Causes Of Long Term Climate
Fluctuation
Outline
the causes of climate change during the Holocene, and discuss some of the main
effects this has had on environmental systems.
The
Holocene refers to the current interglacial period, following the end of the
last glacial around 10, 500 calendar years before present (Roberts 1998). While
this period is primarily marked by the end of glaciation and subsequent warm
conditions, climate during the Holocene has not been uniform. This essay first
gives an overview of climate changes during the Holocene, then outlines their
possible causes and main effects on environmental systems.
While
the climatic periods of the Holocene have been classified in various ways (such
as the Blytt-Sernander sequence based on Danish peat bogs), the broad global
trends in climate are now commonly divided into three phases: the Early
Holocene Amelioration, Climatic Optimum (or Hypsithermal), and Late Holocene
Deterioration (Bell & Walker 1992). The first phase reflects the fairly
rapid amelioration of the climate following the end of the Younger Dryas, the
final cold snap of the last ice age. This phase was marked, for example, by an
increase of up to 1 C per century in North Atlantic winter sea surface
temperatures, and climatic warming in France of 3-4 C every 500 years over
several millennia. By 9.5 kya (thousand calendar years ago) summer temperatures
in Europe were already as warm as the present day and in Eastern North America
were even warmer (Bell & Walker 1992). The period from around 9 kya to 4
kya marks the so-called Climatic Optimum (or Hypsithermal in North America), a
warm and relatively stable period when temperatures in much of the northern
temperate zone were even higher than today (Bell & Walker 1992). Finally,
the period from 4-5 kya to the present was marked first by fluctuating climatic
conditions and then marked deterioration from around 3 kya onwards, referred to
as the Late Holocene Deterioration (Bell & Walker 1992).
The
effects of these trends have not been uniform and have differed geographically
in timing and extent. Moreover climate has been marked by continuous
smaller-scale shifts on the multi-centennial timescale and below (Roberts
1998). During the Holocene there have been at least three rapid climatic
deteriorations followed by a return to previous conditions within a few
centuries, termed climatic crises (Roberts 1998: 122). The most recent and
best-recorded climatic fluctuations have been the Medieval Warm Period or
Little Optimum), a warming period between c. 700-1300 AD; and the Little Ice
Age, a cooler and wetter period from c. 1400-1850. These fluctuations were
relatively small in scale (<3 C) but big enough to have notable
environmental effects (especially in upland environments) and human impacts
(Bell & Walker 1992; Roberts 1998). Figure 1 shows July temperatures in
Norway and Svalbard illustrating both the broad climatic trends and short-term
fluctuations during the Holocene, with the Little Ice Age indicated near the
present:
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