Andrew B. G. Bush

The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance

Understanding the historical dynamics of forest communities is a critical element for accurate prediction of their response to future change. Here, we examine evergreen rainforest distribution in the Sunda Shelf region at the last glacial maximum (LGM), using a spatially explicit model incorporating geographic, paleoclimatic, and geologic evidence. Results indicate that at the LGM, Sundaland rainforests covered a substantially larger area than currently present. Extrapolation of the model over the past million years demonstrates that the current “island archipelago” setting in Sundaland is extremely unusual given the majority of its history and the dramatic biogeographic transitions caused by global deglaciation were rapid and brief. Compared with dominant glacial conditions, lowland forests were probably reduced from approximately 1.3 to 0.8 × 106 km2 while upland forests were probably reduced by half, from approximately 2.0 to 1.0 × 105 km2. Coastal mangrove and swamp forests experienced the most dramatic change during deglaciations, going through a complete and major biogeographic relocation. The Sundaland forest dynamics of fragmentation and contraction and subsequent expansion, driven by glacial cycles, occur in the opposite phase as those in the northern hemisphere and equatorial Africa, indicating that Sundaland evergreen rainforest communities are currently in a refugial stage. Widespread human-mediated reduction and conversion of these forests in their refugial stage, when most species are passing through significant population bottlenecks, strongly emphasizes the urgency of conservation and management efforts. Further research into the natural process of fragmentation and contraction during deglaciation is necessary to understand the long-term effect of human activity on forest species.

Late Holocene ,-..; 1500 yr climatic periodicities and their implications

Fourier and nonlinear regression analysis of a 4000+ yr paleoclimate proxy record in western Canada shows strong periodicities of ∼1500 yr and several weaker century- to millenial-scale periodicities. In conjunction with the 23 708 yr Milankovitch periodicity, these produce a model of climate fluctuation through the postglacial consistent with recognized paleoclimatic fluctuations of the past 15 000 yr in the northern mid-latitudes. These results suggest that postglacial climatic anomalies such as the Little Ice Age and the Younger Dryas were at least in part periodic phenomena rather than the result of unique, aperiodic events. Projecting these periodicities into the future suggests that even in the absence of anthropogenic climate forcing, a natural warming trend will continue until ca. a.d. 2400.

Global Land Ice Measurements from Space (GLIMS): remote sensing and GIS investigations of the Earth's cryosphere

Abstract Concerns over greenhouse‐gas forcing and global temperatures have initiated research into understanding climate forcing and associated Earth‐system responses. A significant component is the Earths cryosphere, as glacier‐related, feedback mechanisms govern atmospheric, hydrospheric and lithospheric response. Predicting the human and natural dimensions of climate‐induced environmental change requires global, regional and local information about ice‐mass distribution, volumes, and fluctuations. The Global Land‐Ice Measurements from Space (GLIMS) project is specifically designed to produce and augment baseline information to facilitate glacier‐change studies. This requires addressing numerous issues, including the generation of topographic information, anisotropic‐reflectance correction of satellite imagery, data fusion and spatial analysis, and GIS‐based modeling. Field and satellite investigations indicate that many small glaciers and glaciers in temperate regions are downwasting and retreating, although detailed mapping and assessment are still required to ascertain regional and global patterns of ice‐mass variations. Such remote sensing/GIS studies, coupled with field investigations, are vital for producing baseline information on glacier changes, and improving our understanding of the complex linkages between atmospheric, lithospheric, and glaciological processes.

The climate of the Last Glacial Maximum: Results from a coupled atmosphere-ocean general circulation model

Results from a coupled atmosphere-ocean general circulation model simulation of the Last Glacial Maximum reveal annual mean continental cooling between 48 and 78C over tropical landmasses, up to 268 of cooling over the Laurentide ice sheet, and a global mean temperature depression of 4.38C. The simulation incorporates glacial ice sheets, glacial land surface, reduced sea level, 21 ka orbital parameters, and decreased atmospheric CO2. Glacial winds, in addition to exhibiting anticyclonic circulations over the ice sheets themselves, show a strong cyclonic circulation over the northwest Atlantic basin, enhanced easterly flow over the tropical Pacific, and enhanced westerly flow over the Indian Ocean. Changes in equatorial winds are congruous with a westward shift in tropical convection, which leaves the western Pacific much drier than today but the Indonesian archipelago much wetter. Global mean specific humidity in the glacial climate is 10% less than today. Stronger Pacific easterlies increase the tilt of the tropical thermocline, increase the speed of the Equatorial Undercurrent, and increase the westward extent of the cold tongue, thereby depressing glacial sea surface temperatures in the western tropical Pacific by ;58 -6 8C.

Expanded and Recently Increased Glacier Surging in the Karakoram

Abstract A review of published literature and satellite imagery from the late 1960s onwards has revealed 90 surge-type glaciers in the Karakoram mountains, of which 50 have not previously been described in detail. These glaciers were identified by a number of surface features indicative of surge-type behavior such as looped moraines, rapid terminus advance, strandlines and rapid changes in surface crevassing. These observations indicate that surge-type behavior is more common and widespread than previously believed on Karakoram glaciers. There is strong spatial clustering of the surge-type glaciers, and a doubling in the number of new surges in the 14 years after 1990 (26 surges) than in the 14 years before 1990 (13 surges). This is coincident with a period of increased precipitation and positive glacier mass balance in this region, and supports previous studies which have found that mass balance has an important control on the frequency of glacier surging.

