Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 160

similar to that in 1950. They further found that the sea ice has cyclic behavior.
A decrease in ice mass is in agreement with observations of mountain glaciers
in the Arctic, the majority of which showed shrinkage. Further, the Greenland
ice sheet, the greatest ice sheet after Antarctica, has lost mass during the last
five decades, during which time systematic observations have been carried
out, and has contributed to the observed sea level increase.
5.8.4 Climate change
The climate of the Arctic has undergone large changes in the past, at times
when human activity could not have influenced the climate. Actual measure-
ments in the Arctic go back some 200 years, but proxy data such as tree-rings
go back for a millennium, and ice core data from Greenland to 100 000 years.
The European Arctic was warm during the Medieval Warm Period (MWP),
the tenth century. It was at this time that the Vikings settled Greenland (see
Section 8.2 ), and from the name one can deduce that vegetation was abun-
dant. Around AD 1300 the Little Ice Age (LIA), lasting some five hundred
years, commenced; the Viking colonies died out in Greenland by around
AD 1500, two hundred years into this cold spell. Going further back, the
ice core analyses of Greenland give detailed information. One astounding
result was the abrupt changes in temperature that occurred, also called
Dansgaard-Oeschger cycles, lasting from centuries to millennia. The last
ice age ended some 13 000 years back, with one 1300-year reversal to cold
temperatures, the Younger Dryas. While long-term temperature cycles are
related to the Milankovitch theory (Section 5.2 ), a full understanding of
shorter cycles and abrupt changes is still missing.
Climate change occurred in the Arctic during the past century; however,
the processes have been anything but linear, as might have been expected
from the fairly steady increase of CO 2 and other greenhouse gases in the
atmosphere. This shows clearly that climate change, along with anthropo-
genic influences, is also affected by natural cycles. The Arctic climate system
appears very complex and is well connected to events at lower latitudes, a
result not surprising considering that the polar regions present the heat sinks
of the global energy budget. The behavior of different climatic parameters
appears rather cyclic, with different time periods. Furthermore, temporal
changes vary widely spatially. Nevertheless, the Arctic is today warmer
than it was 100 years ago, and this warming is more pronounced than that
for lower latitudes. Further, there is less and thinner sea ice, permafrost
temperatures have increased, most of the mountain glaciers are in recession,
and the Greenland Ice sheet has lost mass. A better understanding of these
processes will be a major undertaking for the next decades, combining
ground-based data with remotely sensed observations and models.