Geoscience Reference
In-Depth Information
By the 1980s, acid rain was clearly recognized as an international problem that
required an international solution. In 1979, negotiations to reduce the emissions
of air pollutants began under the auspices of the United Nations Economic
Commission for Europe (UNECE) with the establishment of the Convention on
Long-Range Transboundary Air Pollution (LRTAP) (http://www.unece.org/env/
lrtap) (Bull et al . 2001). Work under the Convention had produced a series of
protocols in which countries agreed to reduce emissions of sulphur and nitrogen
compounds to the atmosphere. The latest protocol, signed in 1999 at Gothenburg,
Sweden, called for about 80% reduction in S and 50% reduction in N emissions
in Europe by the year 2010 relative to levels in 1980. Similar agreements were
reached in North America and have resulted in large decreases in S emissions in
both eastern Canada and eastern United States.
The result has been dramatic reductions in S deposition in both Europe (Fig. 7.1)
and North America. As these protocols began to be implemented, emissions of
acidifying gases in Europe declined from their peak levels in the late 1970s/early
1980s, and by the year 2000, sulphur deposition had decreased by >50% and
nitrogen deposition by about 20% (UNECE 1999). Further decreases are predicted
(Fig. 7.1) for the next 20 years as the Gothenburg protocol and other, national,
legislations are implemented (called the current legislation scenario - CLe).
And it has begun to work. In the 1990s, surface waters in Europe showed the
first signs of recovery in response to lower levels of acid deposition; SO 4
concentrations decreased, pH and acid neutralizing capacity (ANC) increased
and concentrations of Al n + decreased (Stoddard et al . 1999; Evans et al . 2001).
The waters are becoming less toxic for fish and other aquatic organisms (Monteith
et al . 2005), but there is still a long way to go before recovery is complete.
In a modelling study of surface waters in 12 acid-sensitive areas of Europe,
Wright et al . (2005) showed that while many waters have shown chemical
recovery since the peak of acid deposition in the 1980s, even with full
implementation of the Gothenburg protocol, many others will continue to be
acidified for decades to come (Fig. 7.2).
Climate change can affect the chemical and biological recovery of freshwaters
from acidification. Long-term, seasonal and episodic changes in climate all
potentially affect a variety of processes in catchments and surface water bodies. For
example, warming can be expected to increase rates of mineralization of soil organic
matter, which, in turn, might release nutrients such as nitrogen in the catchment in
runoff. This was the result of the CLIMEX (Climate Change Experiment) experiment
in Norway, in which, nitrogen flux in runoff increased following a whole-catchment
warming (Wright 1998). The most biologically damaging effects of acidification
often occur during short acid episodes in lakes and streams. These episodes typically
coincide with climatic extremes, such as droughts, storm events, snowmelt or
periods of winter freezing. Acid pulses following droughts have been documented
from Ontario, Canada (Dillon et al . 1997), and acid pulses following storms with
high winds and inputs of sea salts have been reported from Southern Norway
(Hindar et al . 1994). The prognosis for future climate in Europe is generally
warmer, wetter in the north but drier in the south, and stormier in all areas with
more frequent extremes. Climate change may thus offset or even reverse some of
the ongoing recovery expected as S and N emissions continue to decline.
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