Global Climate Changes and Critical Loads of Sulfur and Nitrogen in the European Ecosystems

Global biogeochemical cycle of carbon and its alterations attracted great attention in the mass media due to CO2 increase in the atmosphere being closely related to the

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Figure 7 Temperature anomalies and methane and carbon dioxide concentrations over the past 220 000 years as derived from the ice-core records at Vostok, Antarctica.

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Figure 7 Temperature anomalies and methane and carbon dioxide concentrations over the past 220 000 years as derived from the ice-core records at Vostok, Antarctica.

various changes in the Earth biosphere. These changes include both climate changes and environmental pollution. Here one should mention a project on the integrated assessment of regional air pollution and climate change in Europe, AIR-CLIM project, which inter alia examined whether climate change will alter the effectiveness of agreed-upon or future policies to reduce regional air pollution in Europe. Climate changes and emission abatement strategy can be estimated using the calculations of pollutants' critical loads and their exceedances.

A critical load has been defined as the maximum input of pollutants (sulfur, nitrogen, heavy metals, POPs, etc.), which will not introduce harmful alterations in biogeochemical structure and function of ecosystems in the long term, that is, 50-100 years according to present knowledge. Starting from this general definition, methodologies have been developed during the 1990s by the working group on effects (WGE) under the long-range transboundary air pollution (LRTAP) convention for calculating and mapping critical loads in Europe. This has been used by European countries to calculate critical loads of pollutants for various ecosystems (forests, surface waters, and seminatural vegetation).

Transboundary air pollution by sulfur, nitrogen, heavy metals, and persistent organic species is not the only environmental problem calling for internationally agreed abatement policies. In developed countries, it is the issue of climate change, which currently attracts most attention and resources, and negotiations under the framework convention on climate change (FCCC) are trying to come up with equitable mitigation policies. To date, climate change policies are mostly discussed in isolation. However, any measures taken (or not taken) to slow down global warming are likely to have an impact on other environmental problems.

Eight scenarios for different combinations of future GHG, sulfur, and nitrogen emissions, covering the years 1990-2100, were developed during the AIR-CLIM project (Table 5).

To assess the risk of ecosystem damage due to a given scenario, critical loads have to be compared with the resulting deposition patterns. Within the integrated assessment framework of AIR-CLIM, deposition fields due to emission scenarios are computed with the source-receptor matrices (SRMs) derived from the EMEP longrange atmospheric transport model. The SRMs derived for the meteorological years 1985-96 were averaged to minimize the effects of interannual variability. With the aid of these SRMs, the sulfur and nitrogen (NOX + NH3) emissions of the European countries and the respective depositions in every grid cell are computed. If the depositions are greater than critical loads, we say the critical loads are exceeded. While in the case of a single pollutant the exceedance can be defined in an obvious manner, for example, Ex(Ndep) = Ndep — CLnut(N), there is no unique exceedance (i.e., amount of deposition to be reduced to reach nonexceedance) in the case of acidifying N and S.

Figure 8 depicts the temporal development of the percentage of forest area for which critical loads of acidity and nutrient nitrogen are exceeded under the eight AIR-CLIM scenarios. The area for which critical loads are exceeded declines under all scenarios, starting from 41% for acidity critical loads and 75% for nutrient N in 1990. The speed decrease after 2010, however, differs between the two sets of scenarios, with larger decreases in the B1-set. In all the cases, the A1-P and the B1-450-A scenarios are the least and most stringent ones, respectively, with the other scenarios giving intermediate results. The most striking conclusion is that acidification (almost) ceases to be a problem, with excee-dance percentages in 2100 between 4.7% (A1-P) and 0.7% (B1-450-A). In drawing this conclusion it has to be borne in mind that considering the in-grid variability of deposition (e.g., by reducing the grid size) would certainly lead to higher exceedances. Furthermore, areas that cease to be exceeded at some point in time are not at once without the risk of adverse effects. The recovery of the chemical, and especially the biological status of the soil is delayed due to finite buffers, which

Table 5 Overview of AIR-CLIM scenarios

Scenario

Greenhouse gas policies

SO2/NOx policies

A1-P

None

Present policies

A1-A

None

Advanced policies

A1-550-P

To achieve 550 ppm CO2 stabilization

Present policies

A1-550-A

To achieve 550 ppm CO2 stabilization

Advanced policies

B1-P

None

Present policies

B1-A

None

Advanced policies

B1-450-P

To achieve 450 ppm CO2 stabilization

Present policies

B1-450-A

To achieve 450 ppm CO2 stabilization

Advanced policies

Scenario A1-P

Scenario B1-P

Scenario A1-P

Scenario B1-P

2100

Figure 8 Temporal development of the percentage of forest area for which the critical loads of acidity and nutrient nitrogen are exceeded for the four scenarios in the A1-set (left) and for corresponding four scenarios in the B1-set (right).

2100

Figure 8 Temporal development of the percentage of forest area for which the critical loads of acidity and nutrient nitrogen are exceeded for the four scenarios in the A1-set (left) and for corresponding four scenarios in the B1-set (right).

have to equilibrate with the lower deposition. Only dynamic models can provide estimates of the times needed for a full recovery.

Eutrophication, on the other hand, continues to be a widespread problem, even under the most stringent scenario, which brings the exceedance hardly down to 15% of the forest area. This confirms the conclusion that nitrogen is the main pollutant in need of future mitigation.

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