Kam-biu Liu

Late Glacial Stage and Holocene Tropical Ice Core Records from Huascarán, Peru

Two ice cores from the col of Huascar�n in the north-central Andes of Peru contain a paleoclimatic history extending well into the Wisconsinan (W�rm) Glacial Stage and include evidence of the Younger Dryas cool phase. Glacial stage conditions at high elevations in the tropics appear to have been as much as 8� to 12�C cooler than today, the atmosphere contained about 200 times as much dust, and the Amazon Basin forest cover may have been much less extensive. Differences in both the oxygen isotope ratio ζ18O (8 per mil) and the deuterium excess (4.5 per mil) from the Late Glacial Stage to the Holocene are comparable with polar ice core records. These data imply that the tropical Atlantic was possibly 5� to 6�C cooler during the Late Glacial Stage, that the climate was warmest from 8400 to 5200 years before present, and that it cooled gradually, culminating with the Little Ice Age (200 to 500 years before present). A strong warming has dominated the last two centuries.

Earliest domestication of common millet (Panicum miliaceum) in East Asia extended to 10,000 years ago

The origin of millet from Neolithic China has generally been accepted, but it remains unknown whether common millet (Panicum miliaceum) or foxtail millet (Setaria italica) was the first species domesticated. Nor do we know the timing of their domestication and their routes of dispersal. Here, we report the discovery of husk phytoliths and biomolecular components identifiable solely as common millet from newly excavated storage pits at the Neolithic Cishan site, China, dated to between ca. 10,300 and ca. 8,700 calibrated years before present (cal yr BP). After ca. 8,700 cal yr BP, the grain crops began to contain a small quantity of foxtail millet. Our research reveals that the common millet was the earliest dry farming crop in East Asia, which is probably attributed to its excellent resistance to drought.

Lake-sediment record of late Holocene hurricane activities from coastal Alabama

Coastal lake sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late Holocene time. Frederic—a category 3 hurricane that struck the Alabama coast on the Gulf of Mexico in 1979—left a distinct sand layer in the nearshore sediments of Lake Shelby as a result of storm-tide overwash of beaches and dunes. Sediment cores taken from the center of Lake Shelby contain multiple sand layers, suggesting that major hurricanes of category 4 or 5 intensity directly struck the Alabama coast at ca. 3.2-3.0, 2.6, 2.2, 1.4, and 0.8 ka ( 14 C yr), with an average recurrence interval of ∼600 yr. The Alabama coast is likely to be struck by a category 4 or 5 hurricane within the next century.

Late pleistocene temperature depression and vegetation change in Ecuadorian Amazonia

Abstract Paleoecological (pollen, phytolith, and wood) analyses of sediments, radiocarbon dated 33,000 to 26,000 yr B.P., from two sites in Ecuadorian Amazonia provide data that suggest a cooling of ca. 7.5°C below present in equatorial lowlands from 33,000 to 30,000 yr B.P. A period of warning followed in which novel species assemblages, a blend of montane and lowland floral components, persisted for at least 4000 years. These data of forest community change, from sites lying within the postulated glacial rain forest Napo refugium, provide the strongest paleoecological refutation of the refugial hypothesis yet obtained. The large temperature depression at ca. 30,000 yr B.P. allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum (LGM) of the northern hemisphere, freezing temperatures would have been experienced in parts of lowland Amazonia between 25,000 and 18,000 B.P.

