A pale white dot

“There’s nothing new under the sun” goes a famous saying and these words are very apt when trying to understand Earth’s climate trends. Thanks to numerous discoveries made about Earth’s ancient past, we now know that our climate has never been static. According to geological and palaeontological records, climate change has affected the Earth throughout geologic time.

In this context, this is the second of a series of articles about climate change over geological time. The first (A warming medieval climate supports a revolution in agriculture by Steven Wade Veatch and Cheryl Bibeau) appeared in Issue 48.

To understand climate change today, researchers study past climates and events that affect climates, such as volcanic activity, solar radiation, sunspot activity, astronomical changes and other factors that influence climate. Once we understand the dominoes that have fallen during the past climate change events, we can understand and predict – to some degree – the kind of patterns that may follow current trends. To do this, scientists piece together clues from past climates provided by rock formations. Scientists likewise examine fossil records that yield climate signals from the past. These fossils range from prehistoric pollen to dinosaurs. Putting both geological and fossil records together reconstructs ancient climates and environments. More recent climate change is studied through climate records held in polar ice caps and ice sheets, ice cores, glaciers, isotopes of elements (like oxygen, carbon and sulphurfur), soil sediments and tree rings.
When we think of the term “ice age”, the picture that immediately comes to mind is early Neanderthals or Homo sapiens wrapped in animal fur, hiking endlessly through snow and ice-covered plains, striking fire, hunting mammoths and surviving in nomadic camps. This image stems from the most recent ice age (Pleistocene Epoch), but evidence reveals more severe ice ages before the last one. Scientists know of at least five major glaciation events (see table 1). And it is speculated that some of the ice ages covered the whole Earth in snow and ice.

Table 1: Five Major Continental Glaciations. There have been five episodes of extensive continental glaciation through geologic time. The Cryogenian Glaciation lasted the longest, producing a “Snowball Earth” (Levin, 2013).
Glaciation Time Period Huronian Glaciation (Paleoproterozoic Era) 2.4 – 2.1 Ga
Cryogenian Glaciation (Neoproterozoic Era) 850 – 635 Ma
Andean-Saharan Glaciation (Ordovician-Silurian Period) 460 – 430 Ma
Karoo Glaciation (Carboniferous-Permian Period) 360 – 260 Ma
Pleistocene Glaciation (Pleistocene Epoch) 2.6 Ma to the present.

Fig. 1. A photomicrograph of Cyanobacteria, Tolypothrix sp. Cyanobacteria produce oxygen as a by-product of photosynthesis and it is thought this process converted Earth’s early, oxygenpoor, reducing atmosphere, into an oxidizing one, causing two major events: (1) the “ Great Oxygenation Event” and (2) the so-called rusting of the Earth. Both events dramatically changed the nature of life forms on Earth and almost led to the extinction of anaerobic organisms. (Image by Matthew Parker, used by permission under Community Commons Licence 3.0.)

Broadly speaking, a number of scientists believe the Earth’s climate, throughout geological time, can be characterised by three climate conditions. First, is that of “Earth as a Greenhouse”, when warm temperatures extend to the poles, eliminating the polar icecaps and all other ice sheets. In some parts of the planet, the climate was like hell in a box. Secondly, is that of “Earth as an Icehouse”, which includes some permanent ice, whose extent varies as glaciers periodically advance and retreat. And lastly, is what is termed “Snowball Earth”, in which the planet’s entire surface is frozen for up to hundreds of millions of years (Walker, 2003). There is credible speculation that there is a fourth state – “Slush House Earth” – in which there is an ice-free zone along the equator (Cowen, 2013). Today’s climate, marked by polar ice caps, is characterised by the second condition, an “Icehouse”. Since primordial times, it has been speculated that the Earth has been cycling between these phases.

The Earth froze completely in defiance to the warmth of the sun between 2.45 and 2.22bya, resulting in Earth’s first Ice Age, known as the Huronian Glaciation (named after Lake Huron in Ontario, Canada). This deep freeze may not have happened just once, but perhaps several times, during the Huronian Glaciation (Levin, 2013).

The cause of this first Snowball Earth event is not known. However, several theories have been proposed, including a decrease in solar output, the Earth passing through so-called space clouds or an extreme cooling caused by a reduction in greenhouse gases (Oceans of Ice: The Snowball Earth Theory of Global Glaciation; see further reading). Some scientists believe a combination of these events could be a reason the Earth became frozen in ice. It seems likely that a sharp drop in carbon dioxide – a greenhouse gas – caused temperatures to plummet. An unimaginably thick, white ice sheet crept down from the poles. Snow, whipped by winds, danced on the crenelated surface of the ice, while the bottom of the ice sheet plucked and ground the rock surface beneath as it crept forward.

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