Ecosystems more sensitive than previously thought

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 sensitivity of various ecosystems to exposure to acidifying and eutrophying air pollutants has been monitored and mapped for more than 25 years, and European countries coordinate this work through the Coordination Centre for Effects (CCE) of the Convention on Long-range Transboundary Air Pollution (CLRTAP).

Recently, the CCE has developed a new set of maps, using updated information from the countries’ national experts. By comparing the critical load maps with data on air pollutant deposition, the CCE has also produced maps that 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 are exceeded.

This new data shows that the areas at risk are greater than previously assumed – the acidity critical loads are now exceeded in eight per cent of the ecosystems in the EU (7% in the whole of Europe). The area exposed to nitrogen overload now extends to 75 per cent of EU ecosystems (62% in Europe). See table and maps.

Table : Area of ecosystems exposed to excess deposition of eutrophying and acidifying air pollutants in 2010 (km²).

Figure :
Areas where critical loads for acidification are exceeded by acid depositions (top) and areas where critical loads for eutrophication are exceeded (bottom) at 2010 emission levels.

Following emission cuts over the last 40 years in the main acidifying air pollutants, especially sulphur dioxide (SO₂), the area of sensitive ecosystems at risk of acidification in Europe has now shrunk to less than 250,000 square kilometres (km2), nearly eight times smaller than it was in 1980. 

Progress is however markedly slower for eutrophication, which is caused by excess nitrogen deposition resulting from emissions of nitrogen oxides (NOx) and ammonia (NH₃). Here the affected area has shrunk by less than 40 per cent over the same time period, and still covers 1.9 million km².

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.

Clearly there is still a long way to go to actually achieve the long-term environmental objectives of the EU’s 7th Environmental Action Programme, one of which is that there should be no exceedance of the critical loads for acidification and eutrophication. The same objective is also enshrined in the CLRTAP Gothenburg Protocol.

The key legal instrument in the EU for cutting emissions of acidifying and eutrophying air pollutants is the National Emissions Ceilings (NEC) directive, which is currently being revised, and negotiations on new emission reduction targets up to 2030 are now ongoing between EU institutions, with the aim of reaching a final compromise by June 2016.

Christer Ågren

Source: Modelling and mapping of the impacts of atmospheric deposition of nitrogen and sulphur: CCE Status Report 2015. By J. Slootweg, M. Posch, and J-P Hettelingh (eds.). RIVM Report 2015-0193, Coordination Centre for Effects, the Netherlands. Link: www.wge-cce.org/Publications/CCE_Status_Reports/Status_Report_2015