A pollen record of Holocene climatic changes from the Dunde ice cap, Qinghai-Tibetan Plateau

Pollen preserved in ice cores from the Dunde ice cap provides a sensitive record of Holocene climatic changes and vegetational response in the northern Qinghai-Tibetan Plateau at time scales ranging from millennia to centuries and decades. Pollen analysis of the annually resolvable ice layers for a 30 yr period (1957‐1986) suggests that total pollen concentration is correlated positively with summer precipitation and negatively with summer temperature; thus it is a sensitive indicator of moisture availability and vegetation density in the steppe and desert regions around Dunde. High pollen concentrations between 10 000 and 4800 yr B.P. suggest that the summer monsoon probably extended beyond its present limit to reach Dunde and westernmost Tibet in response to orbital forcing. The summer monsoon retreated time-transgressively across the Qinghai-Tibetan Plateau during the middle Holocene. Relatively humid periods occurred at 2700‐2200, 1500‐800, and 600‐80 yr B.P., probably as a result of neoglacial cooling. Prominent pollen changes during the Medieval Warm Period (790‐620 yr B.P.) and the Little Ice Age (330‐80 yr B.P.) suggest that the vegetation in the Qinghai-Tibetan Plateau region is sensitive to abrupt, century-scale climatic changes, such as those anticipated in scenarios of greenhouse warming.

Spatial Variations in Major U.S. Hurricane Activity: Statistics and a Physical Mechanism

The authors provide a statistical and physical basis for understanding regional variations in major hurricane activity along the U.S. coastline on long timescales. Current statistical models of hurricane activity are focused on the frequency of events over the entire North Atlantic basin. The exception is the lead author’s previous work, which models the occurrence of hurricanes over the Caribbean Sea, Gulf of Mexico, and the southeast U.S. coast separately. Here the authors use statistics to analyze data from historical and paleoclimatic records to expand this work. In particular, an inverse correlation in major hurricane activity across latitudes at various timescales is articulated. When activity is above normal at high latitudes it tends to be below normal at low latitudes and vice versa. Past research, paleoclimatic records, and historical data hint at the potential of using the North Atlantic oscillation (NAO) as an indicator of where storms will likely track over long timescales. An excited (relaxed) NAO is associated with higher (lower) latitude recurving (nonrecurving) storms. The Gulf (East) Coast is more susceptible to a major hurricane strike during a relaxed (excited) NAO.

Environmental change in the Yangtze River delta since 12,000 years B.P.

Abstract A 52-m core from Qidong at the tip of the Yangtze River (Chang Jiang) delta provides a history of sea-level change, deltaic development, and vegetational and climatic changes during the last 12,000 yr. About 12,000 yr ago, when sea level was about 60 m below the present level, the coring site was situated in the innermost part of the exposed continental shelf. The late-Pleistocene vegetation on the uplands of the Lower Yangtze River valley was a mixed forest of deciduous and broadleaved evergreen trees in which Betula, Ulmus, Tsuga, and Cupressaceae were slightly more abundant than at present. Abies and Picea were probably present as relict populations on mountains bordering the region. Rapidly rising sea level converted the Qidong area to a coastal or estuarine environment between 11,000 and 10,800 yr B.P., leading to widespread development of wetlands dominated by Gramineae and Cyperaceae. During the next 2500 yr coastal erosion due to marine transgression obliterated the sedimentary record. Deltaic sedimentation predominated from 8300 to 3800 yr B.P., as the Yangtze River delta prograded by successively building a series of estuarine sand bars. During the mid-Holocene, the climate was slightly warmer and more humid than at present, allowing the subtropical broadleaved evergreen trees to increase their populations. Pinus and Quercus became more abundant after 3800 yr B.P. in response to climatic cooling. The present deltaic plain in Qidong formed less than 200 yr ago.

Temperature depression in the Lowland Tropics in glacial times

Equatorial air temperatures at low elevations in the New World tropics are shown by pollen and other data to have been significantly lowered in long intervals of the last glaciation. These new data show that long recognized evidence for cooling at high elevations in the tropics were symptomatic of general tropical cooling and that they did not require appeal to altered lapse rates or other special mechanisms to be made to conform with conclusions that equatorial sea surface temperatures (SSTs) were scarcely changed in glacial times. The new data should be read in conjunction with recent findings that Caribbean (SSTs) were lowered in the order of 5 ° C, contrary to previous interpretations. Thus these accumulating data show that low latitudes as well as high were cooled in glaciations. In part the earlier failure to find evidence of low elevation cooling in the lowland tropics resulted from the data being masked by strong signals for aridity given by old lake levels in parts of Africa and elsewhere. Global circulation models used to predict future effects of greenhouse warming must also be able to simulate the significant cooling of the large tropical land masses at glacial times with reduced greenhouse gas concentrations. Plants and animals of the Amazon forest and similar ecosystems are able to survive in wide ranges of temperatures, CO2 concentrations, and disturbance, though associations change constantly.

