Discuss the Possible Mechanisms for Short-term Climate Change within the Quaternary.

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Christopher Richard Wilson. Van Mildert College.  Level 2 Tutorial Essay.  

Discuss the Possible Mechanisms for Short-term Climate Change within the Quaternary

Introduction

The Quaternary period is distinctive for its cyclical nature of climate change in relation to previous geological periods.  Long term changes govern the glacial-interglacial transitions, but these also feature shorter term changes involving a variety of different causal mechanisms, the roles of different mechanisms being a source of detailed debate and intensive research.  Lowe and Walker (1997, p. 361) make the distinction between ‘external’ and ‘internal’ mechanisms, where external means change driven (or forced) by external variables, and internal refers to processes that operate within the terrestrial-ocean-atmosphere system.  The mechanisms to be considered are oceanographic changes, ice sheet/glacier fluctuations, volcanic forcing, solar forcing and atmospheric gas content which will then lead to a discussion of the current debate over the relative roles of the greenhouse effect and solar forcing.  Geomagnetism and geodynamic factors will also be discussed in brief.

Oceanographic Changes

The climate of western Europe contrasts markedly with the much colder climate of eastern Canada, at a similar latitude.  This is almost entirely due to Gulf Stream warming of the UK.  Cores from the North Atlantic Ocean show repeated excursions of polar waters with meltwater influxes from the Laurentide and Fennoscandanian ice sheets over the last 110 ka, which has been correlated with evidence from Greenland ice cores. These have major consequences (Lowe and Walker, 1997) for Gulf Stream operation.  The North Atlantic Polar front also changed position significantly during the last glacial-interglacial transition partly in response to meltwater movements, causing Norwegian Sea surface temperatures to fall by up to 5ºC in 40 years, leading to large and rapid atmospheric changes.  

A major issue in short term climate change is the impact of changes in the Thermohaline Circulation (THC, figure 1).  Crucial here is production of North Atlantic Deep Water (NADW), as saline cold water sinks at high latitudes and is ‘ventilated’, (Lowe and Walker, 1997 p. 362) releasing heat to the atmosphere.  Wally Broecker et. al. suggested the idea that this process could have a ‘turn on/off’ mechanism (Lowe and Walker, 1997 p. 363) that would describe sudden climatic changes over the last glacial-interglacial cycle.  It has also been suggested that it could completely shut down.  NADW formation changes are recorded in carbon isotope ratios or cadmium variations in benthic Foraminifera, dependent on temperature.  At the Last Glacial Maximum (LGM) NADW formation decreased significantly, then decreased further or ceased completely before the climatic warming at 12.8-12.5 ka BP.

 (Figure 1 – Thermohaline Circulation, from the US National Oceanic and Atmospheric Administration http://www.aoml.noaa.gov/graphics/conveyor.gif)

   

Elliott et. al. (2002) discuss abrupt, high-frequency changes in the climate system, found in ocean records and ice cores at a periodicity of 1000-3000 years.  A crucial feature is the occurrence of Dansgaard-Oeschger (DO) temperature oscillations, millennial-scale cycles ending with a shift from cold to warm after a long term cooling, associated with decreased sub-polar North Atlantic Ocean temperatures and increased iceberg discharge into the ocean.  Ice layers rich in Ice-Rafted Debris (IRD), some of which are known as Heinrich Events (HEs), formed as icebergs melted and released rock fragments into the ocean.  HEs are associated with a drastic decrease or possible shutdown of NADW formation.  Ice sheet instabilities are involved in the operation of the THC, but it is said the THC can also influence these as well as being caused by them, the ‘bipolar see-saw’ (Broecker 1997, see Elliott et. al. 2002 p.1153).      

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Elliott et. al. (2002) examine two high resolution records of the benthic Foraminifera δ13C C. wuellerstorfi, which is depleted in cold times associated with HEs.  These records give insights on the role of NADW ventilation changes associated with DO cycles.  The results show drastic decreases of δ13C associated with the HEs reflecting a northward migration of 13C-depleted southern waters to the Irminger Basin (62ºN), and deposition rates alter δ13C values.  Changes of δ13C associated with the cold stadials observed in Greenland ice cores, and of NADW formation, are of lesser amplitude in inter-Heinrich stadials than those during an event.  However ...

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