Sunday, March 25, 2012


date: Thu, 12 Oct 2000 12:12:38 -0400
from: "Raymond S. Bradley" <>
subject: synthesis section 1-revised
to:, jcole@geo.Arizona.EDU,

Can you take a quick glance through this. I've revised it & reorganised
things a bit--regional sections folow this

Chapter 6. The Climate of the Last Millennium
Raymond S. Bradley, Keith R. Briffa, Julia E. Cole and Malcolm K. Hughes

6.1 Introduction
We are living in unusual times. Twentieth century climate was dominated by
near universal warming; almost all parts of the globe had temperatures at
the end of the century that were higher than when it began, and in most
areas temperatures were significantly higher (Parker et al., 1998; Jones et
al., 1999; Wallace et al., 2000). [Do we want a figure here Figure 6.1say
a global map of (1900-1909)-(1990-1999) MAT from instrumental
data?] However the instrumental data provide only a limited temporal
perspective on present climate. How unusual was the last century when
placed in the longer-term context of climate in the centuries and millennia
leading up to the 20th century? Such a perspective encompasses the period
before large-scale contamination of the global atmosphere and global-scale
changes in land-surface conditions. By studying the records of climate
variability and forcing mechanisms in the recent past, it is possible to
establish how the climate system varied under �natural� conditions, before
anthropogenic forcing became significant. Natural forcing mechanisms will
continue to operate in the 21st century, and will play a role in future
climate variations, so regardless of how anthropogenic effects develop it
is essential to understand the underlying background record of forcing and
climate system response.
Sources of information on the climate of the last millennium include:
historical documentary records, tree rings (width, density); ice cores
(isotopes, melt layers, net accumulation, glaciochemistry); corals
(isotopes and other geochemistry, growth rate); varved lake sediments
(varve thickness, sedimentology, geochemistry, diatom and pollen content);
banded speleothems (isotopes). These are all paleoclimatic proxies that
can provide continuous records with annual resolution. Other information
may be obtained from sources that are not continuous in time, and that have
less rigorous chronological control. Such sources include geomorphological
evidence (e.g. from former lake shorelines and glacier moraines) and
sub-fossil macrofossils that indicate the range of plant or animal species
in the recent past. In addition, ground temperature measurements in
boreholes reflect the past history of surface temperatures, with temporal
resolution decreasing with depth. These provide estimates of overall
temperature changes from one century to the next (Pollack et al. 19XX).
Proxies of past climate are generally controlled by a particular aspect of
the system that causes a climate-related signal to be recorded. For some
biological proxies, such as tree ring density or coral growth rate, the
main factor might be temperature or more specifically, the temperature of a
particular season (or even just part of a season). Density and growth rate
might also be influenced by antecedent climatic conditions, or by other
non-climatic factors. Similar issues are important in other proxies, such
as the timing of snowfall events that make up an ice core, or the rate and
timing of sediment transport to a lake. Though we recognize that the
details of such relationships are important, proxies are rarely interpreted
directly in terms of very specific controls, but rather in terms of
temperature or precipitation in a particular season. In many cases the
main climatic signal in a proxy record is not temperature alone. For
example, evidence of a formerly high lake level may indicate higher
rainfall amounts and/or a decrease in evaporation related to cooler
temperatures. Such issues are grist to the paleoclimatologists� mill and
are the subject of numerous studies. Suffice it to say that proxies are
generally selected to optimize a reconstruction of either temperature or
precipitation and it is these studies that provide the basis for our review.
Changes in temperature have large-scale spatial coherence, making it easier
to identify major variations with relatively few records. Precipitation
changes are more local or regional in extent, but they often reflect
circulation changes that may have large-scale significance (as, for
example, in ENSO-related rainfall increases that commonly occur in the
southeastern U.S. during strong El Ni�o events; Stahle et al., 1998). In
this chapter, we focus mainly on temperature variations, but precipitation
and hydrological variability are examined where there is good evidence for
important changes at the regional scale. In particular, we ask two
questions regarding each attribute:
� does the 20th century record indicate unique or unprecedented
� do 20th century instrumental data provide a reasonable estimate of
the range of natural variability that could occur in the near future?
First we deal with the overall pattern of temperature change at the largest
(hemispheric) scale. Then, we examine variability over four large
sub-regions of the globe. These results are placed in the longer-term
perspective of Holocene climatic changes, and finally we examine forcing
factors that may have played a role in the variations that have been

