Evidence for Climate Teleconnections Between Greenland and the Sierra Nevada of California During the Holocene, Including the 8200 and 5200 Climate Events - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 205

North Atlantic controlled the areal extent of droughts, and
that the two modes of variability together controlled the
severity and longevity of droughts.
Though evidence for the teleconnections between the
northern Atlantic Ocean exists, one has to be careful in
discussing the causes for megadroughts in the remote past.
First, the causes for many of the climatic teleconnections in
the remote past, and even today, are not well understood
[Cook et al., 2010]. Second, the relationships between the
in

planet has warmed. Seager et al. [2007] noted that recent
severe droughts experienced in the southwestern United
States may have been associated with a shift of storm tracks
northward and an expansion of the subtropical high into the
midlatitudes.
What is instructive in terms of ongoing human-caused
climate change is how fast climates changed from wetter to
drier and from hotter to colder conditions in the Sierra
Nevada and Greenland during the relatively recent past.
Seager et al. [2007] noted that although ongoing warming
due to the buildup of greenhouse gasses is a relatively grad-
ual change, the paleoclimate records tell us that the transition
to a megadrought can be very abrupt. We know that ACCEs
occurred repeatedly in the past without the influence of hu-
mans. We do not know, however, if the large amounts of
greenhouse gases we are adding to the atmosphere could
somehow trigger a future human-caused ACCE.
We do know that past ACCEs have had serious repercus-
sions for people and the world

c oceans in the past were
not always consistent [Conroy et al., 2009]. These facts
suggest that factors other than the ENSO, PDO, or AMO
might also predispose western North America to severe
drought.
Synchronicity between climate events in the Western
Hemisphere and climate events in Greenland, described here
and elsewhere, suggests that there were shared climate tele-
connections between the western United States and the North
Atlantic in the past. However, it should be noted, with the
possible exception of the 8200 years climate event [Alley et
al., 1997; Barber et al., 1999], that shared climate telecon-
nections between western North America and the North
Atlantic does not require that events in one region (e.g., the
Sierra Nevada) were directly caused by events in another
(e.g., the North Atlantic). Rather, climate teleconnections
suggest that both regions were affected by at least some of
the same climate drivers at some times in the past.

uences of the Atlantic and Paci

s environments. For instance,
a severe drought 8200 years ago reportedly caused the sud-
den collapse of dry land cereal farming in Mesopotamia and
massive loss of human life, leading eventually to the begin-
ning of irrigated agriculture along the Tigris and Euphrates
rivers and the rise of the

'

first phase of Mesopotamian civili-
zation, the Sumerian empire, in Iraq [Weiss, 2003]. The 5200
ACCE also coincided with a severe drought, leading to the
end of the Uruk Phase of Mesopotamia culture [Weiss, 2003]
and the beginning of the First Dynasty of Ancient Egypt.
Therefore, it behooves us to learn about past occurrences of
ACCEs and to pay attention to what we are doing to the
climate and what the repercussions might be.

5.5.3. A possible explanation for climate teleconnections
and the implications. Some researchers have been hesitant to
acknowledge that climate events in California and Greenland
could be correlated with each other unless the mechanisms
for those climate teleconnections, which are still unsatisfac-
tory understood at this point, were known. In section 5.2, I
noted that the evidence from Coburn Lake suggested that the
general locations of the Earth

6. CONCLUSIONS

This study sought to explain the unusually peaked Coburn
Lake macrocharcoal record and its relationship to climate
and

s major precipitation belts
might have shifted abruptly during ACCEs in the past. This
could be one mechanism to explain climate teleconnections
between California and Greenland. Any major relocation of
the general location of the polar jet stream, north or south
away from today ' s location, would have meant fewer storms
passing over the northern Sierra Nevada and drier conditions
during those relocations.
Some evidence suggests that shifting of the Earth ' s major
precipitation belts will occur in the future or may already be
occurring. The Intergovernmental Panel on Climate Change
[2007] suggests that the subtropical high-pressure zones of
the world will become drier and will expand poleward in the
future due to greenhouse gas-induced warming. Archer and
Caldeira [2008] suggested that the northern polar jet stream
might already have moved northward in recent years as the

'

fire events in the Sierra Nevada, eastern Canada, and
Greenland. The entire 8500 year long Coburn Lake charcoal
record was compared with a high-resolution 7400 year long
precipitation record from Pyramid Lake, Nevada [Mensing
et al., 2004]. Severe fires and soil erosion occurred at Coburn
Lake at the beginning of six of the seven most severe
droughts that occurred in the northern Sierra Nevada over
the past 7400 years and at the beginning of every drought
that occurred in the northern Sierra Nevada over the past
1800 years.
Two thousand years of both the Coburn Lake charcoal and
the Pyramid Lake precipitation records were compared with
a 2000 year long Cirque Peak temperature record [Scuderi,
1993]. It was found that temperatures at the beginnings of
droughts in the Sierra Nevada were either high or times of