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What's New on the Paleo Web Pages - 2004

What's New for 2004:
Temperature Reconstructions Climate Over Past Millennia
Jones and Mann Reviews of Geophysics
Vol. 42, No. 2, RG2002, 6 May 2004.

We review evidence for climate change over the past several millennia from instrumental and high-resolution climate "proxy" data sources and climate modeling studies. We focus on changes over the past 1 to 2 millennia. We assess reconstructions and modeling studies analyzing a number of different climate fields, including atmospheric circulation diagnostics, precipitation, and drought. We devote particular attention to proxy-based reconstructions of temperature patterns in past centuries, which place recent large-scale warming in an appropriate longer-term context. Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales.
NH Temperature Reconstruction Merging Information from Different Resources for New Insights into Climate Change in the Past and Future
Huang Geophysical Research Letters
Vol. 31, L13205, 8 July 2004.

An understanding of climate history prior to industrialization is crucial to understanding the nature of the 20th century warming and to predicting the climate change in the near future. This study integrates the complementary information preserved in the global database of borehole temperatures [Huang et al., 2000], the 20th century meteorological record [Jones et al., 1999], and an annually resolved multi proxy model [Mann et al., 1999] for a more complete picture of the Northern Hemisphere temperature change over the past five centuries. The integrated reconstruction shows that the 20th century warming is a continuation to a long-term warming started before the onset of industrialization. However, the warming appears to have been accelerated towards the present day. Analysis of the reconstructed temperature and radiative forcing [Crowley, 2000] series offers an independent estimate of the transient climate-forcing response rate of 0.4 - 0.7 K per Wm-2 and predicts a temperature increase of 1.0-1.7 K in 50 years.
1863 PDSI Reconstruction Long-Term Aridity Changes in the Western United States
Cook et al., Science
Vol. 306, No. 5698, pp. 1015-1018, 5 November 2004.

The western United States is experiencing a severe multi-year drought that is unprecedented in some hydroclimatic records. Using gridded drought reconstructions that cover most of the western US over the past 1,200 years, we show that this drought pales in comparison to an earlier period of elevated aridity and epic drought in AD 900-1300, an interval broadly consistent with the "Medieval Warm Period". If elevated aridity in the western US is a natural response to climate warming, then any trend towards warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America.
Greenland ice core location map by S. Ekholm, Danish Cadastre High-resolution record of Northern Hemisphere climate extending into the last interglacial period
North Greenland Ice Core Project members, Nature
v.431, No. 7005, pp. 147-151, 9 September 2004.

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5°C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.
Western Pacific Salinity Map Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch.
Stott et al., Nature
Vol. 431, pp. 56-59, 2 September 2004.

In the present-day climate, surface water salinities are low in the western tropical Pacific Ocean and increase towards the eastern part of the basin. The salinity of surface waters in the tropical Pacific Ocean is thought to be controlled by a combination of atmospheric convection, precipitation, evaporation and ocean dynamics, and on interannual timescales significant variability is associated with the El Nino/Southern Oscillation cycles. However, little is known about the variability of the coupled ocean-atmosphere system on timescales of centuries to millennia. Here we combine oxygen isotope and Mg/Ca data from foraminifers retrieved from three sediment cores in the western tropical Pacific Ocean to reconstruct Holocene sea surface temperatures and salinities in the region. We find a decrease in sea surface temperatures of ~0.5C over the past 10,000 yr, whereas sea surface salinities decreased by ~1.5 practical salinity units. Our data imply either that the Pacific basin as a whole has become progressively less salty or that the present salinity gradient along the Equator has developed relatively recently.
EPICA Dome C Ice Core data plot Eight glacial cycles from an Antarctic ice core
EPICA Community Members, Nature
Vol. 429, pp. 623-628, 10 June 2004.

