DARK AGES COLD PERIOD
IN NORTH AMERICA
How does our current climate compare to the natural climate of the Dark Ages Cold Period?
SPPI & CO2SCIENCE ORIGINAL PAPER ♦
March 14, 2012
DARK AGES COLD PERIOD IN NORTH AMERICA
Citation: Center for the Study of Carbon Dioxide and Global Change. "Dark Ages Cold Period in North America.” Last
modified March 14, 2012. http://www.co2science.org/subject/d/summaries/dacpnamerica.php.
A key aspect of the debate over potential CO2-induced global warming centers on how much of
the modern rise in temperature is the product of the contemporaneous anthropogenic-induced
rise in atmospheric CO2 concentration and how much is due to natural forcings. The world’s
climate alarmists contend that most of the warming is
due to CO2, while climate skeptics claim that natural
factors dominate.
In promoting their opposing
Considering the need to
positions, climate alarmists frequently turn to climate
models for support. Climate skeptics, on the other
evaluate the climate of the
hand, look to real-world data, arguing that there have
been multiple periods in Earth’s recent history when
modern era within the
temperatures rose to levels that were just as high as,
or even higher than, they are presently, when the air’s
CO2 content was much lower than it is today. And
historical context of natural
since atmospheric CO2 was obviously not responsible
for the higher temperatures of those prior periods of
climate change, we here
warmth, logic suggests it may well have had nothing to
do with the current rise in temperature either. Thus,
examine what researchers
discovering the cause or causes of past climatic
oscillations has become an important goal of
paleoclimate research.
have learned about the
One climatic cycle that has been found to occur
Dark Ages Cold Period,
throughout the globe during both glacial and
interglacial periods alike is a millennial-scale oscillation
specifically focusing on
that regularly alternates between warmer and colder
intervals, the past two cycles of which have brought us
the relative warmth of the Roman Warm Period1, the
North America.
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relative coolness of the Dark Ages Cold Period , the
heat of the Medieval Warm Period3, the global chill of
the Little Ice Age4, plus the welcome warmth of the Current Warm Period. And considering the
need to evaluate the climate of the modern era within the historical context of natural climate
change, we here examine what researchers have learned about the Dark Ages Cold Period,
specifically focusing on North America.
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Starting in the Pacific northwest portion of the continent, Reyes et al. (2006)5 compiled and
analyzed both new and previously published geological data obtained from 17 glacier fields in
coastal and near-coastal British Columbia (Canada) and Alaska (USA). Their results indicate
there was a widespread glacier advance during the first millennium AD along an ~2000-km
transect of the Pacific North American cordillera that was centered on AD 400-700. And “the
synchroneity of this glacier advance and inferred cooling over a large area,” in the words of the
eleven researchers, “suggest a regional climate forcing” that “together with other proxy
evidence for ... regional climate amelioration ca.
AD 850-1200 (Hu et al., 2001) during the
The synchroneity of this glacier
Medieval Warm Period (Cook et al., 2004;
Moberg et al., 2005), and subsequent Little Ice
Age glacier expansion (Larocque and Smith,
advance and inferred cooling
2003; Wiles et al., 2004), are consistent with a
millennial-scale climate cycle in the North Pacific
over a large area suggest a
region.”
Nearby, but working five years earlier, Hu et al.
(2001)6 conducted multiproxy geochemical
analyses of a sediment core obtained from
Farewell Lake in the northwestern foothills of
the Alaska Range, obtaining what they describe
as “the first high-resolution quantitative record
of Alaskan climate variations that spans the last
two millennia,” while stating as the reason for
their study that “knowledge of natural climatic
variability is essential for evaluating possible
human impacts on recent and future climate
changes.”
regional climate forcing [that]
together with other proxy
evidence for ... regional climate
amelioration … are consistent
with a millennial-scale climate
cycle in the North Pacific region.
