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Agents of Natural Change in Watersheds

Natural Change Processes: Climate Change
Although there is strong evidence of human-induced climate change effects, climate change has long been a natural agent of change occurring gradually over broad spatial scales. Climatic conditions and atmospheric CO₂ concentrations have been variable on a scale of glacial-interglacial cycles, as evidenced by analyses of gas bubbles in Antarctic ice, simulations by atmospheric circulation models, estimates of global ice volume, and paleolimnological analyses of pollen grains in lake and bog sediments. These paleo-records of climate clearly indicate that climate has varied due to natural causes on time scales from decades to millions of years. The present warm period began approximately 10,000 years ago with the end of the last ice age. Within this time, minor fluctuations have occurred in climate. The Medieval Warm Period reached its peak in the 12th and 13th centuries. This was followed by considerably colder climate that lasted until about 1890 referred to as the "Little Ice Age". During this period mean global temperatures were approximately 0.5-1.0°C cooler than today. On even shorter time scales (years to decades), the El Niño phenomenon has widespread effects on global climate. In general, the dominant tree species in North America today have probably become common only within the past 10,000 years, and have likely attained their current distribution within the last 3,000 - 5,000 years (Brubaker 1988).

This process has potentially important implications for long-term watershed management, particularly in parks and wilderness areas that have been established to preserve certain species or communities. Over the life span of long-lived species, climate may change to the extent that subsequent generations will not reproduce effectively in protected areas. For example, the oldest populations of giant sequoia in California national parks experience generally drier conditions relative to when they were established, 2,000 - 3,000 years ago. As a result, seedling establishment may be hampered and populations may be in decline. Vegetation responds to climate at the level of the individual species rather than as an intact assemblage of species. As climate changes in the future, we should therefore expect the formation of new communities rather than the simple displacement of existing communities to new locations (Crowley 1996).