Eutrophication of terrestrial ecosystems due to air pollution

Setting the scene

The 7^th EAP (EU, 2013) includes the objective of reducing the impact of air pollution on ecosystems and biodiversity, with the long-term aim of not exceeding critical levels and loads. Currently, the most important impact of air pollution on ecosystems and biodiversity is eutrophication caused by airborne nitrogen deposition to ecosystems. In certain terrestrial sensitive ecosystems such as grasslands, excessive atmospheric loads of nitrogen can alone result in loss of sensitive species, increased growth of species that benefit from high nutrient levels, changes to habitat structure and function, the homogenisation of vegetation types, etc. This briefing addresses ecosystem eutrophication from air-borne sources. There are also other sources that cause ecosystem eutrophication (e.g. use of fertilisers on cropland and pastures if not applied correctly – see AIRS_PO1.2, 2017).

Policy targets and progress

The EU Thematic Strategy on Air Pollution includes a milestone for 2020, relative to 2000, of a 43 % reduction in areas of ecosystems exposed to eutrophication, i.e. areas where eutrophication critical loads are exceeded (EC, 2005a,  2005b). This milestone is in line with the long-term objective of not exceeding critical loads.  

In 2000^[1], the area of ecosystems where the critical load was exceeded was about 78 % of the total in the EU Member States (approximately 60 % in all 33 EEA member countries for which data were available, including the 28 EU Member States) and decreased in 2010 to 63 % in the EU (55 % in all 33 EEA member countries). Assuming that current legislation is fully implemented, the area in exceedance is projected to be 54 % in the EU (48 % in all 33 EEA member countries) in 2020 (EEA, 2017a). The relative reduction is approximately 31 % for the EU, as well as for all the 33 EEA member countries, between 2000 and 2020, which is below the 43 % reduction milestone suggested by the air pollution thematic strategy for this period.

Nevertheless, as illustrated in Figure 1, the magnitude (though not the area) of the exceedance is projected to decline considerably in most areas, except for a few ‘hot spot’ areas, particularly in Belgium, Germany and the Netherlands, as well as in northern Italy.

Figure 1. Exposure of ecosystems to risk of eutrophication due to airborne deposition of nutrient nitrogen – area and magnitude of exceedance in 2000 and 2020.

Sources:
a. EEA – Indicator CSI005
b. CCE (Coordination Centre for Effects), CLRTAP, UNECE.

Note: The maps show areas where critical loads for eutrophication of freshwater and terrestrial habitats are exceeded

The main sources of eutrophication are emissions of nitrogen compounds (i.e. nitrogen oxide, ammonia) to the atmosphere. Nitrogen oxide (NO_x) emissions for the EU decreased by approximately 41 % between 2000 and 2015 (EEA, 2017b). This reduction was primarily due to the introduction of three-way catalytic converters for cars. However, emission reductions from modern vehicles have not been as large as was originally anticipated. Standard diesel vehicles, for example, can emit up to seven times more NO_x in real‑world conditions than in official tests (EEA, 2016).

Ammonia (NH₃) emissions have not fallen by as much. In 2015, they had fallen by approximately 8 % compared with their value in 2000 for the EU (EEA, 2017b). Actually, both in 2014 and 2015 EU ammonia emissions increased by 1.1 % and 1.8 % respectively (AIRS_PO3.2, 2017). Agriculture is the main source of NH₃ emissions; they amount to approximately 95 % of total emissions in the EEA-33 region. Emissions primarily arise from the decomposition of urea in animal wastes and uric acid in poultry wastes.

A key driver behind the observed reductions was the implementation of the National Emission Ceilings Directive (EU, 2001), which set air pollutant emission ceilings to be achieved by 2010 for, inter alia, emissions of the eutrophying air pollutants NO_x and NH₃. However, eutrophying emissions primarily from the agriculture and road transport sectors, but also from shipping and air travel, have been and will remain significant contributors to eutrophication caused by air pollution.

Further reductions in eutrophying air pollutant emissions are expected by 2020, among other things, as a result of the 2012 amended Gothenburg Protocol, which set stricter air pollutant emission ceilings for 2020 (UNECE, 2012) and of the revised National Emission Ceilings Directive which, inter alia, reflected these ceilings into EU legislation (EU, 2016).

Nevertheless, as illustrated at the start of this section, the decreases anticipated for 2020, under a scenario that assumes  current legislation (including the revised National Emission Ceilings Directive) is fully implemented, are not expected to contribute sufficiently to reductions in the ecosystem area exposed to excess atmospheric nitrogen deposition and affected by eutrophication. The relative reduction over the 2000 to 2020 period is expected to be only about 31 %, which is below the EU thematic strategy’s suggested milestone of a 43 % reduction for that period. Deeper reductions could take place through additional specific and targeted (technical) mitigation measures, particularly in the agriculture and transport sectors. Dietary changes resulting in less meat and dairy farming and the reduced use of petrol and diesel in cars could also contribute to reductions.

Country level information

Figure 2 shows the percentage of the area by country where the critical loads for eutrophication were exceeded in 2010 and the areas where exceedance is expected in 2020. Although a decrease is predicted by 2020, if current legislation is implemented, the area showing exceedance will be above 50 % in most countries (see bars). Extremely high magnitudes of exceedance can be found in Denmark, Hungary, Luxembourg, the Netherlands and Switzerland, caused by high deposition rates and/or ecosystems that are very sensitive to an excess supply of nitrogen from the atmosphere (see dots), for example nutrient-poor grasslands. 

Figure 2. The ecosystem area at risk of eutrophication due to airborne nitrogen deposition and the magnitude of exceedance in each country

Note: AAE is the average accumulated exceedance, showing the magnitude of exceedance in equivalents (mol nitrogen/ha per year). The data are based on the revised Gothenburg Protocol emission reduction agreements of 2012 (assuming for the 2020 scenario that current legislation is fully implemented). Data for Serbia and Montenegro are presented as aggregated data.

Outlook beyond 2020

The updated EU air pollution strategy aims, inter alia, to achieve a situation in which the EU ecosystem area exceeding critical loads for eutrophication is reduced by 35 % by 2030, relative to 2005 (EC, 2013a).

The revised National Emission Ceilings Directive not only transposed the amended Gothenburg Protocol 2020 air pollutant reduction commitments, it also set more ambitious air pollutant reduction commitments for 2030. These commitments – in particular the reduction commitments for the two eutrophying air pollutants, NO_x and NH₃ – will contribute to the achievement of the objective of 35 % by 2030.

However, this objective would not be met if only current legislation was fully implemented.

It would be met if the maximum number of technically feasible reduction measures was implemented (EC, 2013b).

Beyond  2030, the EU aspires towards ‘achieving levels of air quality that do not give rise to significant negative impacts on and risks to human health and the environment’ (EU, 2002). 

About the indicator

The indicator shows area and quantitative information for ecosystems where atmospheric nutrient nitrogen deposition is above the critical load. A critical load is a 'quantitative estimate of an exposure to one or more pollutants, below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge' (UNECE, 2015). Deposition loads of eutrophying airborne pollutants above the critical loads are termed an 'exceedance'. 

Exposure in an ecosystem for which information on critical loads is available, is calculated as the average accumulated exceedance (AAE). The AAE is the area-weighted average of exceedances, accumulated over all sensitive habitats (or ecosystem points) defined in a grid cell.