Defra - Department for Environment, Food and Rural Affairs.

Science Search

Science and Research Projects

Return to Science Search homepage   Return to Project List

Critical Load and Dynamic Modelling Umbrella 2007-2010 - AQ0801

This project suppports the development of policy within Defra by predicting the effect that atmospheric pollutants have on Britain's sensitive plants, soils and freshwaters. Sulphur pollution has been, historically, the main component of 'acid rain', which has caused acidification of soils and waters, leading to biological damage (such as fish deaths) in many areas. Although sulphur pollution (mainly from fossil fuel burning) has decreased greatly from its 1970s peak, many damaged ecosystems are slow to recover, and some of the most sensitive ecosystems may remain damaged even under reduced loadings. At the same time, levels of atmospheric nitrogen pollution (mainly from fossil fuel burning and agriculture) have increased. This can also contribute to acidification, and as a key nutrient for plant growth, can cause nutrient enrichment (eutrophication) of semi-natural ecosystems. Many of the characteristic plant species of these ecosystems, including rare species important for biodiversity, are adapted for low nutrient conditions; the 'fertilisation' of these habitats by atmospheric nitrogen deposition can allow other, nutrient-loving species to invade and displace existing species, leading to loss of biodiversity and conservation value.

Using the results of a wide range of scientific studies, including the Defra freshwater and terrestrial research programmes, we calculate 'critical loads' of sulphur and nitrogen deposition for a wide range of UK habitats. If deposition is below the critical load, we predict that this ecosystem should not be damaged, whereas if it the critical load is exceeded, then damage (acidification and biodiversity loss) will eventually occur. By calculating and mapping critical load exceedance, areas of potential damage can be identified, and pollution controls applied to reduce this damage. The UK's critical load data contribute to European level assessments under the UNECE Convention on Long-Range Transboundary Air Pollution, which are used to support international negotiations to limit pollutant emissions, providing the most cost effective balance between maximising environmental protection while limiting costs of pollution control for industry. In addition to calculating critical loads, we also apply dynamic models, which allow us to predict how the condition of an ecosystem will change over time. This approach builds on the critical loads by providing information on the timescales of damage and recovery, either by predicting how different pollution-control scenarios will affect ecosystems in future, or by calculating the change in pollution levels that would be required to achieve recovery in a damaged ecosystem by a given date.

For this proposal, we will continue to develop our models to improve the way they simulate complex chemical and biological processes, and to enhance the linkages between the deposition of sulphur and nitrogen pollutants, the chemical response of soils and waters, and the biological response of plants and aquatic biota. By integrating different models of soil chemistry, plant growth and competition we aim to provide a whole-ecosystem model, so that the effects of many different man-made influences (including pollution, local land management, and climate change) can be assessed in combination. We aim to extend our current modelling approach to cover a wider range of ecosystems at the national scale, including development of an approach to predict changes in lake chemistry across the UK, and also to predict changes in particular areas of conservation importance, such as nature reserves and Sites of Special Scientific Interest (SSSIs). The results of this work directly support policy development by Defra at the national scale, and also provide a resource to support the sustainable management and environmental protection of individual sites and regions within the UK.
The proposed work is intended to address the main Defra priorities in predicting the effects of nitrogen and sulphur deposition on terrestrial and aquatic ecosystems, including risks to sites of conservation importance, at the national scale; to assess policy options for fulfilling obligations under the Habitats Directive, Water Framework Directive and national policy objectives; and to provide support for the review and revision of the Gothenburg Protocol including scenario assessment and target setting via CCE calls for data (the first of which is anticipated in October 2007). The objectives map onto the 2007-2010 workplan of the Working Group on Effects, including: modelling nitrogen effects on biodiversity (ICP M&M); developing target loads for nutrient nitrogen (JEG); assigning critical loads for Natura 2000 sites (ICP M&M); developing dynamic models that account for the effects of climate change (JEG); and developing uncertainty analysis for acidification and eutrophication (ICP M&M).

Objective 1: To enhance existing ecosystem models by incorporating new scientific understanding of the key processes that determine the effects of S and N deposition on soil and water chemistry, and subsequent biological impacts

Objective 2: To develop a model framework that flexibly links biogeochemical, plant growth and plant occurrence models, in order to model the effect of multiple environmental drivers including S and N deposition, management and climate change

Objective 3: To undertake targeted collection of new data required for robust dynamic model testing and application.

Objective 4: To test the models against detailed site measurements and large-scale datasets, and to quantify uncertainties in model predictions

Objective 5: To apply the improved models to predict future change for relevant deposition scenarios at the national scale for terrestrial ecosystems (1km resolution) and aquatic ecoystems (UK lake population)

Objective 6: To maintain the current critical loads database, and develop new methods for assessing potential impacts on designated conservation sites

Objective 7: To develop new methods for calculating biodiversity-based critical loads

Objective 8: To carry out critical load and dynamic model scenario assessments, and to provide the UK National Focal Centre for critical loads and dynamic modelling

Objective 9: To organise and chair the UNECE Joint Expert Group on Dynamic Modelling

Objective 10: To provide the UK National Focal Centre for the International Cooperative Programme on Integrated Monitoring
Time-Scale and Cost
From: 2007

To: 2012

Cost: £768,962
Contractor / Funded Organisations
Centre for Ecology and Hydrology
Air Pollution              
Environmental Protection              
Fields of Study
Air Quality