Clean Air – Ecosystems and Climate

International action on air pollution was initially – in the 1970s and 1980s – motivated by damage to ecosystems, especially acidification of fresh water and wide-spread forest damage. Although the deposition of acidifying sulphur has significantly been reduced over the last 30–40 years, deposition of eutrophying nitrogen still exceeds the critical loads on approximately two thirds of EU ecosystems, and critical levels for ozone are exceeded over the majority of European forestry and agriculture land.

The scientific knowledge about air pollution impacts on ecosystems and biodiversity is solid and well-established. However, current policymaking often relies on economic cost-benefit analyses, and it has for many reasons proven difficult to achieve scientifically and economically robust monetary valuations to ecosystems and biodiversity. As a consequence, political acceptance of ecosystem-oriented cost-benefit analysis is yet to be achieved.

Moreover, there are complex interactions between air pollution, climate change, land-use change and other ongoing processes, which all affects ecosystems and biodiversity. While the evidence of air pollution causing damage to ecosystems and biodiversity is undeniable, further research is needed to deepen our understanding of the above-mentioned interactions. This work should be complemented by efforts to translate the complexities of these processes into clear and transparent facts to assist policymaking.

Nitrogen emissions from agriculture, industry, and transport are decreasing only slowly, and nitrogen deposition still exceeds critical loads in 60–70% of Europe. Nitrogen negatively impacts the environment, human health, and contributes to global warming. Although nearly all reactive nitrogen (Nr) released into the environment is eventually converted back to nitrogen gas (N₂) through denitrification, it can undergo numerous reactions beforehand, causing a range of harmful effects on the atmosphere, hydrosphere, and biosphere. This process is often referred to as the nitrogen cascade. However, nitrogen is also an essential nutrient, and nearly half of the food produced for human consumption relies on nitrogen fertilizers derived from industrial nitrogen fixation.

Reactive nitrogen is present in all organic matter and many materials and products we depend on. The pathways and forms in which Nr leaks into the environment are diverse. To address the complexity of nitrogen cycling, the concept of the National Nitrogen Budget (NNB) was developed by the Air Convention’s Task Force on Reactive Nitrogen (TFRN). This methodology divides national nitrogen budgets into eight key nitrogen pools and defines the major flows between them, proposing methods for quantifying these flows. NNB construction is based on national and international statistics, and where national data is unavailable – such as nitrogen content in certain products – typical values from published sources are synthesized. Sweden is close to completing its NNB using this methodology, while Denmark had already published an NNB before the TFRN approach was formalized. Finland and Norway have started their work on NNBs by focusing on forest nitrogen budgets through a joint project with Sweden and Denmark.

Nitrogen cycling in ecosystems is closely linked to carbon cycling. The size of carbon and nitrogen stocks in various pools depends on the balance between inputs and outputs. While measuring carbon inputs (photosynthesis) and outputs (microbial respiration and fires) is challenging, nitrogen inputs and outputs are easier to quantify. Since nitrogen and carbon are key components of organic matter, nitrogen budgets can help estimate changes in carbon stocks. The work of the TFRN on NNBs, therefore, has the potential to reduce uncertainties in national carbon budgets. This opens up opportunities for beneficial cooperation between the United Nations Framework Convention on Climate Change (UNFCCC) and the Air Convention.

Surface water acidification remains an issue in the Nordic countries, which is reflected in continued exceedances of critical loads, monitoring data and also by significant annual spending on liming measures. The main culprit for surface water acidification in acid-sensitive (e.g., base-cation poor catchment soils) areas is elevated sulphate deposition along with long-term depletion of base cations from decades of mobilization and export of base cations from soils to surface waters. Natural acidity from dissolved organic matter (organic acidity) is beginning to be a more dominant driver of surface water acidity now that sulphate deposition has been strongly reduced.

Climate exerts a control on surface water acidification through enhanced soil base cation depletion and mobilization of organic acidity during high-precipitation events and long-term trends. Additionally, sea salt deposition associated with stormy weather can lead to episodic reacidification. Land use, especially in the form of forestry, can also lead to local acidification and to longer-term base cation depletion if all biomass is exported from the catchment. Long-term increases in biomass and longer growing seasons may lead to lower leaching of nitrate and thus oligotrophication of surface waters. Thus, surface water quality and acidity are controlled by multiple stressors.

