Geoscience Reference
In-Depth Information
N. Hemisphere
E. Europe
N. Hemisphere
N. Hemisphere
C
Global
C
W
C
W
C
W
W
C
W
0
2
1
4
5
4
3
6
200,000
4
7
2
10,000
5
3
50,000
500
400,000
1
50
5
(c)
600,000
20,000
(b)
100,000
1000
10°C
800,000
6
1.5°C
(e)
900,000
1.
2.
3.
4.
5.
6.
7.
Thermal maximum of 1940s
Little Ice Age
Younger Dryas Cold Interval
Present Interglacial (Holocene)
Last Glacial (Weichselian)
Penultimate Interglacial (Eemian)
Penultimate Glaciation (Saalian)
(d)
7
10°C
150,000
(a)
100
10°C
0.5°C
Figure 13.4
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: From Understanding Climatic Change: A Program for Action(1975). By permission of the National Academy Press, Washington, DC.
Table 13.2
The four categories of climatic variable subject to anthropogenically induced change.
Variable changed
Scale of effect
Sources of change
Atmospheric composition temperature
Local-global
Release of aerosols and trace gases
Surface properties; energy
Regional
Deforestation; desertification; urbanization
budgets
Wind regime
Local-regional
Deforestation; urbanization
Hydrological cycle components
Local-regional
Deforestation; desertification, irrigation;
urbanization
sharp warming trend (
Figure 13.4
). The Holocene
(our present interglacial) is considered to begin at
11.5ka, after the close of the Younger Dryas event.
Based on assessments of Milankovich forcings,
the present interglacial should last for at least
another 30,000 years. A particularly striking
feature of the last glacial cycle is rapid millennial-
scale changes between warm and cold conditions,
known as Dansgaard-Oeschger (D-O) cycles.
The Younger Dryas event is considered to be the
last of these D-O cycles. As is evident in a number
of proxy records (see
Figure 13.5
), the onset
and termination of the Younger Dryas cold event,
with a switch from near glacial to interglacial
2 The last glacial cycle and post-
glacial conditions
The last interglacial, known as the Eemian, peaked
about 125ka. The last glacial cycle following the
Eemian was itself characterized by periods of
extensive ice (known as stades) and less extensive
ice (known as interstades), Maximum global ice
volume (the Last Glacial Maximum, or LGM)
occurred around 25-18ka. The LGM ended with
abrupt warming between about 15 and 13ka,
depending on latitude and area, interrupted
by a cold regression called the Younger Dryas
(13-11.7ka). This was then followed by a renewed
