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Origin and Evolution of Earth: Research Questions for a Changing Planet
actual cause may have been the different locations of the continents. The continents were all situated at low to midlatitudes where temperatures are warmest, allowing silicate weathering to proceed rapidly and draw down CO2 levels, even as the global surface temperature dropped and polar ice accumulated (Marshall et al., 1988; Donnadieu et al., 2004). Alternatively, CH4 concentrations may have been high during the mid-Proterozoic and then dropped as O2 levels increased (for a second time; see Question 8) near the end of this time (Pavlov et al., 2003). In either case, as ice cover increased, the albedo and thus cooling would have increased until the planet plunged into an extreme “icehouse” condition. Surface temperatures calculated for this hard snowball Earth are about −20°C at the equator and about −40°C averaged ver the globe (Pollard and Kasting, 2004).
The existence of a snowball Earth must be inferred from geological evidence. Translation of such evidence into a hypothesis about Earth’s climate and evaluation of the hypothesis using modern climate models and concepts provide an interesting example of the scientific challenges inherent in reconstructing Earth’s past conditions. The rock assemblage now considered indicative of the snowball period was initially difficult to decipher. There are marine glacial deposits that formed near the equator, suggesting glaciation in the tropics and hence exceptionally cold conditions; banded iron formations, suggesting anoxic conditions in the oceans; and stratigraphically above and below the glacial deposits there are limestones, which suggest warm conditions (Figure 3.5; Hoffman and Schrag, 2000). In some cases there are nonmarine deposits, which suggest that sea level dropped, and there is carbon isotopic evidence suggesting that photosynthesis all but stopped.
The warm conditions following the snowball Earth period may have arisen because volcanism would have continued through the snowball period, contributing CO2 to the atmosphere that could not be removed by rock weathering because the rocks were covered with ice. Once extreme levels of CO2 were reached (~400 times the modern preindustrial level; Caldeira and Kasting, 1992), the greenhouse effect would have been strong enough to overcome the high albedo, melt the ice, and swing Earth to exceptionally warm conditions (~40°C global average in this model) before weathering processes could catch up and remove the atmospheric CO2. The temporarily high atmospheric CO2 would probably have made the rain especially acidic, enhancing chemical weathering and causing a large amount of calcium to be delivered to the oceans by rivers; this may explain the unusual, rapidly deposited limestone layers that cap most Neoproterozoic glacial deposits (Hoffman and Schrag, 2000). A recent three-dimensional climate simulation by Pierrehumbert (2004) has cast doubt on this scenario, however. The new calculations indicate that even 0.2 bars of CO2 (700 times the preindustrial level) could not have deglaciated a hard snowball Earth. Given the many uncertainties involved in applying climate models to the Proterozoic Earth, it is not yet clear whether the hypotheses or the models are incorrect.
Indeed, there are many arguments against the snowball Earth hypothesis. Even supporters of this theory disagree about significant issues. One is the survival of photosynthetic algae through the plunge in temperatures. How was this possible if the ice was a kilometer thick everywhere as some models have it? Could photosynthetic life have survived in local volcanic hot spots, like modern Iceland? Or did other refuges exist? One variant of the snowball hypothesis, the so-called thin-ice model (McKay, 2000), suggests that the ice in the tropics was only about 1 to 2 m thick, allowing enough penetration of sunlight for photosynthesis. In addition, there would likely be leads and lanes of open water in very thin ice. This model allows Earth to deglaciate at a much lower CO2 level, only about 30 times the present level (Pollard and Kasting, 2005). However, there are questions as to whether such a solution can be stable, given that sea ice can flow from the poles to the equator, where it would melt (Goodman and Pierrehumbert, 2003). Clearly, much more work is required if the snowball Earth hypothesis is to become an established chapter in Earth’s climate history. Nevertheless, even the most moderate of interpretations of the Neoproterozoic evidence for glaciation suggest that it was the coldest period in the past 2 billion years. By comparison, the glaciations that have affected Earth in more recent times have had comparatively little effect on the global carbon cycle.
What Triggers Abrupt Climate Change?
Abrupt climate events are unusual, but they provide insights on the rates at which the climate system is