Global Climate Change and Human Impacts on Forest Ecosystems

Abstract

We are experiencing climatic changes that may have no precedent in recent human history, and the forced greenhouse warming caused by human activities may outweigh natural climatic fluctuations unless some very unusual natural events occur. Thus, the prevention of dangerous anthropogenic interference with the climate system has become a challenge (Nisbet 1991; IPCC 1995). The climate of the Earth is undergoing natural variations, from diurnal cycling to variations during time spans of millions of years. Variations that occur over longer time scales (more than 1000 years) are related to, among other things, changes in solar radiation (Schönwiese et al. 1994), time-dependent changes of Earth-orbit parameters, the configuration of continents and oceans, and changes in the chemical composition and optical properties of the atmosphere. However, variability on a several-year time scale appears to be dominated by natural variability of the atmosphere, while variability on decadal and century-long time scales is most likely caused by internal variability of the coupled ocean-atmosphere system (Jöhannesson et al. 1995). The latter has probably been influenced notably by anthropogenic impacts during the past Century, forcing the so-called greenhouse effect.