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.
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.