Over the last 700,000 years, the climate has operated on a relatively predictable schedule of 100,000-year glaciation cycles. Each glaciation cycle is typically characterized by 90,000 years of cooling, an ice age, followed by an abrupt warming period, called an interglacial, which lasts 10,000-12,000 years. The last ice age reached its coolest point 18,000 to 20,000 years ago when the average temperature was 9-12.6° F cooler than present. Earth is currently in a warm interglacial called the Holocene that began 10,700 years ago.
Although precise temperature readings over the entire period of geologic history are not available, enough is known to establish climatic trends. During the Holocene, there have been about seven major warming and cooling trends, some lasting as long as 3000 years, others as short as 650. Most interesting of all, however, is that the temperature variation in many of these periods averaged as much as 1.8° F, .3° F more than the temperature increase of the last 150 years. Furthermore, of the six major temperature variations occurring prior to the current era, three produced temperatures warmer than the present average temperature of 59° F while three produced cooler temperatures.
