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Renewed interest in the sun
Conventional wisdom has generally held that decadal-to-millennial scale changes
in the sun's luminosity are too small to have major effects on Earth's climate.
Several recent studies challenge that notion. Drew Shindell and coworkers used
a general circulation model to show that reduced irradiance during the Maunder
Solar Minimum (mid-1600s to early 1700s) may have cooled the Northern Hemisphere
continents by 1 to 2 degrees Celsius during winter (Science, v. 294,
p. 2149-2152). This cooling occurs via feedbacks with stratospheric ozone and
a shift in the sea-level pressure pattern known as the Arctic Oscillation/North
Atlantic Oscillation (AO/NAO).
Dave Hodell and colleagues used lake-level proxies to reconstruct the history
of drought on the Yucatan Peninsula over the past 2,600 years (Science,
v. 292. p. 1367-1370). They found a dominant periodicity of 208 years, which
they equated to the 206-year solar cycle previously inferred from cosmogenic
nuclide records (beryllium-14 and -10) preserved in tree rings and ice cores.
An early Holocene proxy record of rainfall in Oman (stalagmite oxygen isotopes)
generated by Ulrich Neff and others bears a strong resemblance to the tree ring
carbon-14 record, suggesting a link between irradiance and monsoon intensity
(Nature, v.411, p. 290-293). Finally, Gerard Bond and colleagues demonstrated
a striking correspondence between the nuclide records and the abundance of ice-rafted
sediments in the North Atlantic, with both exhibiting oscillations lasting about
1,000-2,000 years throughout the Holocene (Science, v. 294, p. 2130-2136).
Reduced irradiance was associated with a southward expansion of drift-ice-laden
polar waters. They further suggested that since the regional distribution of
cooling during these events appears to be inconsistent with the AO/NAO pattern
predicted by Shindell and colleagues, reduced North Atlantic thermohaline overturning
may have acted as an amplifier.
Tropics getting their due
The North Atlantic has long been recognized as a hotbed of climate variability
on glacial-interglacial and millennial timescales. Increased attention is now
being focused on tropical climate dynamics, especially the El Niño -Southern
Oscillation (ENSO), the largest source of interannual variability in the modern
climate system. Alexander Tudhope and colleagues used oxygen isotopes in corals
from the western equatorial Pacific Warm Pool (Papua New Guinea) to reconstruct
ENSO variability during 15 multidecadal time slices scattered over the past
130,000 years (Science, v. 291, p. 1511-1517). They found that variance
in the typical ENSO frequency band (2.5-7 years) was present during all intervals
but was almost always weaker than today, particularly during the last glaciation
(42,000-38,000 years ago) and mid-Holocene (6,500 years ago). Luc Beaufort and
coworkers reconstructed primary productivity across the tropical Indian and
Pacific oceans using calcareous nannoplankton assemblages (Science, v.
293, p. 2440-2444). Over the past 180,000 years, two patterns emerge: a spatially
coherent glacial-interglacial cycle and a precessional scale (approximately
23,000-year periodicity) east-west gradient linked to ENSO dynamics. The latter
observation is consistent with previous modeling by Amy Clement and coworkers
of the ENSO response to low-latitude insolation forcing. Such forcing may also
explain the southward migration of the Intertropical Convergence Zone (ITCZ)
since the early-middle Holocene, as inferred from terrigenous sediment accumulation
in the Cariaco Basin (off Venezuela) by Gerald Haug and others (Science,
v. 293, p. 1304-1308). A late Holocene intensification of El Niño could
have amplified the southward shift.
Global teleconnections
Recent years have witnessed the discovery of climate records from various parts
of the globe that closely resemble air temperatures recorded in Greenland ice
cores, characterized by millennial scale "Dansgaard-Oeschger" (D-O)
oscillations. The search for mechanisms that can explain such distant connections
remains the topic of intense research. Herman Kudrass and colleagues showed
that planktonic foraminiferal oxygen isotopes in the northern Bay of Bengal
mainly reflect surface salinity, as forced by runoff from the Indian summer
monsoon (Geology, v. 29, p. 63-66). The similarity to Greenland air temperature
over the past 80,000 years led them to suggest that the monsoon amplifies or
even drives D-O cycles in the Northern Hemisphere via the greenhouse effect
of water vapor. In the high-latitude North Pacific, Thorsten Kiefer and coworkers
inferred D-O-like millennial scale warmings of about 2.5 to 4 degrees Celsius
on the basis of planktonic foraminiferal oxygen isotopes and faunal assemblages
(Paleoceanography, v. 16, p. 179-189). An attempt to synchronize these
records to the North Atlantic using the history of geomagnetic field intensity
suggests that temperature changes in the two oceans may be out of phase. The
authors proposed a link between North Atlantic thermohaline overturning (which
warms the North Atlantic) and Pacific Deep Water upwelling (which cools the
North Pacific). More precise was Thomas Blunier and Ed Brook's use of methane
(which is relatively well mixed in the atmosphere) to synchronize air temperature
records from Greenland and Antarctica over the past 90,000 years (Science,
v. 291, p. 109-112). They determined that major millennial-scale warmings in
Antarctica (seven total) coincided with cold periods in Greenland, a pattern
that can be explained by variations in the interhemispheric transport of heat
by North Atlantic thermohaline circulation.
Climates of the more distant past
Climate reconstructions for warm periods such as the Cretaceous suggest that
while polar regions were much warmer than today, the tropics were unchanged
or even cooler than now, a situation that defies climate models. Paul Pearson
and colleagues have suggested that many previous sea-surface temperature estimates
based on planktonic foraminiferal oxygen isotopes are biased by diagenetic recrystallization
(Nature, v. 413, p. 481-487). Their measurements of pristine foraminifera
extracted from clay-rich sediments point to tropical temperatures exceeding
30 degrees Celsius during the Late Cretaceous and Eocene, potentially resolving
the "cool tropics paradox." Paul Wilson and Dick Norris reached a
similar conclusion for the mid-Cretaceous and also documented a collapse of
upper-ocean stratification associated with a global oceanic anoxic event occurring
approximately 99 million years ago (Nature, v. 412, p. 425-429). The
latter observation argues against theories that call upon ocean stagnation to
explain the widespread formation of Cretaceous black shales. Finally, Mark Cane
and Peter Molnar proposed that the aridification of eastern Africa around 3-4
million years ago was caused not by the closing of the Isthmus of Panama, as
previously suggested, but rather by a reconfiguration of the Indonesian seaway
(Nature, v. 411, p. 157-162). The northward movement of New Guinea may
have shifted the source of through-flow waters from the warm South Pacific to
the cooler North Pacific, resulting in cooler Indian Ocean surface temperatures
and thus reduced rainfall over eastern Africa. The consequential shift from
forests to grasslands is believed to have profoundly influenced the evolution
of hominids.
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