A 1,000‐Year History of Typhoon Landfalls in Guangdong, Southern China, Reconstructed from Chinese Historical Documentary Records

In China, the abundance of historical documentary records in the form of Fang Zhi (semiofficial local gazettes) offers an extraordinary opportunity for providing a high-resolution historical dataset for the frequency of typhoon strikes. We have reconstructed a 1,000-year time series of typhoon landfalls in the Guangdong Province of southern China since AD 975 based on data compiled from Fang Zhi. Even though the 571 typhoon strikes recorded in the historical documents probably underrepresent the total number of typhoon landfalls in Guangdong, calibration of the historical data against the observations during the instrumental period 1884–1909 suggests that the trends of the two datasets are significantly correlated (r= 0.71), confirming that the time series reconstructed from historical documentary evidence contains a reliable record of variability in typhoon landfalls. On a decadal timescale, the twenty-year interval from AD 1660 to 1680 is the most active period on record, with twenty-eight to thirty-seven typhoon landfalls per decade. The variability in typhoon landfalls in Guangdong mimics that observed in other paleoclimatic proxies (e.g., tree rings, ice cores) from China and the northern hemisphere. Remarkably, the two periods of most frequent typhoon strikes in Guangdong (AD 1660–1680, 1850–1880) coincide with two of the coldest and driest periods in northern and central China during the Little Ice Age. Conceivably, the predominant storm tracks shifted to the south during these cold periods, resulting in fewer landfalls in Japan and the east-central Chinese coast but more typhoons hitting Guangdong. Spectral analysis of the Guangdong time series reveals an approximately fifty-year cycle in typhoon landfall frequency. While the physical mechanism remains to be identified, it is tempting to relate this periodicity to the pentadecadal variability identified in the North Pacific Index (NPI) time series.

Quaternary history of the temperate forests of China

Abstract Pollen data from 80 sites in North China and Northeast China are reviewed to document the Quaternary history of the deciduous forest and the temperate mixed conifer-hardwood forest of China. During the Late Tertiary the forest in North China consisted of an admixture of temperate deciduous hardwoods, subtropical broadleaved evergreen elements, and ancient conifers of tropical and subtropical affinities. Pollen evidence from long boreholes through Quaternary deposits indicates four or five glacial/interglacial climatic cycles. The first glacial episode, known as the Nangou Cold Period, led to the development of a spruce-fir forest of no modern analogue in North China. Deciduous forest was re-established in North China during the intervening interglacials. The subtropical evergreen elements and Tertiary relicts were successively eliminated during the subsequent glacials. In the last glacial, spruce-fir forests which are now confined to mountain slopes above 1500 m descended to the lowlands of North and Northeast China, implying a temperature depression of at least 8–10°C. Cold steppe occurred at least locally near Beijing in the last glacial maximum, but the data base is insufficient to delimit the spatial and temporal extent of this and other paleovegetation types on a regional scale, or to reconstruct the locations of glacial refugia and the dynamics of plant migration. The Holocene pollen stratigraphies suggest a tripartite division, with a period of maximum warmth, the Hypsithermal, in the mid-Holocene. In both North and Northeast China, the forest became more diverse during the mid-Holocene when thermophilous hardwoods expanded at the expense of pine and, in more southerly locations, birch. Unequivocal evidence for intensified summer monsoon during the early Holocene, as predicted by the Kutzbach model based on orbital parameter changes, remains to be found from the pollen records of the temperate forest regions of China.