Energy Indicators for Sustainable Development: Guidelines and Methodologies International Atomic Energy Agency United Nations Department of Economic and Social Affairs International Energy Agency Eurostat European Environment Agency

(c) International Conventions and Agreements: The following agreements are 
relevant to this indicator: the United Nations Economic Commission for Europe 
(UNECE) Convention on Long-range Transboundary Air Pollution (CLRTAP) 
(Geneva, 1979) and its protocols to reduce emissions of sulphur (Helsinki, 1985, 
Oslo, 1994, Gothenburg, 1999) and nitrogen oxides (Sofia, 1988, Gothenburg, 1999). 
These protocols are widely accepted as a major step in combating environmental 
acidification in Europe. A new multi-pollutant, multi-effect protocol on acidification, 
eutrophication and ground-level ozone was signed by the European Union (EU) 
Member States in Gothenburg in 1999. Exceedance of critical loads is also covered by 
the Acidification Strategy of the EU. 
(d) International Targets/Recommended Standards: No specific targets have been 
defined; however, the goal worldwide should be to reduce the area of soil affected by 
acidification and/or to reduce the severity of acidification. In the EU in the long term, 
the target is to cut acidifying emissions to levels whereby critical loads will not be 
exceeded anywhere. 
(e) Linkages to Other Indicators: The indicator is linked to other environmental 
indicators such as air pollutant emissions from energy systems, which includes 
emissions of sulphur oxides (SO
x
) and nitrogen oxides (NO
x
). 
METHODOLOGICAL DESCRIPTION 
(a) Underlying Definitions and Concepts: The environment’s ability to withstand 
acid precipitation is measured by the concept of a critical load, which is now accepted 
as a basis for political decisions on reductions of emissions of sulphur and nitrogen. A 
critical load represents a quantitative estimate of a long-term exposure to acid 
pollutants that the environment (ecosystem), according to present knowledge, can 
absorb without sustaining damage or, in other words, the pollution load that the 
environment can withstand. The area where critical loads are exceeded provides an 
indication of the ecosystem area in which damage could occur. Exceedance of critical 
loads is a complex function of the deposition of various pollutants and the natural 
buffering ability of the waterway or soil in question. Long-range transboundary air 
pollution plays a significant role in areas where the critical load is exceeded. The 
number of exceedances of critical loads has therefore come to be adopted as a proxy 
measure for the level of ecosystem protection. 
It is important to distinguish between the concepts of ecosystem protection and the 
concept of exceedance of critical loads. Targets for reducing acidification are mainly 
aimed at addressing the gap between the present level of exceedances and a ‘zero’ 
level of exceedances; that is, the gap closure approach. Both the EU Acidification 
Strategy and the UNECE protocols use the gap closure approach; both have the long-
term goal of reducing the number of unprotected ecosystems to zero. The main 

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difference between the two is in the planned pace of attainment, with the EU targets 
being required to be reached on a faster time scale.