When scientists in the 1960s investigated the causes of the die-back of forests, the so-called ‘Waldsterben’, and acidification of lakes with associated fish loss, they found that air pollution, often emitted thousands of kilometres away, was the culprit. This research formed the basis for the deliberations on the 1979 UNECE Convention on Long-range Transboundary Air Pollution.
60 years on, researchers are continuing to study air pollution and the associated impacts on water quality in the framework of the Convention’s International Cooperative Programme for assessment and monitoring of the effects of air pollution on rivers and lakes (ICP Waters). At this year’s meeting of ICP Waters (28-29 April 2021), experts discussed recent findings in relation to trends in acidification of lakes and rivers. As confirmed by a recent report, data from 500 lakes and rivers in Europe and North America over almost 30 years (1990-2016) show improved water chemistry, which implies improved conditions for fish and other aquatic life. Specifically, the data shows trends in recovery from acidification, which can be attributed to reductions of sulphur dioxide emissions under the Convention over the same period. Recovery has slowed down in Europe, but accelerated in North America since the early 2000s.
Land use and climate change are possible factors for a delay in chemical and biological recovery from acidification. Climate change extends the growing season, makes forests grow faster and lets the upper forest boundary expand in many mountainous regions. At the same time, climate change can make forests more vulnerable to drought and insect attacks, which in some cases can lead to forest decline or even dieback in some areas. What is more, is that climate mitigation measures can also lead to land use changes, through e.g. intensified forestry, to sequester carbon or to substitute fossil fuels with biofuel. All of these changes can, in turn, alter the nutrients in soils, which, for its part, may affect chemical and biological recovery of freshwaters in response to reduced acid deposition. Effects of land use and climate change on recovery from acidification could become more prominent where declines in atmospheric pollutants are levelling off. These effects hence need to be further studied as the changing climate is increasingly affecting ecosystems.