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deepice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long-28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.
A Paleo Perspective on Abrupt Climate Change A Paleo Perspective on Abrupt Climate Change

Paleoclimate evidence from ice cores, tree rings, and other natural recorders reveals that rapid and dramatic changes in climate have occurred in the past at global and regional scales. The Perspective discusses what we know, and what we don't know, about the causes and effects of these abrupt climate changes.
Red Sea fossil coral head Increased seasonality in Middle East temperatures during the last interglacial period.
Felis et al., Nature
Vol. 429, pp. 164-168, 13 May 2004.

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.
Dongge and Hulu Cave stalagamite isotope records Timing, Duration, and Transitions of the Last Interglacial Asian Monsoon.
Yuan et al., Science
Vol. 304, Issue 5670, 575-578, 23 April 2004.

Thorium-230 ages and oxygen isotope ratios of stalagmites from Dongge Cave, China, characterize the Asian Monsoon and low-latitude precipitation over the past 160,000 years. Numerous abrupt changes in 18O/16O values result from changes in tropical and subtropical precipitation driven by insolation and millennial-scale circulation shifts. The Last Interglacial Monsoon lasted 9.7 ± 1.1 thousand years, beginning with an abrupt (less than 200 years) drop in 18O/16O values 129.3 ± 0.9 thousand years ago and ending with an abrupt (less than 300 years) rise in 18O/16O values 119.6 ± 0.6 thousand years ago. The start coincides with insolation rise and measures of full interglacial conditions, indicating that insolation triggered the final rise to full interglacial conditions.
Caribbean Salinity Links between salinity variation in the Caribbean and North Atlantic thermohaline circulation
Schmidt et al., Nature
Vol. 428, No. 6979, pp. 160-163, 11 March 2004.

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bolling/Allerod warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.
Figure 2 14C Activity and Global Carbon Cycle Changes over the Past 50,000 Years
Hughen et al., Science
Volume 303, Number 5655, 9 January 2004.

A series of 14C measurements in Ocean Drilling Program cores from the tropical Cariaco Basin, which have been correlated to the annual-layer counted chronology for the Greenland Ice Sheet Project 2 (GISP2) ice core, provides a high-resolution calibration of the radiocarbon time scale back to 50,000 years before the present. Independent radiometric dating of events correlated to GISP2 suggests that the calibration is accurate. Reconstructed 14C activities varied substantially during the last glacial period, including sharp peaks synchronous with the Laschamp and Mono Lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments. Simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14C production rate together with plausible changes in deep-ocean ventilation and the global carbon cycle during glaciation.
Greenland ice core records of CH4 and N2O for Dansgaard-Oeschger Event #12 N2O and CH4 variations during the last glacial epoch: Insight into global processes
Flückiger et al., Global Biogeochemical Cycles
Vol. 18, GB1020, January 2004.

Greenhouse gas measurements along polar ice cores provide important insight into the former composition of the atmosphere, its natural variations, and the responses to fast climatic changes in the past. We present high-resolution nitrous oxide (N2O) and methane (CH4) records measured along two ice cores from central Greenland covering part of Marine Isotope Stages 3 and 4 in the last glacial epoch. The N2O data confirm the hypothesis that N2O shows variations in phase to fast climatic changes observed in the Northern Hemisphere, the so-called Dansgaard-Oeschger (D-O) events. Variations exist not only for events with a long duration (1500 years and more) but also for the shorter ones. The comparison with CH4 unveils interesting differences between the response of CH4 and N2O to D-O events. While the average amplitudes of CH4 oscillations associated with D-O events are similar to those of the Northern Hemisphere summer insolation, the magnitude of the N2O concentration change instead correlates with the duration of the D-O events. The records give further insight into the timing of concentration changes at the beginning of D-O events. They show that for long-lasting events the N2O concentration starts to increase before both the sharp increase in the CH4 concentration and the temperature reconstructed for Greenland.
Villars Cave, Southwestern France Precise dating of Dansgaard-Oeschger climate oscillations in western Europe from stalagmite data
Genty et al., Nature
Volume 421, pp. 833 - 837 (20 February 2003).

The signature of Dansgaard-Oeschger events - millennial-scale abrupt climate oscillations during the last glacial period - is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of 14C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 ±0.6 and 67.4 ±0.9 kyr ago.

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8 December 2004