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Home page Solar and galactic influences Climate change Sea level change Continental drift Volcanic activity Asteroid impact Pathogens Other species
About 70% of our planet is covered by water. Most people know that there are high and low tides as well as being aware that heavy rains, storms and tsunamis can cause extensive flooding. With the possible exception of tsunamis these are not likely to cause extinctions, but sea level changes which result in flooding or desertification can have serious consequences for the food web.
 Although the amount of water may have remained relatively constant throughout geological history, the level of sea water relative to the land has changed substantially over time. This has been due to three main causes: Tectonic plate movement, water temperature and glaciation. Tectonic plate movements are dealt with more fully under Continental Drift, below, but have been a source of sea level rises and fallings which have been associated with many extinctions. Water expands when heated and when water temperature rises so does sea level and when water temperature falls sea level goes down. It has been calculated that a rise of 1ºC in surface sea water temperature would cause a rise of 40.3mm in sea level. Water goes through a cycle:  The amount of water available to determine sea level depends on glaciation and the retention of water in the form of ice and snow in the polar regions and on the tops of mountains. To some degree this is seasonal, but over the history of the Earth there have been many periods, which we have called 'Ice Ages', when average sea level was much lower because so much water was retained in great glaciers.  There have been four ice age periods in the last 540 million years. The diagram above shows the long-term evolution of oxygen isotope ratios19 during the Phanerozoic eon as measured in fossils. Such ratios reflect both the local temperature at the site of deposition and global changes associated with the extent of continental glaciation. As such, relative changes in oxygen isotope ratios can be interpreted as rough changes in climate. Quantitative conversion between this data and direct temperature changes is a complicated process subject to many systematic uncertainties, however it is estimated that each 1 part per thousand change in d18O represents roughly a 1.5-2°C change in tropical sea surface temperatures. Recent research has suggested that these 'Ice Ages' might be caused by the movement of the Earth, as part of our solar system, through the spiral arms of our galaxy20. It is worth noting that we are currently in an 'Ice Age' which has been going on for the last 70 million years. This concept of 'Ice Age' requires recognition that the temperature, glacial thickness and sea level are anything but stable and contain considerable variation as seen on this graph of the last 2.5 million years
These fairly dramatic illustrations clearly show a sequence of 'ice ages' separated by interglacials. However, the length of both the 'ice ages' and interglacials is varied. Although there seems to be a rough cycle of 100k between the glacial maximums (at the bottom of the last graph), they are sufficiently out of synchronisation for several determining forces to be at work. The same applies to the interglacials which show little stability despite all seemimgly having one very high point - itself only an average spanning perhaps a few thousand years. Our current interglacial spanning 10,000 years does seem particularly marked by the relatively sudden high rate of CO2 concentration. In the discussion on our Holocene epoch we can look at the more recent changes in detail.
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