Geoscience Reference
In-Depth Information
Figure 13.5
Main trends in global climate during the past million years or so. (A) Northern hemisphere, average land-air temperatures.
(B) Eastern Europe, winter temperatures. (C) Northern hemisphere, average land-air temperatures. (D) Northern hemisphere,
average air temperatures based partly on sea-surface temperatures. (E) Global average temperatures derived from deep-sea cores.
Source
: Data taken from
Understanding Climatic Change: A Program for Action
(1975), National Academy Press, Washington, DC. Courtesy of
the US National Academy of Sciences, Washington, DC.
2 Late glacial and post-glacial conditions
Information on past climatic conditions is obtained
from many proxy records. For example, the advance
and retreat of glaciers represents a response to winter
snowfall and summer melt. The history of vegetation,
which indicates temperature and moisture conditions,
can be traced from pollen types preserved in lake
sediments and peatbogs. Former lake shorelines indicate
changes in moisture balance. Estimates of seasonal
climatic elements can be made from studies of annual
snow/ice layers in cores taken from polar ice sheets,
where no melt occurs. These layers also record past
volcanic events through the inclusion of micro-particles
and chemical compounds in the ice, while gas bubbles
trapped in the ice provide data on the past composition
of the atmosphere. In forest biomes where trees form an
annual growth layer, the ring width can be interpreted
through dendroclimatological studies in terms of mois-
ture availability (in semi-arid regions) and summer
warmth (near the polar and alpine tree lines). Ice cores
document the past 150,000 years in Greenland and
450,000 years in Antarctica. Pollen sequences and lake
level records usually span the past 10,000 to 20,000
years, while tree rings rarely extend more than 1000
years. For the last millennium, historical documents
often record crop harvests or extreme weather events
(droughts, floods, river and lake freezing, etc.).
The 100-ka glacial cycles of the last 0.8 Ma show
characteristic abrupt terminations. The last glaciation,
for example, ended with abrupt warming about 14,700
to 13,000
BP
, interrupted by a short cold regression
(the Younger Dryas, 12,900 to 11,600
BP
), followed by
a renewed sharp warming trend (see Figure 13.4). These
shifts clearly involve some non-linear processes and
threshold effects, but many details remain unresolved.
Early Holocene warmth around 10,000
BP
is attrib-
uted to July solar radiation being 30 to 40W m
-2
greater
than now in northern mid-latitudes, due to the combined
orbital effects. Following the final retreat of the con-
tinental ice sheets from Europe and North America
between 10,000 and 7000 years ago, the climate rapidly
ameliorated in middle and higher latitudes. In the sub-
tropics this interval was also generally wetter, with high
lake levels in Africa and the Middle East. A
thermal
maximum
was reached in the mid-latitudes about
5000 years ago, when summer temperatures were 1 to
2°C higher than today (see Figure 13.5B) and the Arctic
tree line was several hundred kilometres further north in
Eurasia and North America. By this time, subtropical
desert regions were again very dry and were largely
abandoned by primitive peoples. A temperature decline
set in around 2000 years ago with colder, wetter