It is difficult to assess to which status the ecosystems can be expected to return in an era of low air pollution but increasingly warmer and wetter climatic conditions. Especially estimation of biological status is fraught with uncertainty, since more factors than air pollution have changed since the onset of the industrial revolution. Very little is known about historical biological communities, and some populations may now be extinct. Thus, what may be expected in terms of a recovered ecosystem requires a solid understanding of species diversity and dynamics in healthy ecosystems, and predictive ability of changes in biogeochemical cycling.

In the perspective of the green transition and the biodiversity crisis, valuation of costs and benefits from changes in pressures to ecosystems is crucial. This presentation shows results from two NMR-funded projects revolving around this topic.

The project Benefit Nature has the goal to derive the methodology for a new Nature component to the EVA system, which previously has focused solely on human health effects and corresponding socioeconomic costs. The anthropogenic driver is reactive nitrogen deposition, the receptor is terrestrial sensitive ecosystems with biodiversity measured as similar plant species richness as the key metric, and the valuation of species loss/gain is based on Danish conditions.

The project Nordic Nature & Nitrogen has the goal to implement a more complex (and more true to natural processes) dry deposition parameterisation in the three Nordic regional models DEHM, EMEP, and MATCH, and to evaluate the impact on reactive nitrogen deposition to sensitive terrestrial nature areas. The new parameterisation takes into account the saturation effects in plants, leaves, water and soil, which occur in high deposition regimes, which potentially redistributes the reactive nitrogen deposition with increased deposition further away from sources.

The results from the two (ongoing) research projects, will give recommendations for policy-relevant future research projects that can take advantage of existing data developed within the projects.

The complexity of valuing nature in economic terms was brought up as well as the intrinsic value of a thriving, healthy environment (clean air, clear water, productive soils, etc.). The latter could be set in contrast to the utilitarian view of “ecosystem services”. Time of recovery, and the difference between chemical and biological recovery, were highlighted. Could “cost of restoration” be used as an alternative way of valuation? Which leads to the question of what recovered/restored ecosystems can we expect as a result? Links to the CBD, the EU’s Nature Restoration Law and the impacts of climate change were also discussed.

The Nordic cooperation on air quality should aim to:

Ensure the continuity of ecosystems monitoring. Long-term data collection and analyses are essential for establishing the links between biodiversity and air pollution, and also needed for economic valuation of nature. Monitoring is of particular relevance for the protection of the vast and sensitive Nordic ecosystems.

Further develop exposure and valuation metrics for biodiversity. While exposure metrics already exist and are used for policy, there is still room for improvement of these, e.g. with respect to delay functions for exposure. Also, the implications of biodiversity metrics for science and policy advice need to be analysed qualitatively. In addition, research is needed for development of biodiversity valuation metrics. Promoting such developments and qualitative analyses will benefit Nordic countries that have vast ecosystem areas, many of which are stressed by air pollution, but not yet beyond rescue.

Promote better exchange of knowledge. Link Nordic scientific knowledge on air pollution (especially nitrogen) impacts on ecosystems, as well as Nordic work on national nitrogen budgets, to relevant working groups supporting the Intergovernmental Panel on Climate Change (IPCC) and CBD.

Encourage integrated nitrogen-carbon management approaches. Nordic leadership in piloting integrated nitrogen-carbon strategies could set a model for broader international collaboration, creating a more coherent approach to managing nitrogen and carbon pollution, reducing overlap, improving both nitrogen and carbon emissions reporting, and thus improving policy outcomes both nationally and internationally.

Assess effects of ozone on (semi)natural ecosystems. Background ozone is still a threat to ecosystems and human health, and there is clear risk that ozone concentrations may increase if methane emissions increase. As ecosystem impacts of background ozone levels are not so well known, an assessment of current knowledge with respect to Northern ecosystems would be useful.