|Permo-Triassic climate change|
Climates through the Permian were hot, and temperatures were rising. All continents were joined together as a single supercontinent, Pangaea, and there was apparently no ice at either pole. The physical environmental changes that followed from the massive Siberian Traps eruptions appear to have caused further dramatic global warming. Reconstructing ancient climates is based on several lines of evidence, and the details are still unsettled. Much of the text here is from Benton and Newell (2014).
Sedimentological evidence for ancient climatesSedimentary rocks are the key to accurate reconstruction of palaeoclimates. Key evidence comes from coals, evaporites, aeolian dunes, glacial deposits and fossil plants, and full details of methods are here.
Palaeogeography and climate in the Permo-TriassicPangaea (see below) was surrounded by the Panthalassa Ocean and a deep oceanic gulf, Tethys, which was latitudinally confined to the tropical-subtropical belt, and contained several Asian landmasses (Roscher et al. 2011). The impact on climate of this peculiar plate configuration has been investigated by modelling (Kutzbach and Gallimore 1989; Kiehl and Shields 2005). The concentration of exposed land at low and mid-latitudes, and the presence of a warm sea-way would have maximised summer heating in the circum-Tethyan part of the continent and created a strong monsoonal regime. Furthermore, extreme continentality, with hot summers and relatively cold winters, is expected. Polar regions were ice-free, and their latest Permian deposits contain coals, plants and soils typical of cool temperate latitudes. The polar regions were temperate, not only because of elevated CO2 levels (Royer et al. 2004), but also from the unrestricted ocean transport of heat toward the poles and a reduced albedo arising from the lack of permanent land ice (Kiehl and Shields, 2005).
Permian-Triassic Pangaean palaeogeography, with the location of the main PTr terrestrial basins and boundary sections indicated, (1) West Siberian/ Kuznetsk Basin, (2) Precaspian/Urals foreland basin/Russian Platform, (3) Central European Basin, (4) Iberian Basin, (5) South China, (6) Paraná Basin, eastern South America (7) Karoo, South Africa, (8) Satpura/ Raniganj basins, central India, (9) Bowen Basin, western Australia, and (10) Victoria Land and the central Transantarctic Mountains, Antarctica. From Roscher et al. (2011). Late Permian climate zones are generalised from Schneebeli-Hermann (2012).
The harsh hot-house climatic conditions that characterised the Late Permian were probably maintained (Preto et al. 2010) or exacerbated during the Early Triassic with, for example, the northward and southward expansion of low-latitude arid belts into the vast, formerly humid basins of European Russia and the Karoo (Chumakov and Zharkov 2003; Royer et al. 2004).
Numerical climate modelsThe peculiar plate configuration of Pangaea around the PTB (see above) attracted some pioneering work on energy balance models and atmospheric general circulation models (e.g. Kutzbach and Gallimore 1989). These models demonstrated the likelihood of extreme seasonal temperature variation within the large continental interiors of Pangaea and the probability of arid conditions on the western side of Gondwana and Laurussia. The early models were also successful in reproducing the expected high precipitation and strong monsoonal circulation of the Tethys coast. However, there appeared to be discrepancies in polar and high-latitude regions where the models generally predicted much colder conditions than suggested by the presence of coal forests in the Antarctic (Retallack et al. 2007).
More recent models (Kiehl and Shields 2005), which include coupling of atmospheric and oceanic circulation, have been more successful at reproducing cool temperate polar regions (see image below) by allowing a flow of warm water into high latitudes, which, because of the configuration of Pangaea, was relatively unrestricted in high latitudes. Models are now being turned toward understanding climate change events across the PTME. The climate models of Roscher et al. (2011) show that an episode of global cooling around the PTB was more effective at producing the observed changes in climate belts than global warming.
This image shows annual mean surface temperatures in degrees Celsius at the time of the Permian extinction. It is based on a computer simulation generated by the Community Climate System Model at NCAR. Note how the very hot tropical belt extends much further north and south than today, but there are cool polar regions in this model. (Illustration courtesy Jeff Kiehl, NCAR.)
Coupled climate-carbon cycle models by Winguth and Winguth (2012) have been used to explore oceanic anoxia. These authors found it hard to generate complete anoxia, but that the oxygen minimum zone expanded considerably, while the deep Panthalassa Ocean remained ventilated. Further, these models suggested that upwelling of toxic water was probably not a global phenomenon, and so probably not a major player in the PTME. The effects of increased weathering and enhanced nutrient input into the oceans was felt only in the Early Triassic, according to the models, and so probably contributed to the delayed recovery of life.