Critical loads and critical
levels

CONCAWE supports the use of
critical loads and critical levels in the assessment and management of
athmospheric polluants, but believes that their implications must be
understood.

Critical levels (CLLs) and critical loads (CLs) currently represent the
only quantitative methodology for assessing the regional-scale
environmental impact of atmospheric pollution on ecosystems both directly
and through deposition. The approach can be applied to effects as diverse
as the reduction of crop yields due to ozone, damage to ancient monuments
by sulphur dioxide, or the deposition of acidifying pollutants on soils.
As such, critical levels and critical loads are essential tools in the
quest for an objective and cost-effective approach to environmental
protection. Critical loads are a fundamental component of the integrated
assessment modelling currently being undertaken under the UN-ECE1 Convention on Long-Range Transboundary Air Pollution
(CLRTAP) and the EU Acidification and Ozone Strategies. Indeed, they are
central to the determination of the levels of emission abatement required
by these initiatives. In addition, critical levels form the basis for the
air quality limit values being proposed to protect ecosystems under the EU
Ambient Air Quality Assessment and Management Directive.

UN-ECE* defines Critical Levels and Critical Loads as:

Critical Levels (CLLs)
The concentration of pollutants
in the atmosphere above which direct adverse effects on receptors,
such as plants, ecosystems or materials, may occur according to
present knowledge.

Critical Load (CL)
A quantitative estimate of exposure
to one or more pollutants below which significant harmful effects on
sensitive elements of the environment do not occur according to
present knowledge.

* United Nations Economic Commission
for Europe

Critical loads are used in environmental policy development but,
despite this, they are widely misunderstood and misrepresented. The use of
the word 'critical' is misleading: they are not sharp thresholds, nor does
their being exceeded necessarily mean that the environment is actually
deteriorating. Rather, it indicates that, with current conditions
prevailing, deposition will lead to the implicit environmental quality
standard being breached at some time in the future. Moreover, exceedance
of a CL may still be compatible in the longer term with environmental
improvement. Therefore, the impression given by some that exceedance
always implies environmental deterioration, is not correct. To base policy
on the notion that harm is automatically occurring when a CL is exceeded
is therefore wrong in principle. Immediate action does not, necessarily,
need to be taken, although it may be prudent to do so where the current or
future level of actual damage is considered unacceptable or where the pace
of improvement is considered too slow.

As currently calculated, critical loads have the characteristics more
of a long-term environmental quality standard rather than a damage
threshold. They give an indication of long-term sustainability, a
benchmark above which cumulative detrimental effects on the environment,
however defined, will occur at some time in the future if emissions are
not reduced. That time could be years, decades or centuries hence.
Moreover, the character of detrimental effects which may occur in the
future occupy a wide range, from minor reductions in productivity of
plant/animal populations, to serious challenges to ecosystem vitality. It
follows that critical loads tell us nothing about when an emissions
control policy needs to be put into effect or if it is worthwhile doing so
from a cost/benefit point of view.

The manner in which individual CLs are calculated also supports the
view that they do not represent sharp thresholds. This is unsurprising
since thresholds do not exist as such in the natural environment where
boundless variation prevails in almost every dimension. Examples include:
variation in response to pollutants between individuals within the same
species, variation between species, variation in response due to other
stress factors such as drought or because plant/animal colonies are
surviving close to their natural limits of altitude or latitude, variation
in geophysical conditions, random variation and the unequal influence of
human activity. All play a part in ensuring living organisms never show a
sharply defined response to a specific level of pollution deposition.
Response always occurs over a range, which is often wide. However, as
currently used and partly of necessity, CLs do not reflect this natural
variation in response. Instead, the same 'fixed' standard is often applied
irrespective of actual, local, environmental need. For instance, some
countries such as Norway, Sweden and Germany have used a single CL to
represent all woodland, regardless of tree species, altitude, latitude or
soil.

