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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 3: Climate and Atmospheric Deposition Patterns and Trends

Warren E. Heilman, John Hom, and Brian E. Potter
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 earth’s 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 circulation models.

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 vegetation.

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 (a) January, (b) April, (c) July, and (d) October based on maximum temperature observations from 1950-1993.

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