The Late Cretaceous: Simulation with a coupled atmosphere-ocean general circulation model

Results are presented for the climate of the late Cretaceous period (∼75–65 Ma) as simulated by a global climate model that is interactively coupled to a primitive equation global ocean model. Increased values of atmospheric CO2 and altered land surface albedos are invoked to produce the warm Cretaceous temperatures that have been proposed from biogeographic reconstructions. For comparison, a control simulation of the present climate is performed. The globally averaged atmospheric temperature in the Cretaceous simulation stabilizes after 20 years of integration at a value that is 4°C greater than that of the present day. The lower troposphere in high latitudes contributes a majority of the globally averaged warming as a result of the elimination of the Antarctic and Greenland ice sheets. Nevertheless, equatorial surface temperatures are raised by ∼5°C above those of the control simulation and offset somewhat the reduction in near-surface baroclinicity caused by the absence of the high-latitude ice sheets. In the Cretaceous simulation, global precipitation is approximately 10% greater than in the present day, with the only region of reduced precipitation occurring beneath the south Eurasian monsoon. Additionally, the amplitude of the seasonal cycle in near-surface temperatures is smaller in the Cretaceous and, in conjunction with increased mean annual temperatures, precludes the presence of any year-round snow or ice in the simulation. In high latitudes, however, there are regions that seasonally drop below freezing. The temperatures in these regions are warmer than have been previously observed in atmosphere-only simulations as a result of poleward heat transport by the oceans surface currents.

Glacier velocities across the central Karakoram

Abstract Optical matching of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite image pairs is used to determine the surface velocities of major glaciers across the central Karakoram. The ASTER images were acquired in 2006 and 2007, and cover a 60×120km region over Baltoro glacier, Pakistan, and areas to the north and west. The surface velocities were compared with differential global position system (GPS) data collected on Baltoro glacier in summer 2005. The ASTER measurements reveal fine details about ice dynamics in this region. For example, glaciers are found to be active over their termini even where they are very heavily debris-covered. The characteristics of several surge-type glaciers were measured, with terminus advances of several hundred meters per year and the displacement of trunk glaciers as surge glaciers pushed into them. This study is the first synthesis of glacier velocities across this region, and provides a baseline against which both past and future changes can be compared.

Assessing the impact of Mid-Holocene insolation on the atmosphere-ocean system

A sequence of numerical simulations was performed in order to assess the role of early- to mid-Holocene orbital forcing on the coupled atmosphere-ocean system. Results from both an atmosphere-only general circulation model (GCM) forced by specified sea surface temperatures and a coupled atmosphere-ocean GCM consistently suggest a stronger south Asian monsoon and a strengthened Pacific Walker circulation. The latter feature interacts dynamically with the equatorial ocean in the coupled model to produce enhanced Pacific upwelling, a more pronounced cold tongue, and an even stronger monsoon. Results suggest that the climate of the equatorial Pacific was more similar to the La Nina phase of the modern Southern Oscillation rather than the El Niiio phase.

Tropopause Folds and Synoptic-Scale Baroclinic Wave Life Cycles

Abstract The linear stability of three midlatitude zonal mean states of varying baroclinicity and barotropy is examined using a primitive equation stability analysis under the anelastic approximation. Each of these basic states includes both tropospheric and stratospheric regions with realistic properties compared with those of the earths atmosphere in midlatitudes. In all three cases, the fastest growing synoptic-scale modes delivered by the linear stability analysis have wavelengths of approximately 4000 km. The nonlinear evolution of these synoptic-scale modes is explored using a three-dimensional anelastic finite-difference model whose zonal scale is chosen to be equal to the wavelength of the fastest growing mode of linear theory. During the nonlinear evolution of the wave, deep tropopause folds are shown to form (generically) and the depth to which the fold penetrates the troposphere is seen to increase with the baroclinicity of the mean state. These folds, descending along the sloping frontal zone...

A comparison of simulated monsoon circulations and snow accumulation in Asia during the mid-Holocene and at the Last Glacial Maximum

Abstract Tropical climatology through the last glacial cycle is believed to have ranged from colder, windier conditions at the Last Glacial Maximum (LGM) to relatively warm, stable conditions during the Holocene. Changes in strength of the South Asian monsoon have previously been determined from a variety of proxy data and have been attributed primarily to changes in radiative forcing, although tropical sea surface temperature (SST) is known to play a fundamental role in regulating monsoon strength and is also believed to have changed throughout the late Quaternary. In this study, the monsoons simulated in a coupled atmosphere–ocean general circulation model (GCM) configured for the mid-Holocene (6000 years B.P.) and for the LGM (21,000 years B.P.) are compared. The colder and windier conditions simulated for the LGM produced a summer monsoon whose westerly winds are stronger and whose precipitation and snowfall into the eastern Himalaya are increased, with drier conditions over the rest of the Indian subcontinent and over most of southwest Asia. The mid-Holocene monsoon circulation is stronger than today, and annual mean snow accumulation is increased over the northwestern Himalaya. These changes in precipitation and snow accumulation are analyzed in terms of the altered atmospheric circulations, which are in turn driven by changes in radiative forcing, sea surface temperatures, and sea surface height. All of these factors are therefore demonstrated to be important in governing the spatial distribution of snow and ice deposition in the Himalaya during the late Quaternary, and are likely to have contributed to the observed asynchroneity of Himalayan glaciation and Northern Hemisphere ice sheet volume.