6.2 Temperatures over the last millennium
Most high resolution paleoclimate records (i.e. those with annual
resolution and a strong climate signal) do not extend back in time more
than a few centuries. Consequently, while there are numerous paleoclimate
reconstructions covering the period from the 17th century to the present,
the number of high resolution millennium scale records is very
limited. Continuous records are restricted to ice cores and laminated lake
sediments, where the climatic signal is often poorly calibrated, and to a
few long tree ring records, generally from high latitudes. Inevitably,
this leads to large uncertainties in long-term climate reconstructions that
attempt to provide a global or hemispheric-scale perspective. Bearing this
in mind, what do current reconstructions tell us about the last millennium?
Figure 6.2 shows a reconstruction of northern hemisphere mean annual
temperature for the last 1000 years, based on a set of over [100?]
well-distributed paleoclimatic records spanning the past 600 years but a
much smaller number of data sets (12) for the period prior to A.D. 1400
(Mann et al., 1998, 1999). The paleoclimatic proxies were calibrated in
terms of the main modes of temperature variations (eigenvectors)
represented in the instrumental records for 1902-1980. Variations across
the network of proxies, for the period before instrumental records, were
then used to reconstruct how the main temperature patterns (i.e. their
principal components) varied over time. By combining these patterns,
regional, hemispheric or global mean temperature changes, as well as
spatial patterns over time were reconstructed (Mann et al., 2000). To
accurately reproduce the spatial pattern requires that the proxy data
network is extensive enough to capture many of the principal eigenvector
patterns. With the data available, regional patterns of temperature
variation could only be meaningfully reconstructed for 250 years, although
the large-scale (hemispheric) mean temperature could be reconstructed for a
longer period. This is possible because the proxy data network, even at
its sparsest, exhibits a coherent response to variability at the largest
scale. Thus a reconstruction of hemispheric mean temperature back 1000
years is possible, using a quite limited network of data, albeit with
ever-increasing uncertainty (i.e. expanding confidence limits) the further
back in time one goes (Figure 6.2Mann et al 1000yr reconstruction with
errors). This reconstruction shows an overall decline in temperature of
~0.2�Cfrom A.D. 1000 until the early 1900s when temperatures rose
sharply. Superimposed on this decline were periods of several decades in
length when temperatures were warmer or colder than the overall
trend. Mild episodes, lasting a few decades, occurred around the late
10th/early 11th century and in the late 13th century, but there was no
period when mean temperature was comparable with levels in the late 20th
century. Coldest conditions occurred in the 15th century, the late 17th
century and in the entire 19th century.
Other attempts to assess northern hemisphere temperatures have taken a
simpler approach, either averaging together normalized paleo-data of
various types (Bradley and Jones, 1993; Jones et al., 1998) or averaging
data scaled to a similar range (Crowley and Lowery 2000). Such approaches
do not provide an estimation of uncertainty, and indeed may lead to rather
arbitrary combinations of very diverse data (often having different
temporal precision). Nevertheless, the resulting time series from all of
these studies are similar, at least for the first 400-500
years. Thereafter, some series indicate especially cold conditions, from
the late 16th century until the 19th century, but still generally within
the 2 standard error confidence limits of Mann et al. (1999) (Figure
6.3composite showing low frequency records from Mann et al, 1999; Jones et
al. * Crowley & Lowery + Mann et al uncertainties--from Briffa et al 2000
Fig. 10, without green line[Briffa 2000] and including both sets of
uncertainty �curtains�). The differences between them may be explained by
the different seasons and spatial coverage of data used.
Each reconstruction represents a somewhat different spatial domain. In the
Mann et al. studies, the �northern hemisphere mean� series is the same
geographical domain as the gridded instrumental data set available for the
period 1902-80. This means that some regions within the northern
hemisphere (in the central Pacific, central Eurasia and regions beyond
