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

44
The U.S. Climate Science Program Chapter 3
METERS
Wm –2
δ 18 O(PERCENT)
PERCENT
–20
–40
–60
–80
–100
–120
–140
540
520
500
480
460
440
–34
–36
–38
–40
–42
100
80
60
40
20
0
0
0
fossil record indicates that a certain climate milestone
was reached, such as exceeding a minimum
summer temperature threshold for successful
growth or a winter minimum temperature of
freezing tolerance (Figure 3.8). This methodology
was developed early in Scandinavia (Iversen,
1944); Matthews et al. (1990) used indicator
species to constrain temperatures during the last
interglaciation in northwest cAnAdA, and Ritchie
et al. (1983) used indicator species to highlight
early Holocene warmth in northwest cAnAdA.
This technique has also been used extensively
with fossil insect assemblages.
Methodologies for the numerical estimation
of past temperatures from pollen assemblages
0 10 20 30 40 50 60 70 80 90 100
0
1
10 20 30 40 50 60 70 80 90 100
AGE (KYR BP)
3,510±90 13,700±80 26,270±280
Spores
Trees and shrubs
H 1 H 2 H 3 H 4 H 5 H 6
2
3 8 12 14
4 56 1617
7 10
11 15
13
18
9
Herbs
200 400 600 800
DEPTH, IN CENTIMETERS
Ice-volume equivalent sea level
19 20
June Insolation at 60°N
21
22
GISP2
23
Elikchan 4 Lake Pollen
Figure 3.8 Upper three panels reflect changes as follows: top, sea level
(Lambeck et al., 2002); middle, June insolation at 60°N (Berger and Loutre,
1991); bottom, air temperature over Greenland Ice Sheet (numbers refer
to interstadials) based on the δ 18O record in ice (Grootes et al., 1993)
during the past 100 ka (ages in calendar years). Arrows denote timing of
Heinrich events. The lowermost panel shows changes in the percentages
of tree and shrub pollen, herb pollen, and spores at Ilikchan 4 Lake in the
Magadan region of Chukotka, Russia (Lozhkin and Anderson, 1996). Lake
core x-axis is depth, not time; the base of the core dates to about 60 ka
BP. Between approximately 55 ka and 72 ka, treeline recovered for short
intervals to nearly Holocene conditions. These conditions could reflect
warm interstadials that also appear in the Greenland ice core record about
the same time.
follow one of two approaches. The first is the
inverse-modeling approach, in which fossil data
from one or more localities are used to provide
temperature estimates for those localities (this
approach also underlies the relative estimates
of temperature described above). A modern
“calibration set” of data (in this case, pollen assemblages)
is related by equations to observed
modern temperature, and the functions thus
obtained are then applied to fossil data. This
method has been developed and applied in Scandinavia
(e.g., Seppä et al., 2004). A variant of the
inverse approach is analogue analysis, in which
a large modern dataset with assigned climate
data forms the basis for comparison with fossil
spectra. Good matches are derived statistically,
and the resulting set of analogues provides an
estimate of the past mean temperature and accompanying
uncertainty (Anderson et al., 1989,
1991).
Inverse modeling relies upon observed modern
relationships. Some plant species were more
abundant in the past than they are today, and
the fossil pollen spectra they produced may
have no recognizable modern counterpart—socalled
“no-analogue” assemblages. Outside the
envelope of modern observations, fossil pollen
spectra, which are described in terms of pollen
abundance, cannot be reliably related to past climate.
This problem led to the adoption of a second
approach to estimating past temperature (or
other climate variable) called forward modeling.
The pollen data are not used to develop numerical
values but are used to test a “hypothesis” about
the status of past temperature (a key ingredient
of climate). The hypothesis may be a conceptual
model of the status of past climate, but typically it
is represented by a climate-model simulation for
a given time in the past. The climate simulation
drives a vegetation model that assigns vegetation
cover on the basis of bioclimatic rules (such
as the winter minimums or required warmth
of summer growing temperatures mentioned
above). The resultant map is compared with a
map of past vegetation developed from the fossil
data. The philosophy of this approach is described
by Prentice and Webb (1998). Such data
and models have been compared for the Arctic by
Kaplan et al. (2003) and Wohlfahrt et al. (2004).
The great advantage of this approach is that underlying
the model simulation are hypothesized
climatic mechanisms; those mechanisms allow
not only the description but also an explanation
of past climate changes.