Extreme glacial events, the so-called “snowball Earth” intervals, are by far the coldest periods in Earth history, with evidence of ice sheets existing even in tropical regions. By trying to identify the key climate forcings that could have led to such cold conditions, we seek to understand the full natural range of climate variability on Earth.
In this report, we simulate the effects of major changes to certain climate forcings in an effort to reproduce, with our model, the climatic conditions suggested by the geologic record. We find that while the combination of reduced solar luminosity, atmospheric CO2 levels, and ocean heat transports does cool the planet significantly, it is still not sufficient to reproduce the first-order characteristics of the Sturtian snowball Earth glaciation.
With the debate over global warming capturing the attention of many, it is not surprising that a great deal of current climate research is aimed at understanding the causes and effects of warmer climates. However, despite the likelihood that the 21st century will be an exceptionally warm century we are technically still in the midst of an ice age – the Pleistocene – that has persisted for nearly two million years. The Pleistocene ice age has been the focus of many climate studies that have helped us to better understand not only cold climates, but the Earth’s climate system in general. However, just as there have been periods in Earth history that were warmer even than what we expect from global warming, the Earth has experienced ice ages that were far colder than the Pleistocene.
Between 542 million and 1 billion years ago, during the Neoproterozoic Era, the Earth twice dipped into deep freezes that most geologists consider to have been among the coldest climates in the history of our planet. A variety of evidence suggests that Earth experienced two broad intervals of widespread glaciation: the Sturtian glaciation, which occurred around 750 Ma (Ma=million years ago), and the Marinoan glaciation, which occurred around 635 Ma (see Figure 1). One of the more remarkable features of these glaciations is the determination, based on the characteristics and distribution of certain sedimentary rocks (see, e.g., Figure 2) and other geological data, that continent-scale ice sheets existed at sea level within 10 degrees of the equator – equivalent to the modern-day latitude of Costa Rica. Because the extreme cold conditions are thought to have produced snow and ice cover over much of the Earth’s surface, the Sturtian and Marinoan glaciations have become known as “snowball Earth” intervals.
The possible occurrence of ice sheets in the tropics was a controversial topic until fairly recently, when data clearly supporting their existence came to light (Sohl et al., 1999). Still controversial is the question of whether the tropical oceans were also totally covered with sea ice during these extreme glacial intervals (e.g., Hoffman et al., 1998; Hoffman and Schrag, 2002). Proponents of total or near-total freeze-over of the tropical oceans – the “hard snowball” scenario – have argued that such occurrences had an enormous impact of the subsequent evolution of multicellular life on Earth, and in fact may have been the trigger for the “Cambrian explosion” of life forms that were the ancestors of much of modern multicellular life (Figure 3). Other researchers (e.g., Hyde et al., 2000; Chandler and Sohl, 2000) have argued for a very cold but less extreme climate, the “slushball” scenario, in which ice cover would have been extensive but broad areas of open ocean would have remained at mid to low latitudes. The broader ramifications of the glaciations’ effects have thus provoked a great deal of interest in understanding just what climatic conditions might have been like during the Neoproterozoic glacial intervals, and what climate forcings may have been reponsible for producing such extremely cold conditions.
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