70�N) were not represented. However, the global eigenvector patterns that
were reconstructed are based on data from low latitudes and parts of the
southern hemisphere. Other reconstructions generally do not include data
from sub-tropical or tropical regions, and this may explain the colder
periods in the latter half of the Jones et al. and Crowley and Lowery
records, if higher latitudes were particularly cold at that time compared
to the Tropics.
Another reason for the differences in Figure 6.3 may be because each
reconstruction represents a somewhat different season. In the Mann et al.
(1998, 1999) reconstruction, mean annual temperature data were used for
calibration, since data from both hemispheres were used to constrain the
eigenvector patterns and data from different regions may have had stronger
signals in one season than in another. For example, some data from western
Europe might contain a strong NAO (winter) signal, whereas data from
elsewhere might carry a strong summer precipitation signal related to
ENSO. Both data sets nevertheless help to define important modes of
climate anomalies that themselves capture large scale annual temperature
patterns (Bradley et al., 2000). Other reconstructions are for summer
months (April-September) and this may also explain some of the differences
between the series, for example, if summers were particularly cool in
extra-tropical regions, in the 17th-19th centuries.
Another critical question in any long-term reconstruction is to what extent
does the proxy adequately capture the true low frequency nature of the
climate record. Given that most of the long-term data used in all of these
paleotemperature reconstructions are from tree-rings, it is important to
establish that they are not contaminated by biological growth trends. This
matter is especially critical when individual tree ring records, of
differing record lengths (often limited to a few hundred years) are patched
together to assess long-term climate changes. Briffa et al. (2000) have
carefully evaluated this problem, using a maximum ring width density data
set that is largely independent of that used by Mann et al. (1998,
1999). By combining sets of tree ring density data grouped by the number
of years since growth began at each site, Briffa et al. provide a
methodology that largely eliminates the biological growth function
problem. They also estimate confidence limits through time (Figure 6.4Fig
10 from Briffa et al 2000 on its own, 2SE limits).
The Briffa et al series shows similar temperature anomalies as Mann et al.
in the 15th century (though no sharp decline in temperatures around
A.D.1450) but markedly colder conditions from A.D. 1500 to ~A.D. 1800 (cf.
Figures 6.2 and 6.4). The early 19th century is also colder in the Briffa
et al. series. The Mann et al. and Briffa et al. series (Figure 6.3)
bracket all other paleotemperature estimates for the northern hemisphere,
such as those by Bradley and Jones (1993), Jones et al. (1998) Briffa et
al., (1998), Overpeck et al. (1998) and Crowley and Lowery (2000). The
Briffa et al reconstruction describes a well-defined minimum in
temperatures from ~A.D. 1550-1850 that conforms with the consensus view of
a �Little Ice Age� (Bradley and Jones,1992). Though this period was not
uniformly cold and temperature anomalies differed regionally, overall it
was significantly below the 1881-1960 mean (by as much as 0.5�C for most of
the 17th century) in the regions studied by Briffa et al.
(2000). Independent reconstructions derived from borehole temperatures
suggest even colder temperatures about 400 to 500 years ago, and/or even
greater warming in the 20th century.[elaborate] To what extent these
differences in reconstructed temperatures are related to the effect of land
use change on borehole temperatures remains to be resolved, but it suggests
that land use change may be another factor, in addition to changes in
atmospheric trace gases and aerosols, that may have to be taken into
account to realistically simulate past (and future) climate change.
Although the Mann et al. and Briffa et al reconstructions have much in
common, they are clearly not identical. One explanation for the
differences may again lie in the geographical distribution of data used in
each analysis. The study of Briffa et al. is strongly weighted towards the
northern treeline (60-75�N) where temperatures were particularly low in the
17th century; the study by Mann et al. includes data from lower latitudes,
