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.
About 700 million years ago, during the Cryogenian glaciation, runaway glaciers made Earth look like a snowball. The last ice age led to the rise of the woolly mammoth and the vast expansion of glaciers. But it's just one of many that have chilled Earth throughout the planet's 4.5-billion-year history. So, how often do ice ages happen, and when is the next freeze expected to begin? The answer to the first question depends on whether you're talking about big ice ages or the little ice ages that happen within those larger periods. Earth has undergone five big ice ages, some of which lasted for hundreds of millions of years. In fact, Earth is in a big ice age now, which explains why the planet has polar ice caps.