Responses of Northern U.S. Forests
to Environmental Change
Chapter 3: Climate and Atmospheric Deposition Patterns
Warren E. Heilman, John Hom, and Brian E.
Atmospheric CO2 has increased by 25% in the last century
and is expected to increase further, to a doubling or more of historical
concentrations. Coupled with increases in other greenhouse gases,
these atmospheric changes are expected to cause significant warming
of the earths surface. How such climate change would affect
the northern region will depend in large part on changes in the
development and behavior of weather systems affecting the region.
However, projecting these changes is highly uncertain because of
the extreme complexity of earth-atmosphere interactions, and an
inability to account for small-scale weather phenomena in global
Global-scale model simulations of future climate conditions suggest
a global average temperature increase of between 1.0 and 4.5oC with
a doubling of atmospheric CO2. The greatest warming is expected
over land and at higher latitudes. On a seasonal basis, warming
is expected to be most significant in late autumn and winter. All
models project an increase in global precipitation, particularly
in the winter over northern and mid-latitudes. Global models currently
do not have the capability to make predictions of changes in extreme
weather events, although such changes are theoretically possible.
Over North America, the most significant observed temperature changes
over the last 40 years have occurred from the North Central U.S.
through Northwestern Canada into Alaska. Average surface temperatures
have increased from 0.25oC to 1.5oC over this region. On a seasonal
basis, the most significant changes occurred during the winter and
spring. Some areas within the northern region, particularly in the
west and north, have experienced a large number of extreme maximum
and minimum temperature events over the last 40 years. Other notable
temperature events include late spring freezes and midwinter thaws.
Thaws followed by rapid freezing can be particularly damaging to
Precipitation patterns are highly variable and more unpredictable
than temperature changes. Precipitation in the northern region has
increased an average 2-5% per decade since 1900. Extreme precipitation
events and droughts occur periodically in different parts of the
region. Such extreme events are manifestations of atmospheric circulation
patterns which can be associated with probabilities of event occurrence.
A well-known example of such a circulation pattern is the El Nino/Southern
Oscillation, driven by changes in Pacific ocean surface temperatures.
El Nino events have the largest effect on the U.S. during the winter
and early spring months. In the Northern region, the risk of extreme
precipitation in the winter months is particularly high in the central
and southern Great Plains, based on observed precipitation patterns.
Atmospheric deposition patterns are determined by air pollution
concentration gradients associated with emission sources, meteorological
conditions, topography, and prevailing air transport patterns. The
Northeast and North Central regions of the U.S. contain a high concentration
of pollution sources, but the pattern of pollution exposure varies
markedly within the region from the highest national exposure levels
to unpolluted background levels.
Acid deposition patterns reflect the emissions source areas in
the Ohio river valley and the Midwest, and the prevailing winds
which deposit the highest acidity to a region including Eastern
Ohio, Northern West Virginia, Western Pennsylvania, and Western
New York. Other areas of high acid deposition include the Adirondack
and Catskill Mountains of New York. Nitrogen deposition, of particular
interest because of effects on forest processes, ranges from approximately
2.5 kg/ha in Northern Maine to 8.5 kg/ha in Pennsylvania.
Recent changes in deposition have resulted from the passage of
the 1990 Clean Air Act. Since the Act targeted sulfur emissions,
deposition of sulfur compounds has decreased but deposition of nitrogen
compounds has not changed significantly.
Ozone concentrations over urban/industrial areas of the Northern
U.S. average about twice background levels, or 80 ppb. Highest levels
occur in the Washington-New York corridor, and lowest levels in
Northern Minnesota and Northern Maine. Ozone concentrations vary
greatly across time and space. There is a tendency for areas of
high ozone exposure to also have high deposition of N and S compounds.
Average daily maximum temperatures (oC)
in the region during the months of
July, and October based on maximum
temperature observations from 1950-1993.
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