As we bask in the heat of the late Los Angeles summer, it seems most appropriate to discuss those curious times in Earth’s geologic past when cold and ice dominated the landscape.
Icecaps and ice sheets have waxed and waned across the earth’s oceans and continents for millennia — billions of years, to be more precise. The cryosphere even left its mark on California. The iconic Yosemite Valley was carved via three major glaciations, from about 1 million years ago until 11,000 years ago, giving it its characteristic “U” shape. There were still relicts (though very small relicts) of these once-great rivers of ice until the 1980s and 1990s.
The most unusual epochs of glaciation in Earth’s past sound like the subject of a cheesy science-fiction-horror-drama: Snowball Earth episodes. Picture a plain white marble, perhaps with an ocean-blue equatorial stripe. This is precisely what scientists believe the view of Earth from space may have looked like at several points in the Earth’s 4.5 billion year history.
The first known “snowball Earth” episode occurred around 2.4 billion years ago, and geologic evidence suggests that ice extended to sea level at the tropics. Known now for their tropical island paradises and warm climates, the equatorial latitudes at one point hosted immense continent-sized ice sheets — another vacation plan foiled by snow and ice. The earth repeatedly descended into an icy snowball state 1.9 billion, 700 million and 500 million years before present.
Evidence for these episodes comes primarily from sedimentary and volcanic rocks that store information about the Earth’s very distant past. As glaciers or ice sheets erode the ground over which they flow, they incorporate chunks of rock at their base ranging from dust-sized particles to house-sized boulders. Icebergs at the end of the glacier then break off into the ocean, and as they melt, the boulders fall to the ocean floor and are preserved in the sedimentary layer as “dropstones.” When considered alongside volcanic rocks whose magnetic signatures suggest they formed at low latitudes, the sedimentary sequences are distinct indicators of snowball earth conditions.
Scientists are conflicted about how the earth descends into such a state, though. Many believe it to be a combination of a “faint young sun,” low atmospheric carbon dioxide concentrations and the cycle of feedbacks instigated by immense quantities of ice. Because the surface of the ice is white, it reflects most of the energy from the sun back to space rather than absorbing it, therefore cooling the earth. As more ice accumulates, the earth cools, allowing more ice to form, cooling it further, and so on. By the same token, a little melting would increase the earth’s energy, leading to more melting, less ice, and the eventual dissipation of the snowball.
Though global warming may take all climate headlines these days, there were times when Earth was quite a chilly place.
