Query: As a result, the polar ice caps were quite likely more extensive than they had been since at least the 1920s.

Relevant passages:
Polar ice cap <=SEP=> A polar ice cap or polar cap is a high-latitude region of a planet , dwarf planet , or natural satellite that is covered in ice .   There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap , nor any geological requirement for it to be over land ; only that it must be a body of solid phase matter in the polar region . This causes the term `` polar ice cap '' to be something of a misnomer , as the term ice cap itself is applied more narrowly to bodies that are over land , and cover less than 50,000 km ² : larger bodies are referred to as ice sheets .   The composition of the ice will vary . For example , Earth 's polar caps are mainly water ice , whereas Mars 's polar ice caps are a mixture of solid carbon dioxide and water ice .   Polar ice caps form because high-latitude regions receive less energy in the form of solar radiation from the Sun than equatorial regions , resulting in lower surface temperatures .   Earth 's polar caps have changed dramatically over the last 12,000 years . Seasonal variations of the ice caps takes place due to varied solar energy absorption as the planet or moon revolves around the Sun . Additionally , in geologic time scales , the ice caps may grow or shrink due to climate variation .

Ice age <=SEP=> 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 . Within a long-term ice age , individual pulses of cold climate are termed `` glacial periods '' ( or alternatively `` glacials '' or `` glaciations '' or colloquially as `` ice age '' ) , and intermittent warm periods are called `` interglacials '' . In the terminology of glaciology , ice age implies the presence of extensive ice sheets in both northern and southern hemispheres . By this definition , we are in an interglacial period -- the Holocene -- of the ice age . The ice age began 2.6 million years ago at the start of the Pleistocene epoch , because the Greenland , Arctic , and Antarctic ice sheets still exist .

Cambrian <=SEP=> The Cambrian Period ( -LSB- pronˈkæmbriən -RSB- or -LSB- ˈkeɪmbriən -RSB- ) was the first geological period of the Paleozoic Era , of the Phanerozoic Eon . The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago ( mya ) to the beginning of the Ordovician Period mya . Its subdivisions , and its base , are somewhat in flux . The period was established ( as `` Cambrian series '' ) by Adam Sedgwick , who named it after Cambria , the Latinised form of Cymru , the Welsh name for Wales , where Britain 's Cambrian rocks are best exposed . The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits , sites of exceptional preservation where `` soft '' parts of organisms are preserved as well as their more resistant shells . As a result , our understanding of the Cambrian biology surpasses that of some later periods .   The Cambrian marked a profound change in life on Earth ; prior to the Cambrian , the majority of living organisms on the whole were small , unicellular and simple ; the Precambrian Charnia being exceptional . Complex , multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian , but it was not until this period that mineralized -- hence readily fossilized -- organisms became common . The rapid diversification of lifeforms in the Cambrian , known as the Cambrian explosion , produced the first representatives of all modern animal phyla . Phylogenetic analysis has supported the view that during the Cambrian radiation , metazoa ( animals ) evolved monophyletically from a single common ancestor : flagellated colonial protists similar to modern choanoflagellates .   Although diverse life forms prospered in the oceans , the land was comparatively barren -- with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm . Most of the continents were probably dry and rocky due to a lack of vegetation . Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia . The seas were relatively warm , and polar ice was absent for much of the period .   The United States Federal Geographic Data Committee uses a `` barred capital C '' character similar to the capital letter Ukrainian Ye to represent the Cambrian Period .  The proper Unicode character is .