Each system of memory has its own forms of forgetting which are crucial to their operation; and memory systems interact in complex ways – sometimes in ways that the memory of one system depends on increasing the amnesia of another system; sometimes in forms of hypomnesis, in which the memory of one system is increased by placing memory outside of itself, in another system.
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Sixthly, memory also includes forgetting. Forgetting is not just privative, the lack of memory. Justlike death presupposes life, forgetting presupposes memory: only things which can remember canforget in any interesting sense. Forgetting is an active process,a capacity, a skill. And specific kindsof memory can
require forgetting, selection, erasure. When later I talk about the atmosphere having memory, about the air ‘affecting’ itself over time, when for example a hurricane forms, the capacity of the hurricane to have ‘working memory’ depends on parcels of air forgetting their origins before they were taken up into the hurricane. If the carbon recycled into our bodies remembered too much about its origins in the animals and vegetables we eat, or in the organisms whose matter they absorbed, this would affect our capacity to remember ourselves and what we know.
But at other times atmosphere does not seem to have any memory, and forecasting is impossible.Why does the air forget? This is partly due to the topography of the Earth below it: the very active system of tectonics which as we will see helps the rocks remember also produces features on the Earth’s which make the atmosphere forget . Other planets, such as the gas giants in the outer solar system, Jupiter, Saturn, Uranus and Neptune, have atmospheres with a much longer memory, with vortices that can last for hundreds of years or longer – Jupiter’s red spot, for example, or the hexagonal cloud over Saturn’s north pole. But on Earth the mountain ranges wipe the memory of the air as it moves over them, rather like the erase head on an old-fashioned tape recorder.But the forgetfulness of the air on the Earth is also partly due to the clouds. The one part of the atmosphere with a longer memory is the stratosphere, because here there are no clouds to make the atmosphere forget. So for example there is a water vapour memory in the stratosphere, whereby seasonal high levels of water vapour at 15km altitude in the tropics slowly rise, five years later reaching 40km altitude. In the troposphere by contrast, clouds wipe memory.But of course the atmosphere is not completely chaotic. The air could be said to rely on its ownmne motechnics, its own hypomnesia. Its largest features, the overturning Hadley, Ferrell and Polarcells that determine the direction of trade winds and westerlies, are themselves huge dissipative systems that therefore have their own internal self-organisation and memory. But their internal memory is very short; if the sun went out, for example, the cells would lose their form over a fewdays. However, the atmosphere is reminded every minute ‘who’ it is and how it behaves by the Earth’s placement 93 million miles from the Sun, its shape, tilt and spin(Paltridge 1979).