Ice ages, when large areas of land are covered by continental glaciers, have occurred repeatedly during the Earth's history. Changes in climate account for the advance and retreat of the glaciers.
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.
If there is a volcanic eruption in the Arctic or the Antarctic, the dust from it does not spread so far from the poles. But if there is a tropical volcanic eruption, the dust spreads outwards towards the poles, causing global cooling. A sufficiently large volcanic eruption can have a dramatic effect on the weather. When Toba, in Sumatra, exploded into life 73,500 years ago, it put out enough material to accelerate the onset of the last ice age. In July 1783, after Mt. Laki in Iceland blew up, Benjamin Franklin suggested that the cold winter that followed in the USA could have been caused by it. In 1815, Tambora, in Indonesia, erupted, the second biggest (after Toba) in the past million years. It is reckoned to have thrown out five times more dust than Pinatubo in 1991. 1816 was called "the year without summer". There were June frosts in New England and the French wine harvest was the latest for five centuries. Krakatoa, perhaps the most famous and apocalyptic volcano, was followed by cold, wet summers through Europe for years.
The last time glaciers covered large parts of North America and Europe was from approximately 132,000 years ago until 10,000 years ago. Moderate ice ages appear around every 100,000 years. Little ice ages seem to occur once every 1,500 years, the last began in the 13th Century and continued into the 17th Century.
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.