The Cretaceous hothouse climate and the Pleistocene icehouse climate represent opposite extremes of the Earth's typical climate state of the past half billion years. Going back further in time, the Snowball Earth represents an ultra-extreme on the cold end; going further afield in space, the runaway greenhouse represents an ultra-extreme on the hot end, though one that evidently never occurred on Earth. The Earth has experienced many individual hothouse and icehouse episodes in the past half billion years. For times earlier than 70 million years ago one cannot rely on <518O records from sea-floor sediments to provide a cross-check on ice volume. Instead, one must look for evidence of glaciation or temperate polar climate preserved as glacial features or fossil animal and plant occurrences on land and in near-coastal environments. This record is far less complete and well-preserved. An accurate long term sea level record would make it possible to estimate ice volume in the distant past, but recovering and interpreting sea level has proved difficult. It is certainly possible to detect the existence of major glaciations, but good estimates of ice volume are not available for pre-Mesozoic glaciations. The periods between major glaciations - tentatively labeled as hothouse climates below - could well have undetected episodes of polar ice embedded within them. The known episodes of major and minor land glaciation are summarized in Fig. 1.10.
The Cretaceous hothouse conditions extended back throughout the entire Mesozoic, and well into the late Permian. There is evidence that some of these periods, notably the early Jurassic, may have been even warmer than the Eocene. To find another era of glaciation to rival that of the Pliocene and Pleistocene icehouse, one needs to go back to late Carboniferous and early Permian. The sixty million year period centered on the Permo-Carboniferous boundary 300 million years ago was a time of very extensive glaciation, reaching to lower latitudes even than the Pleistocene ice ages, though not attaining Snowball proportions. This period is a crucial one for the CO2 theory of Phanerozoic climate, since it is a time when there is quite strong evidence for low CO2. The earlier Phanerozoic exhibits comparatively minor glaciations at the end of the Devonian, in the mid-Silurian and for a brief period in the mid-Cambrian, but so far as the Phanerozoic goes, the
Permo-Carboniferous glaciation is the big one to explain.
The changing paleogeography, shown in Fig. 1.11, is likely to have influenced climate. In particular, it is bound to be easier for a glacier to accumulate on land if there is land at or near one or both of the poles. Certainly there is land at the South Pole during the Carboniferous glaciation and the current glaciation that began in the mid-Cenozoic. However, this is clearly not the whole story, as there was plenty of land at the South Pole already 400 million years ago, but the Carboniferious glaciation didn't set in until nearly a hundred million years later. Likewise, Antarctica was already near the South Pole during the Cretaceous, but Antarctic glaciation only took off in the mid-Cenozoic. Most likely, fluctuations in CO2 - probably itself affected by continental configuration - play a crucial role in the timing of glaciations.
A general theme in the evolution of paleogeography is the assembly and breakup of supercontinents. From 500 million years ago to 400 million years ago one can see the Southern supercontinent Gondwana near the South Pole, though there are a few leftover bits of land that are not part of Gondwana. By 300 million years ago, Gondwana has merged with these bits to form the global supercontinent Pangea, which then breaks up into the present continents over the course of the rest of the Phanerozoic. The interiors of supercontinents are isolated from the moderating effects of the oceans on climate, and so could be expected to experience harsh seasonal swings in temperature. Do we expect supercontinent interiors to be deserts or steaming, moist fern forests? This is another of the Big Questions.
Is there a clear dominance of hothouse or icehouse conditions over the past half billion years? The record of the past hundred million years certainly supports the notion that the largely ice-free hothouse is the preferred state of the Earth's climate, but going further back in time it is harder to say whether the apparent dominance of hothouse conditions is an artifact of poor preservation of the polar deposits where glaciers are most likely to have occurred. Some of the episodes we think of as hothouse climates could well have had significant amounts of ice.
In any event, the delineation of the circumstances which favor icehouse or hothouse climates, and the factors governing the transition between the two, constitutes one of the Big Questions of climate science. It seems likely that if the hothouse/icehouse transition of the past 70 million years can be understood, similar mechanisms could be applied to the rest of the Phanerozoic. Variations on the theme would include a greater range of different continental configurations - notably the breakup of supercontinents - as well as biological innovations such as the colonization of land and the evolution of deepwater carbonate shell-forming micro-organisms, both of which can affect the global carbon cycle.
During the Phanerozoic, life on Earth went through a number of mass extinctions rivaling or exceeding the end-Cretaceous event. The biggest mass extinction of all occurred at the end of the Permian, wiping out 96% of all marine species, 70% of all land vertebrates, and a large fraction of all land plants and invertebrates. It is the only mass extinction that included insects to any great extent. There is no clear evidence for a bolide impact at this time, though it remains possible that an impact occurred but failed to leave a trace in the fossil record. In any event, the cause of the end-Permian mass extinction ranks as one of the Big Questions.
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