206 ATMOSPHERE EVOLUTION of oxygen through their exoskeletons: to be big they need to live in conditions of high atmospheric oxygen, so that sufficient oxygen passively diffuses into their blood to power muscles for flight Reduced greenhouse forcing and glaciation at the end of the Palaeozoic with the assembly of Pangaea reduced organic-matter burial, and there was a slow rise in carbon dioxide levels to around six times PAL in the Permian and Triassic Levels of carbon dioxide gradually decreased, and stabilized near present-day levels in the Mesozoic Oxygen tends to track carbon dioxide inversely; geological evidence and palaeoclimate models suggest a maximum of near 35% oxygen in the atmosphere at the beginning of the Permian During the Permian, the oceans were highly stratified, with carbonate-rich water at depth that was depleted in oxygen This system was unstable: ocean hypoxia could occur if ocean circulation intensified enough to mix deep-water carbon dioxide and hydrogen sulphide into surface waters A protracted (20 Ma) whole-ocean hypoxia event is considered to be a major mechanism in the Permian–Triassic extinction event, which wiped out 90%–95% of all marine species (see Palaeozoic: End Permian Extinctions) Figure Phanerozoic carbon dioxide and oxygen concentra tions Carbon Dioxide and Climate Changes High-resolution information about changes in atmospheric chemistry over the past 160 000 years can be obtained by studying the record of trapped gases in ice cores from the Greenland and Antarctic ice-caps Furthermore, oxygen isotopic values from marine sediments, marine planktonic and benthonic fossils, cave deposits, and other sources can be used to estimate marine palaeotemperatures Direct measurements of carbon dioxide and methane concentrations in ice cores permit assessment of past atmospheric levels of these gases, providing factors to incorporate into models of past air temperatures and sea-levels Data from deep ice cores taken in polar regions, coupled with complex palaeoclimate models (Figure 9), show large fluctuations in atmospheric carbon dioxide, oxygen and methane levels, leading to long-term temperature changes of the order of Ỉ6 C or more There is a strong correlation between levels of atmospheric greenhouse gases and palaeotemperature The periodicities in these data provide clear evidence of the role of Milankovitch forcing by changes in the Earth’s orbital parameters The two strongest Milankovitch cycles observed correspond to the 26 000 year precession of the equinoxes and the 100 000 year period of rotation of the Earth’s orbital axis (see Earth: Orbital Variation (Including Milankovitch Cycles)) Changes in greenhouse-gas concentrations appear to follow rather than guide long-term climate, suggesting that Milankovitch Figure 10 Changes in the concentration of atmospheric carbon dioxide over the last 60 years as measured at Mauna Loa, Hawaii Data provided by D Keeling and T Whorf cycles are the prime mechanism bringing the Earth into a greenhouse or icehouse condition Greenhouse gases provide positive feedback at the beginning of temperature changes by boosting insolation Anthropogenic emissions of greenhouse gases are not governed by Milankovitch cycles and represent a separate and increasingly important climate-forcing mechanism Figure shows that carbon dioxide concentrations changed by almost 100 ppm(vol.) towards the end of the last glaciation Modern levels of carbon dioxide are near 370 ppm(vol.) and rising Almost all of this change has occurred since the Industrial Revolution, and high-resolution monitoring at the Mauna Loa observatory shows clear diurnal and annual cycles in carbon dioxide levels (Figure 10), with a mean annual increase of 1.16 ppm(vol.) year This is over one hundred times the rate of increase of carbon dioxide levels inferred from all available