What they found, in the words of the five
scientists, “suggest that at Farewell Lake SWT [surface water temperature] was as warm as the
present at AD 0-300 [during the Roman Warm Period], after which it decreased steadily by
~3.5°C to reach a minimum at AD 600 [during the depths of the Dark Ages Cold Period].” And
from that point in time, they say “SWT increased by ~3.0°C during the period AD 600-850 and
then [during the Medieval Warm Period] exhibited fluctuations of 0.5-1.0°C until AD 1200.”
Completing their narrative, they say that “between AD 1200-1700, SWT decreased gradually by
1.25°C [as the world descended into the depths of the Little Ice Age], and from AD 1700 to the
present, SWT increased by 1.75°C,” the latter portion of which warming initiated the Modern
Warm Period.
Also in Alaska, Wiles et al. (2008)7 used “comparisons of temperature sensitive climate proxy
records with tree-ring, lichen and radiocarbon dated histories from land-terminating, non5
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surging glaciers for the last two millennia” to “identify summer temperature as a primary driver
of glacial expansions,” based on “field and laboratory work over the past decade” that yielded
“five new or updated glacier histories,” one each for Bear Glacier (Kenai Mountains), Marathon
Mountain Cirque (Kenai Mountains), Amherst Glacier (Chugach Mountains), Crescent Glacier
(Chugach Mountains) and Yakutat Glacier (St. Elias Mountains), all located just above the Gulf
of Alaska (about 60°N) between approximately 140 to 150°W. And according to the four
researchers, they detected “general glacier expansions during the First Millennium AD” that
experienced their “strongest advance” at AD 600, which latter cold interval -- with ice extent
“as extensive as [the] subsequent Little Ice Age” – occurred during the Dark Ages Cold Period.
In addition, their record revealed an alternating warm-cold-warm-cold-warm sequence during
the past 2000 years, which they say “is consistent with millennial-scale records of ice-rafted
debris flux in the North Atlantic and Northern Hemisphere temperature reconstructions.” And
noting that “variable Holocene solar irradiance has been proposed as a potential forcing
mechanism for millennial-scale climate change,” they
conclude that “this is supported by the Southern
The past century’s lead-in
Alaskan glacial record,” which conclusion implies that
the past century’s lead-in to the Current Warm
to the Current Warm Period
Period may well have been similarly orchestrated
and have had essentially nothing to do with the
concomitant increase in the air’s CO2 content.
may well have been
similarly orchestrated and
have had essentially
nothing to do with the
concomitant increase in the
air’s CO2 content.
In one final study from Alaska, Clegg et al. (2010)8
conducted a high-resolution analysis of midge
assemblages found in the sediments of Moose Lake
(61°22.45’N, 143°35.93’W) in the Wrangell-St. Elias
National Park and Preserve of south-central Alaska
(USA), based on data obtained from cores removed
from the lake bottom in the summer of AD 2000 and
a midge-to-temperature transfer function that
yielded mean July temperatures (TJuly) for the past six
thousand years.
The results of this study are portrayed in the figure
below, where it can be seen, in the words of Clegg et
al., that “a piecewise linear regression analysis identifies a significant change point at ca 4000
years before present (cal BP),” with “a decreasing trend after this point.” And from 2500 cal BP
to the present, there is a clear multi-centennial oscillation about the declining trend line, with
its peaks and valleys defining the temporal locations of the Roman Warm Period (RWP), the
Dark Ages Cold Period (DACP), the Medieval Warm Period (MWP), the Little Ice Age (LIA) -during which the coldest temperatures of the entire interglacial or Holocene were reached -and, finally, the start of the Current Warm Period (CWP), which is still not expressed to any
significant degree compared to the Medieval and Roman Warm Periods.
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Mean July near-surface temperature (°C) vs. years before present (cal BP)
for south-central Alaska (USA). Adapted from Clegg et al. (2010).
