Continents have moved and altered shape during geological times. For instance, Antarctica and Australia were joined until 55 million aBP. Following the breakup, Australia moved equatorward, becoming more arid, and Antarctica moved poleward, becoming colder. An ice cap first appeared on Antarctica about 36 million aBP, and the Antarctic ice sheet has maintained its current volume since 5 million aBP.
The Quartenary is characterised by oscillations of the extent of land ice in the northern hemisphere. Glacial periods were interrupted by brief interglacials about every 100-200 thousand years. The Holocene corresponds to the most recent interglacial. Conditions have been colder than the 20th century during at least 90% of the Quaternary. The latest 160,000 years of the Quaternary consisted of the last full cycle of glacial advance and retreat. The warmest time of the Pleistocene probably occurred at about 120,000 aBP, as shown by oxygen-isotope ratios in sediments of the Pacific Ocean north of Australia, and sea-level records from raised coral reefs of Papua New Guinea
The atmosphere is nowadays thought to have been created at the time the Earth was being formed, about 4.5 billion years ago (4.5 GaBP). Asteroids struck the growing planet and caused degassing, chiefly steam, but also hydrogen, nitrogen, carbon dioxide and carbon monoxide. It is now believed that much of this primordial atmosphere was removed quite early after the formation of the Earth by the impact of a body about the size of Mars, which incidentally created the Moon. More outgassing resulted from continued impacts, and at the same time the Earth started to cool. Water vapour condensed and the Earth became covered by oceans. Gravity was insufficient to hold the lightweight outgassed hydrogen (H2), so that escaped to outer space. Most of the carbon dioxide combined with calcium and other minerals to form carbonate rock, but there was enough left for a ?greenhouse effect?, preventing the oceans from freezing. CO2 concentrations were perhaps 300 times what they are now. The consequence was that the Earth's atmosphere was warmer than today, and no polar ice caps existed prior to about 2.5 GaBP. The early Earth?s atmosphere was not very different from that of Mars or Venus, except that only on Earth water existed in a liquid state.
The upward transport of boundary-layer air within a deep convective cloud also lifts pollutants such as nitric and sulphuric-acid gases from the PBL into clouds, where they acidify the droplets or ice crystals, leading to acid rain some distance downwind (1).
In addition, deep convection mixes another pollutant, ozone, into the troposphere. Boundary-layer ozone is formed photochemically in areas of intense industrial, urban or biomass burning (Note 14.G). These processes generate the gases required to form ozone, i.e. the 'precursors'. These are nitrogen monoxide, hydrocarbons, and to a lesser extent carbon monoxide. Ozone concentrations in the upper troposphere have been found to increase following the outbreak of thunderstorms over areas rich in lower-tropospheric ozone in Brazil, the Atlantic and South Africa. Some ozone is also created by the ultraviolet radiation generated by lightning flashes. So it is difficult to quantify the effect of thunderstorms on lower-tropospheric ozone.
The upward transport of boundary-layer air within a deep convective cloud also lifts pollutants such as nitric and sulphuric-acid gases from the PBL into clouds, where they acidify the droplets or ice crystals, leading to acid rain some distance downwind (1).
In addition, deep convection mixes another pollutant, ozone, into the troposphere. Boundary-layer ozone is formed photochemically in areas of intense industrial, urban or biomass burning (Note 14.G). These processes generate the gases required to form ozone, i.e. the 'precursors'. These are nitrogen monoxide, hydrocarbons, and to a lesser extent carbon monoxide. Ozone concentrations in the upper troposphere have been found to increase following the outbreak of thunderstorms over areas rich in lower-tropospheric ozone in Brazil, the Atlantic and South Africa. Some ozone is also created by the ultraviolet radiation generated by lightning flashes. So it is difficult to quantify the effect of thunderstorms on lower-tropospheric ozone.