It follows that several methods exist for calculating CLs for each
ecosystem and there is a range of subordinate options within each method.
Work under the UN-ECE CLRTAP allows individual countries to choose which
ecosystems to include in their calculations of CLs, which methods of
calculation to adopt and how to parametrize those methods. This has the
advantage of permitting countries to take into account specific features
of their own environment, but it also has the disadvantage of being open
to bias from cultural or political preference. The result is that the
calculation of CLs is inconsistent between countries. A rigorous appraisal
of the inconsistencies is infeasible since not all countries have made
public the choices and assumptions used in calculating CLs or the
receptors they regard as being at risk. Table 1 illustrates some of the
differences between countries. It can be seen that Finland, Italy and
Sweden have nominated more than 90 per cent of their country's area as
ecosystems which need to be protected by critical loads for acidity,
whereas less than 10 per cent of Belgium, Croatia, Denmark, Hungary,
Ireland or The Netherlands have been identified as sensitive to acidity.
The differences can only be partly explained by differences in geology. It
can also be seen that the number of ecosystems identified in the different
countries varies widely, ranging from about 299 000 in the UK to 34 in
Croatia.

Number of critical loads and spacial density provided by
country

Country

Total area of
country (000km2)

Number of
ecosystems

Percentage of
ecosystem
cover

Average ecosystem
area (km2)

EU Countries

Austria
Belgium
Denmark
Finland
France
Germany

Italy
Netherlands
Spain
Sweden
United
Kingdom

84
30
43
334
548
357
69
302
36
499
445
244

6 444
652
18 784
4 533
442
94
943
26 383
738
127 490
3 409
5 053
298
983

58
5
9
96
25
24
10
100
9
17
89
36

7.6
2.4
0.2
71.0
307.7
0.9
0.3
406.8
0.03
25
79
0.3

non-EU countries

Croatia
Czech
Republic
Estonia
Hungary
Norway
Poland
Russian
Fed.
Switzerland

56
79
46
93
318
311
5
090
41

34
29 418
140
42
3 025
3
914
14 251
12 349

5
34
41
3
70
57
74
30

79
0.9
135
68
73
46
265
1

Source: UN-ECE CLRTAP Coordinaion Centre for
Effects-Status Report 1997

Defining environmental damage is another problem. The quality standard
represented by a CL may, in some areas, represent conditions that can
never have existed naturally. To take an example, it is not difficult to
show that the more sensitive Norwegian lakes can never have had an acid
neutralizing capacity (ANC) which the critical load methodology imposes on
them. In such locations, the critical loads could not be reached even if
anthropogenic sources of acid deposition ceased. Defining damage is thus a
matter of choosing an arbitrary standard whether it be ANC, fish
densities, biomass production rates or crop yields. Again, this ought to
be differentiated according to what the system could support under natural
conditions.

Similar problems arise with critical levels. The difficulties in
defining damage are reflected in the often inconsistent interpretation of
basic dose/response data by experts. Take, for example, the critical level
adopted for protecting arable crops from ozone. A value of 5300 ppb/hrs
AOT40 (accumulated hours of exposure over a threshold of 40 parts per
billion) was identified at a meeting of experts in Bern in 1993. Three
years later, at a workshop in Kuopio, experts met again and
'reinterpreted' essentially the same dose/response relationship to give a
value of 3000 ppb/hrs AOT40. The impact of this change in critical levels
on the predicted area of exceedance in the UK is shown in Figure 1:
cropland at risk rises from 29 per cent to 91 per cent. Clearly this is a
change with profound implications for emission control policy, but one
which simply reflects the immaturity of the science base.

Figure 1

Comparison of critical level exceedance maps fot
arable crops based on the recommandations of a) the Bern workshop
and b) the Kuopio workshop.

Maps provided
by environment Information Centre,
ITE

CONCAWE believes that the concept of critical loads and critical levels
represents a useful first step towards the quantitative assessment of
environmental damage but that the science is presently in its infancy and
not yet able to provide more than a qualitative base for policy
development. The key position occupied by critical loads in current
legislative initiatives may reflect an incorrect belief amongst policy
makers that they represent damage thresholds.
CONCAWE encourages
further research and development of this important field and better
awareness of the strengths and limitations of the critical loads and
critical levels approach.