Nitrogen overload still harms ecosystems

Two-thirds of EU ecosystems are currently exposed to more nitrogen deposition than they can cope with and one-tenth is receiving too much acid fallout. Significant additional reductions in the emissions of ammonia, nitrogen oxides and sulphur dioxide are needed to correct the situation.

Critical loads are scientific estimates of the amounts of pollutants that various ecosystems can tolerate without being harmed. They are sometimes referred to as the limits on what “nature can tolerate.” If pollutant depositions exceed the critical load limit, damage to sensitive ecosystems will by definition occur sooner or later.

The maps show the extent to which European ecosystems are exposed to more air pollutant depositions than they can tolerate in the long term without damage, i.e. where the critical load limits for acidification and eutrophication (nutrient nitrogen) are exceeded, and how the situation has changed over time.

It should be noted that the maps give a snapshot of deposition versus ability to resist at a given point in time – they do not really reflect the environmental situation right now. Environmental monitoring, experiments and calculations show that there may be considerable time lags, and that the damage that has already been caused by excess air pollutant inputs will persist for decades, in some places even for centuries.

Following the emission cuts over the last 40 years in the main acidifying air pollutants, especially sulphur dioxide, the area of sensitive ecosystems at risk of acidification in Europe is now less than 250,000 square kilometres (km2), nearly eight times smaller than it was in 1980 (see table).

Table: Change in area of ecosystems exposed to excess deposition of eutrophying and acidifying air pollutants from 1990 to 2010 and projection for 2020 assuming full implementation of the revised Gothenburg Protocol (million km2).

Progress is however markedly slower for eutrophication, which is caused by excess nitrogen deposition. Here the affected area has shrunk by less than 40 per cent over the same time period, and still covers 1.6 million km2.

Assuming that all the European countries that agreed in May last year to adopt the revised Gothenburg Protocol under the Convention on Long-range Transboundary Air Pollution (see AN 2/12) actually implement their new emission reduction commitments by 2020, some further improvements are to be expected. But there is still a long way to go to actually achieve the long-term environmental objectives of the Protocol, one of which is that there should be no exceedance of the critical loads for acidification and eutrophication.

If all countries were to implement readily available technical emission abatement measures by 2020, the area at risk of acidification would shrink to only one per cent in Europe (three per cent in the EU) and that at risk of eutrophication to 22 per cent in Europe (38 per cent in the EU).

These figures on the expected environmental effects of the revised Gothenburg Protocol come from a recent scientific assessment by the Convention’s Coordination Centre for Effects (CCE). The figures and maps presented here are based on the 2008 critical loads database and a geographical mapping resolution of 50 x 50 kilometre squares.

More recently, deposition and critical loads data with a higher resolution (28 x 28 km) has also been developed, and some preliminary results of using this newest data are also included in the CCE analysis. These show that areas at risk remain largely the same when using the new set of data, and while the total area where acidity critical loads are exceeded turns out to be somewhat smaller, the area exposed to nitrogen overload is calculated to be even greater.

A tentative assessment of changes to ecosystem biodiversity in a number of classified nature areas (such as certain grasslands, scrublands and woodlands) in the EU indicates that in 1990 the area at risk – which is defined as the area where air pollutant deposition triggers more than a five-per-cent change in species diversity – covered around 290,000 km2. Full implementation of the Gothenburg Protocol commitments could reduce this area to 68,000 km2 by 2020.

Christer Ågren

Source: Modelling and mapping of atmospherically induced ecosystem impacts in Europe: CCE Status Report 2012. By M. Posch, J. Slootweg, J-P Hettelingh (eds). RIVM Report 680359004, Coordination Centre for Effects, Bilthoven, the Netherlands. www.rivm.nl/cce

Figure: Areas where critical loads for acidification are exceeded by acid depositions (left) and areas where critical loads for eutrophication are exceeded by nutrient nitrogen depositions (right) caused by emissions between 1980 (top) and 2020 (bottom), the last projected under the Revised Gothenburg Protocol (RGP).

How much do we need to cut emissions?

When evaluating the environmental improvements that are expected to result from the revised Gothenburg Protocol by 2020, scientists at the CCE also made a rough estimate of what additional reductions in acidifying and eutrophying emissions are needed to achieve levels of depositions that no longer exceed the critical load limits.

This was done using the very simplified approach of assuming uniform (same percentage) gradual emission reductions for all European countries and for international shipping. The starting point was the emission levels projected for 2020, assuming full implementation of the revised Gothenburg Protocol.

For eutrophication it was shown that an additional 70 per cent reduction in total nitrogen emissions (nitrogen oxides and ammonia) would bring the exceedance of the critical loads for nutrient nitrogen close to zero. The total area at risk of eutrophication in Europe would still cover about two per cent, but the magnitude of exceedance in these areas would be quite low.

As regards acidification, exceedance of the critical loads approached zero at an additional 60 per cent combined reduction in emissions of sulphur dioxide, nitrogen oxides and ammonia. The remaining area at risk in Europe would come down to less than one per cent. No detailed analysis was made regarding the locations of the remaining areas at risk (e.g. their spread between countries) or what types of ecosystems (e.g. nature protection areas) would still be exposed to excess deposition.