and incorporates temperature reconstructions from both marine and
terrestrial regions in calculating a hemispheric mean. If this is the
explanation for the differences, it suggests that low latitude regions
(equatorward of ~35�N) did not experience a drop in temperatures in the
latter half of the last millennium comparable to higher northern
latitudes. This further suggests an increase in the Equator-Pole
temperature gradient during that time.
One thing that all reconstructions clearly agree on is that northern
hemisphere mean temperature in the 20th century is unique, both in its
overall average and in the rate of temperature increase. In particular the
1990s were exceptionally warm -- probably the warmest decade for at least
1000 years (even taking the estimated uncertainties of earlier years into
account). The last ~50 years also appear to have been the warmest period
by far (Table 1??). A caveat to this conclusion is that the current
proxy-based reconstructions do not extend to the end of the 20th century,
but are patched on to the instrumental record of the last 2-3
decades. This is necessary because many paleo data sets were collected in
the 1960s and 1970s, and have not been up-dated. Furthermore, in the case
of tree rings from some areas (especially at high latitudes) the climatic
relationships prevalent for most of the century appear to have changed in
recent decades, possibly because of increasing aridity &/or snowcover
changes at high latitudes that have altered the ecological responses of
trees to climate (cf. Jacoby et al; Briffa et al; Vaganov et al.,
1999). Consequently, it must be recognized that an assessment of the
unusual nature of the 1990s is necessarily based on a direct comparison of
instrumental data with long-term proxy-based
reconstructions. Nevertheless, the conclusion that temperatures rose at
unprecedented rates in the 20th century, reaching levels by the end of the
century that were unprecedented within the last millennium seems to be an
extremely robust result from these studies. (cf. Pollack et al. 1998??).
Confidence that an accurate reproduction of the recent instrumental record
would be possible if all the available paleoclimatic data were updated to
the present is provided by Figure 6.5 (Mann et al. v. instrumental,
1850-1980). This shows that a set of proxy data calibrated against the
1902-1980 period of instrumental data captured mean annual temperatures
well both during this period and during the preceding 50 years for which an
independent set of instrumental data is available. The excellent fit over
the late 19th century test period provides confidence that an updated set
of proxy data would also accurately reproduce recent changes.
Figure 6.3 also shows that the overall range in temperature over
the last 1000 years has been quite small. For example, the range in
50-year means has only been ~0.5�C [?cf. both records] (from the coldest
period in the 15th , 16th and 19th centuries, to the warmest period of the
last 50 years: Table 1??). Within that narrow envelope of variability, all
of the significant environmental changes associated with the onset and
demise of the �Little Ice Age� (~1450-1850) took place. This puts into
vivid context the magnitude of projected future changes resulting from
greenhouse-gas increases and associated feedbacks (Figure 6.6 Mann et al +
others? + IPCC projections). Even the low end of model estimates suggest
additional temperature increases on the order of 1-2�C by the end of the
21st century (Intergovernmental Panel on Climate Change 1996 or 2000?).
The discussion so far has focused exclusively on the northern hemisphere
record because there are insufficient data currently available to produce a
very reliable series for the southern hemisphere. Data from the Mann et
al. (1998) reconstruction (back to A.D. 1700) averaged for those parts of
the southern hemisphere that were represented in the instrumental
calibration period, show a similar temporal pattern to that of the northern
hemisphere, but generally warmer (less negative anomalies). However, much
more work is needed on southern hemisphere proxy records to extend and
verify that result.

Raymond S. Bradley
Professor and Head of Department
Department of Geosciences
University of Massachusetts
Amherst, MA 01003-5820

Tel: 413-545-2120
Fax: 413-545-1200
Climate System Research Center: 413-545-0659
Climate System Research Center Web Page:
Paleoclimatology Book Web Site (1999):


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