In discussing their results, the seven scientists write that “comparisons of the TJuly record from
Moose Lake with other Alaskan temperature records suggest that the regional coherency
observed in instrumental temperature records (e.g., Wiles et al., 1998; Gedalof and Smith,
2001; Wilson et al., 2007) extends broadly to at
least 2000 cal BP,” while noting that (1) climatic
The Dark Ages Cold Period in
events such as the LIA and the MWP occurred
“largely synchronously” between their TJuly
record from Moose Lake and a δ18O-based
North America was a time of
temperature record from Farewell Lake on the
northwestern foothills of the Alaska Range, and
both relative coolness and
that (2) “local temperature minima likely
associated with First Millennium AD Cooling
wetness, much like the Little Ice
(centered at 1400 cal BP; Wiles et al., 2008) are
evident at both Farewell and Hallet lakes
(McKay et al., 2008).”
Age was in this part of the world.
Dropping down to the central part of the
conterminous United States, Follett et al. (2004)9 “used 13C:12C ratio (δ13C) and 14C dating to
evaluate relationships and changes in warm (C4) versus cool season (C3) plant signatures with
age of soil organic carbon” across “three soil temperature regimes and three soil moisture
regimes within the historic grasslands [of the] US Great Plains and Western Corn Belt” for the
entire period of the current interglacial or Holocene. They report that their data “indicate a
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change from C4 plants to increasing C3 plant dominance (as a surrogate of cooler temperature)
at ~1,500 yr BP.” More specifically, they say that “the yr BP when δ13C was least negative was
1560,” and that “δ13C was more negative before or after that time,” which places this
significantly colder interval of time well within the Dark Ages Cold Period.
Moving to the east coast of the United States, Willard et al. (2003)10 examined “the late
Holocene (2300 yr BP to present) record of Chesapeake Bay and the adjacent terrestrial
ecosystem in its watershed through the study of fossil dinoflagellate cysts and pollen from
sediment cores.” They report that “several dry periods ranging from decades to centuries in
duration are evident in Chesapeake Bay records.” The first of these periods of lower-thanaverage precipitation, which spanned the period 200 BC-AD 300, occurred during the Roman
Warm Period. The next such period (~AD 800-1200) “corresponds to the ‘Medieval Warm
Period’, which has been documented as drier than average by tree-ring (Stahle and Cleaveland,
1994) and pollen (Willard et al., 2001) records from the southeastern USA.” In addition, they
note that “mid-Atlantic dry periods generally correspond to central and southwestern USA
‘megadroughts’, described by Woodhouse and Overpeck (1998). Hence, it would appear that
the intervening Dark Ages Cold Period was a time of relatively greater wetness throughout
much of the United States.
A similar pattern of alternating multi-century wet and dry regimes was found by Campbell
(2002)11, who analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta,
Canada to provide a high-resolution record of climate variability for this part of North America
over the past 4000 years. Periods of both increasing
and decreasing grain size (moisture availability) were
There is no compelling
noted throughout the 4000-year record at decadal,
centennial and millennial time scales. The most
prominent of the major dry periods once again
reason to believe that
occurred during the Roman Warm Period (abut 900100 BC) and Medieval Warm Period (about AD 700 to
th
20 -century warming was
1300), while the major wet periods occurred during
the Dark Ages Cold Period (about 100 BC to AD 700)
in any way related to the
and Little Ice Age (about AD 1500 to 1900).
In considering the above studies, it appears that the
Dark Ages Cold Period in North America was a time of
both relative coolness and wetness, much like the
atmosphere’s CO2
Little Ice Age was in this part of the world. It is also
instructive to note, in this regard, that the climatic
concentration.
transition from the Little Ice Age to the Current Warm
Period was of the same type and magnitude as the
climatic transition from the Dark Ages Cold Period to the Medieval Warm Period. And because
the earlier of these two warming eras occurred at a time when the air’s CO2 content was stable
and much lower than it is today, there is no compelling reason to believe that 20 th-century
concurrent increase in the
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warming was in any way related to the concurrent increase in the atmosphere’s CO 2
concentration. Quite to the contrary, it was most likely the result of whatever drives the
natural millennial-scale cycling of Earth’s climate that has operated for eons throughout glacial
and interglacial periods alike (Oppo et al., 1998; McManus et al., 1999).
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Cover photo of the highest waterfall in North America,
Yosemite Falls,in California, USA, as uploaded by
MikeSFArea to wunderground.com.
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