* Astronomy

A shift in tropical ocean circulations could explain a historical shift in global climate.

Peter deMenocal has been pondering the ancient natural history of eastern Africa for more than a decade, documenting evidence of desiccation as well as the gradual shift toward open savannahs, grass-eating fauna and the rise of the ancestors of modern humans. Now, the marine geologist at Columbia University in New York and his colleagues think they have pinned down a culprit for the shift in climate: the tropical oceans. deMenocal previewed his team's findings at the American Geophysical Union conference in Santa Fe last week.
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Ancient Hyperthermals

Bursts of intense global warming that have lasted tens of thousands of years have taken place more frequently throughout history than previously believed, according to evidence gathered by a team led by Scripps Institution of Oceanography, UC San Diego researchers.
Richard Norris, a professor of geology at Scripps who co-authored the report, said that releases of carbon dioxide sequestered in the deep oceans were the most likely trigger of these ancient "hyperthermal" events. Most of the events raised average global temperatures between 2° and 3° Celsius (3.6 and 5.4° F), an amount comparable to current conservative estimates of how much temperatures are expected to rise in coming decades as a consequence of anthropogenic global warming. Most hyperthermals lasted about 40,000 years before temperatures returned to normal.
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Geoscientists at Oregon State University have received a five year, $4.4 million grant from the National Science Foundation to lead an international consortium in developing ice core exploration technology that will provide a better understanding of past climate than ever before.
Working with 12 other institutions, the program may ultimately provide a continuous view of Earth's climate for the past 800,000 years, based upon analysis of dust and trace gases found in ancient ice cores taken from Antarctica and Greenland.
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Title: Geochronology and paleoclimatic implications of the last deglaciation 2 of the Mauna Kea Ice Cap, Hawaii
Authors: Anslow, Faron S., Clark, Peter U., Durz, Mark D., Hostetler, Steven W.

We present new 3He surface exposure ages on moraines and bedrock near the summit of Mauna Kea, Hawaii, which refine the age of the Mauna Kea Ice Cap during the Local Last Glacial Maximum (LLGM) and identify a subsequent fluctuation of the ice margin. The 3He ages, when combined with those reported previously, indicate that the local ice-cap margin began to retreat from its LLGM extent at 20.5 ± 2.5 ka, in agreement with the age of deglaciation determined from LLGM moraines elsewhere in the tropics. The ice-cap margin receded to a position at least 3 km upslope for ~5.1 kyr before readvancing nearly to its LLGM extent. The timing of this readvance at ~15.4 ka corresponds to a large reduction of the Atlantic meridional overturning circulation (AMOC) following Heinrich Event 1. Subsequent ice-margin retreat began at 14.6 ± 1.9 ka, corresponding to a rapid resumption of the AMOC and onset of the Bølling warm interval, with the ice cap melting rapidly to complete deglaciation. Additional 3He ages obtained from a flood deposit date the catastrophic outburst of a moraine-dammed lake roughly coeval with the Younger Dryas cold interval, suggesting a more active hydrological cycle on Mauna Kea at this time. A coupled mass balance and ice dynamics model is used to constrain the climate required to generate ice caps of LLGM and readvance sizes. The depression of the LLGM equilibrium line altitude requires atmospheric cooling of 4.5 ± 1 oC, whereas the mass balance modelling indicates an accompanying increase in precipitation of as much as three times that of present. We hypothesise (1) that the LLGM temperature depression was associated with global cooling, (2) that the temperature depression that contributed to the readvance occurred in response to an atmospheric teleconnection to the North Atlantic, and (3) that the precipitation enhancement associated with both events occurred in response to a southward shift in the position of the inter-tropical convergence zone (ITCZ). Such a shift in the ITCZ would have allowed midlatitude cyclones to reach Mauna Kea more frequently which would have increased precipitation at high elevations and caused additional cooling.

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