The modern era of climate is probably the most difficult to understand because of the combined influences of natural and human-induced activities and, therefore, climate understanding and prediction pose an immense challenge to science. While intense efforts are underway to determine the history and significance of human influences on climate, the chemistry of the atmosphere, and on resource depletion, our understanding of climate change is still hampered by lack of knowledge of the natural controls on climate. Mounting evidence points to the significance of a variety of natural climate forcing agents such as solar variability, planetary orbital cycles,volcanic activity, ice dynamics, and changes in thermohaline circulation of the ocean. Without question, human activities have affected the composition of the global atmosphere, and the magnitude of human disturbance to biogeochemical cycles may now be approaching a critical level. This increase has produced serious concern regarding the heat balance of the entire planet.
Greenhouse gases are, however, only part of the human-induced problem.
Sulfur aerosols, ozone and dusts, to mention only a few, can either reinforce or counteract greenhouse gas effects on local to regional scales.