Indicator description
Critical loads are thresholds above which significant harmful
effects occur on sensitive UK habitats. Approximately
78,000km2 of terrestrial habitat areas are sensitive to
acid deposition. About 73,000km2 are sensitive to
eutrophication; many of these areas are sensitive to both.
A three-year average is used to calculate the figures for each year
represented, and so the assessment is based on a direct comparison
between the earliest and latest data points.
The percentage of sensitive habitat area on which acid deposition
exceeded critical loads fell from 73 per cent in 1996 to 60 per
cent in 2000. It has subsequently shown little overall change, with
59 per cent of the habitat area exceeded in 2005. During the
same period the percentage area of sensitive habitats where
eutrophying pollutants (i.e. nutrient nitrogen) exceeded critical
loads fell from 75 per cent in 1996, to 69 per cent in 2000 and
then it increased to 71 per cent in 2007, with a peak of 73 per
cent in 2005. Both measures are assessed as improving in the
long-term, as is the area affected by acidity since 2000. The area
affected by nitrogen is assessed as showing little overall change
since 2000.
Relevance
The air pollutants sulphur dioxide, nitrogen oxides and ammonia
can contribute to acidification, and nitrogen oxides and ammonia
can contribute to terrestrial eutrophication. Both problems can
adversely affect semi-natural ecosystems. Exceeding the critical
load for acid deposition may have direct effects, for example by
altering the soil pH and therefore the range of associated species,
or indirect effects, for example by increasing dissolved aluminium
concentrations in soil and water. Excess nitrogen as a nutrient can
lead to leakage of nitrogen from soil systems, causing
eutrophication of inland waters or affect species composition, for
example, by triggering accelerated growth of some species at the
expense of others.
Background and data sources
Critical loads are thresholds above which significant harmful
effects on sensitive UK habitats are known to occur, given current
levels of scientific understanding. Critical loads are established
separately for acidification and nutrient nitrogen (eutrophication
effects). The pollutants causing acidification and eutrophication
mainly arise as a result of emissions from burning fossil fuels and
from livestock waste.
There are three main steps in the assessment of the area of
sensitive habitat exceeding critical loads: the calculation of
critical loads for each of the sensitive habitats; the mapping of
the habitats and the identification of the area of habitat where
deposition exceeds the critical load.
Critical loads for acidity and nutrient nitrogen are calculated for
13 broad habitats (table 10ii) considered sensitive to
acidification and/or eutrophication. A variety of methods are
used to calculate critical loads, based either on empirical
(observational or experimental data) or mass balance (input/output)
data. Further information on how critical loads are
calculated are given on the Critical Loads and Dynamic Modelling
website (see below).
To identify the area exceeding critical loads, deposition maps
based on a 5x5km grid covering the UK are produced based on the sum
of wet, dry and cloud deposition. These deposition data are
overlain on maps of critical loads for each habitat to
calculate critical load exceedances and the areas of habitat
exceeded.
In general the area of sensitive habitat where critical loads are
exceeded for both acidity and eutrophication is lower in Scotland
than elsewhere in the UK (Table 10i). Detailed
critical load
exceedance maps are available on the Centre for Ecology and
Hydrology website.
Since 2002 (2001-3), the inclusion of nitric acid deposition in the
assessment has increased the area of estimated critical load
exceedance compared to earlier periods. The deposition values from
2003 (2002-4) additionally include aerosol deposition of
NH
4+, NO
3- and SO
4-. In all years,
the 3-year average deposition is used to smooth substantial
year-to-year variability.
Table 10i. Percentage of area of sensitive UK habitats
exceeding critical loads for acidification and eutrophication for
2007 (2006 to 8)
|
|
Acidification (%)
|
Eutrophication (%)
|
|
UK
|
54
|
71
|
|
England
|
68
|
97
|
|
Wales
|
80
|
94
|
|
Scotland
|
43
|
54
|
|
Northern Ireland
|
70
|
89
|
As new research data become available the
methodology for calculating critical loads are being reviewed and
updated periodically. This has lead to an interim revision for this
indicator in 2011. As a result of this
revision the results for exceedance of acidity critical loads
remain unchanged from those published earlier. The results for
all years for exceedance of nutrient nitrogen critical loads have
been updated using the recently updated critical loads of nutrient
nitrogen. Details of the revision can be found in the annex
accompanying this indicator.
Table 10ii. The 13 habitats considered sensitive to
acidification and /or eutrophication for which critical loads are
calculated
|
Habitat
|
|
Acid grassland
|
|
Calcareous grassland
|
|
Dwarf shrub heath
|
|
Bog
|
|
Montane
|
|
Coniferous woodland
(managed)
|
|
Broadleaved woodland (managed)
|
|
Fagus woodland
(unmanaged)
|
|
Acidophilous oak
(unmanaged)
|
|
Scots Pine (unmanaged)
|
|
Other unmanaged woodland
|
|
Dune grassland
|
|
Saltmarsh
|
Further development planned
Dynamic models for acidity are also being applied to
selected habitats to estimate the timescales involved in recovery
of the chemical status of habitat soils.
This methodology is currently being applied to the protected site
network in the UK – to allow for the production of ‘site relevant
critical loads’.
Web links for further information
|
Reference
|
Title
|
Web site
|
|
Centre for Ecology and Hydrology
|
CEH critical loads
modelling and mapping
|
|
|
United Nations
|
Convention on Long Range Transboundary Air Pollution
|
http://www.unece.org/env/lrtap/ |
| Netherlands Environment |
Critical Loads Coordination |
www.mnp.nl/cce/ |
|
Assessment Agency
|
Effects Centre (CEC)
|
|