Background color for WARNING safety signs, labels, and tags. Marking hazardous parts of machines with may cut, crush, or otherwise injure. Emphasizing hazards when enclosure doors are open or when gear, belt, or other guards around moving equipment are opened or removed, exposing unguarded hazards. Marking exposed edges of
Bibcode:1995GBioC...9..377C. doi:10.1029/95GB01107. An ice age is a period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers.
The theoretical extent of these ice sheets during the ice age 1.5 mya is shown in white on the map below. (White area = maximum glacial extent 1.5 million years ago in the Pleistocene Epoch - Microsoft Encarta) Ice ages are said to have occurred every 150 million years, and last for at least 1 million years. According to the ice age theory, our current warm period will end with the beginning of another cold phase in 23,000 years time. Geologists claim that there have been 5 major ice ages. During ice ages the boundary of the ice sheet is hypothesised to spread and contract in cycles.