Critical loads - Possibilities and constraints

Since critical thresholds for acidic deposition were first considered in the 1970s, the critical loads approach has rapidly evolved into a practical tool for addressing the problems of pollution control. It has gained acceptance from many scientists and policy makers, bridging the gap between science and policy and proving its use with the acidifying effects of the atmospheric pollutants of sulphur and nitrogen. For this there are examples at local, national and international scales and there appears to be scope for further applications as methods and databases improve. The UN/ECE used the approach to underpin the second Sulphur Protocol; for this, emission reductions were recommended on the basis of environmental effects as well as the costs of control measures. Although limited by a relatively coarse transport modelling scale, the advantages over non-effect-orientated approaches were evident. At a more local scale, the approach has often lacked information linking some of the chemical effects on soils and freshwaters to harmful effects to biota. In addition, the important consideration of rates of change, in particular recovery, of ecosystems with changing pollutant loads has still to be addressed nationally and internationally. The critical loads approach is a first step towards effects orientated pollution control. Already sophisticated methods, based upon similar dose-response data, are being developed; these lend themselves better to economic evaluation of damage. Despite limitations, the critical loads approach has proved a practical method for deriving pollution control strategies whose success will be judged in the years ahead.