Geography Reference
In-Depth Information
current line of thinking and it is widely believed that the North
Atlantic is a very sensitive part of the Earth's climate system and
changes there can have global impacts. Once increased solar energy
starts to warm the planet as we move out of a glacial period then
positive feedbacks could result in the rapid warming indicated by
the sawtooth pattern in the evidence. This may be related to lower
albedo as ice retreats and the sudden switching on of the thermo-
haline circulation system.
As well as the long-term cycles during the Quaternary there
have also been shorter cycles some of which are just a few decades
long. Many of these fast-changing periods, however, seemed to
have occurred during glacials rather than interglacials. This was
probably related to interactions between ice sheet dynamics, ocean
circulation and biological productivity. So far we have only found
evidence for a few rapid changes in climate during interglacial
periods and it should be noted that we currently live in an intergla-
cial. Therefore, the rapid climate changes that seem to be occurring
at the present time are highly unusual. Any evidence for rapid
climate change during interglacials of the past (e.g. around 8,200
years ago) is therefore being actively explored at the moment as
these changes may provide a guide to possible climate behaviour in
the near future.
Recent climate change
Over the last century glaciers and ice caps have been receding,
snow cover has reduced and sea levels have been rising. The Earth
has warmed by about 0.7°C, particularly over the last 15 years
(Figure 2.3). The concentrations of greenhouse gases such as
carbon dioxide and methane in the atmosphere are greater than at
any time in roughly the last half million years and their concentra-
tions have risen sharply in recent decades. The current concentra-
tion of atmospheric greenhouse gases is equivalent to carbon
dioxide levels of around 430 parts per million compared with only
280 parts per million before the Industrial Revolution (this is cal-
culated by determining what each greenhouse gas equates to in
terms of its warming role and expressing it in units of carbon
dioxide equivalents). The rate of increase in temperature is faster
now than previously, even at the end of glacial periods. And we
