Past Climate Variability and Change in the Arctic and at High Latitudes

NUMBER OF
DATES PER YEAR TEMPERATURE (°C)
–25
–35
–45
25
20
15
10
5
0
June insolation
Alley 2000
GISP2 temperature
Russian Treeline Macrofossils
North American HTM Sites
for most of the Holocene throughout the Arctic.
Consequently, the mid- to late-Holocene record
allows evaluation of the range of natural climate
variability and of the magnitude of climate
change in response to relatively small changes
in forcings.
3.4.9a thE holocEnE thErmal maximum
Many of the Arctic paleoenvironmental records
for the Holocene Thermal Maximum (HTM)
appear to have recorded primarily summertime
conditions. Many different proxies have
been exploited to derive these reconstructions
by use of biological indicators such as pollen,
diatoms, chironomids, dinoflagellate cysts,
and other microfossils; elemental and isotopic
geochemical indexes from lacustrine sediments,
marine sediments, and ice cores; borehole temperatures;
and age distributions of radiocarbondated
tree stumps north of (or above) current
treeline, marine mollusks, and whale bones
(Kaufman et al., 2004).
Past Climate Variability and Change in the Arctic and at High Latitudes
1
20,000 16,000 12,000 8,000 4,000 0
CAL YEAR BP
3
2
January insolation
all genera
1. Bebula
2. Larix
3. Picea
4. Pinus
JUN JAN
470
440
410
FREQUENCY
A recent synthesis of 140 Arctic paleoclimatic
and paleoenvironmental records extending
from berinGiA westward to icelAnd (Kaufman
et al., 2004) outlines the nature of the Holocene
Thermal Maximum in the western Arctic (Figure
3.32). Fully 85% of the sites included in the
synthesis contained evidence of a Holocene
thermal maximum. Its average duration extended
from 2,100 years in berinGiA to 3,500 years
in GreenlAnd. The interval 10–4 ka contains
the greatest number of sites recording Holocene
Thermal Maximum conditions and the greatest
spatial extent of those conditions in the western
Arctic (Figure 3.32B). In the western Arctic the
timing of this thermal maximum begins and
ends along a strong geographic gradient (Figure
3.32C). The thermal maximum began first
in berinGiA, where warmer-than-present summer
conditions became established at 14–13 ka.
Intermediate ages for its initiation (10–8 ka) are
apparent in the cAnAdiAn Arctic islands and
in central GreenlAnd. The Holocene Thermal
Maximum on icelAnd occurred a bit later,
30
20
10
0
70
40
10
Insolation 70°N
(Wm –2 )
Figure 3.31. The Arctic Holocene Thermal Maximum. Items compared, top to bottom: seasonal
insolation patterns at 70°N. (Berger and Loutre, 1991) and reconstructed Greenland air temperature
from the GISP2 drilling project (Alley, 2000); age distribution of radiocarbon-dated fossil remains
of various tree genera from north of present treeline (MacDonald et al., 2007) and the frequency
of Western Arctic sites that experienced Holocene Thermal Maximum conditions (Kaufman et al.,
2004). [Reprinted with permission of Philosophical Transactions of the Royal Society.]
4
Fully 85% of the
sites included in the
synthesis contained
evidence of a Holocene
thermal maximum.
75