Geoscience Reference
In-Depth Information
Younger Dryas
-35
-40
100
50
0
400
200
0
15
14
13
Time (1000s years BP)
12
11
Figure 13.5
The late glacial to interglacial transition (14.7 to 11.6ka) as indicated by d
18
O (ppm), electrical
conductivity (microamps) and calcium concentrations (ppb) in the Greenland Ice Sheet Program (GISP) ice
core from central Greenland.
Source: Reprinted by permission of Grootes (1995). Copyright © National Academy of Sciences. Courtesy of the National
Academy of Sciences, Washington, DC.
climate conditions and back again, apparently
occurred within a five-year time interval for both
transitions! The processes driving D-O events like
the Younger Dryas are still incompletely under-
stood, but likely in some way involve massive
discharges of fresh water from melting ice sheets
to the North Atlantic that disrupted the Atlantic
thermohaline circulation (see
Figure 7.32
).
Early Holocene warmth around 10ka is
attributed to July solar radiation being 30-40W
m
-2
greater than now in northern mid-latitudes,
again due to Milankovich effects. Following the
final retreat of the continental 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 subtropics this
interval was also generally wetter, with high lake
levels in Africa and the Middle East. A Holocene
Thermal Maximum (HTM) was reached in the
mid-latitudes about 5000 years ago, when summer
temperatures were 1-2
hundred kilometers 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 conditions in Europe
and North America. Although temperatures
have not since equaled those of the HTM (we
are getting close), a relatively warmer interval (or
intervals) occurred between the ninth and mid-
fifteenth centuries AD. Summer temperatures
in Scandinavia, China, the Sierra Nevada
(California), Canadian Rocky Mountains and
Tasmania exceeded those that prevailed until the
late twentieth century.
3
The past 1000 years
Temperature reconstructions for the Northern
Hemisphere over the past millennium are based
on several types of proxy data, but especially
dendrochronology, ice cores and historical
C higher than today (see
Figure 13.5B
) and the Arctic tree line was several
°
