, which contains SO2, is released into the sea, leading to the acidification of the water.

JRC researchers have looked at the effect that the acid wash water released by ships in the North Sea has on the acidification of sea water, and compared it with the impact of CO2 emissions on ocean acidification.

The researchers confirmed that in overall terms, CO2 emissions are the leading cause of ocean acidification in the North Sea. On average, CO2 emissions cause ocean acidification about eight times more than the release of acid water from ships. However, JRC researchers found that in certain areas where ship traffic is very intense, the impact of SO2 emissions on ocean acidification can be up to 20 times greater than the average impact calculated for the whole North Sea area.  In these areas, the impact of acid water release from ships is also two times higher than that of the increasing CO2 emissions.

The researchers also found that the increased SO2 levels influence the capacity of sea water to resist the changes in pH levels. Consequently, for every tonne of SO2 released into the water, the North Sea absorbs about half a tonne less of CO2, with respect to its usual capacity.

The study indicates that there might be potential problems related to the surface water quality – in particular with regard to water acidification – in some critical areas, i.e. ports, estuaries and coastal waters, that are subject to the current EU water and marine regulations.

What is ocean acidification?

Ocean acidification – sometimes referred to as climate change's evil twin – refers to the ongoing decrease in the pH of the Earth's oceans. It is the consequence of substances such as CO2 and SO2 that dissolve into the ocean and change the ocean’s chemistry. This is a concern because biological life in the oceans is very vulnerable even to small pH changes, as marine life has not developed protective mechanisms against fluctuating pH values.

Current legislation

Ocean acidification and techniques dealing with it have implications for the EU Member States' obligations under the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD). These directives aim to achieve a "good environmental status" of the EU marine waters by 2020, and to protect the marine resources on which economic and social activities depend.

As shipping is an international industry, environmental, security and safety standards are developed by the International Maritime Organization (IMO), a United Nation specialised agency. The guidelines of the International Maritime Organization (IMO) require scrubber users to show that wash water used to reduce sulfur oxides emissions is not above a certain level of acidity before being discharged into the sea.

In the EU, the sulphur limits established by the IMO are transposed by Directive 2012/33/EU as regards the sulphur content of marine fuels. As of 1 January 2015, EU Member States have to ensure that ships in the Baltic, the North Sea and the English Channel are using fuels with a sulphur content of no more than 0.10%. Higher sulphur contents are still possible, but only if the appropriate exhaust cleaning systems are in place.

Relative annual decrease of pH
Relative annual decrease (in %) of pH due to SO2 inputs compared to the decrease caused by rising atmospheric CO2 concentrations. Critical regions are identified in green and red.
© EU, 2016