Ice ages come in cycles, of which several have been recognized. Geological evidence indicates that major (extreme) ice ages occur approximately every 250 million years. There is also evidence of a number of minor glacial advances (other than those occurring every 100-200 years). One cycle of 70-90,000 years has been noted. Another cycle appears to recur every 11,000 years. Such periods are accompanied by a general lowering of the average temperature which may result in the extinction of many species.
In June 1991, Mt. Pinatubo in the Philippines thrust 30 million tonnes of dust into the stratosphere, forming a cloud around the world which many scientists believe slowed down global warming. The eruption threw dust and sulphur dioxide some 12 miles into the upper atmosphere, above the clouds, where it could not be removed by rain or winds. This "aerosol", as climate researchers call it, encircled the earth in three weeks.
2. It has been argued that the energy to build up ice age glaciers comes from a greenhouse effect which transfers tropical moisture to the higher latitudes during winter. Studies in 1979 of pollen beds from the previous ice age indicate that the final shift from a warm interglacial climate to that ice age took place in less than 20 years. Considering that European forests now seem to be dying in a similarly precipitous way it is possible that the planet may be well into a comparable period of rapid climate change, and therefore only a few years from the next ice age. The processes of natural climate change are this time being accelerated by humanity's contribution to the greenhouse effect through destruction of the remaining forest cover and combustion of remains of ancient forests.