This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.

Abstract. Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30ø08'S, 69ø55'W) revealed a strong enrichment in the 2H and •80 content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of 82H and 8•80 help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2-4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.

A climate–glacier model was used to reconstruct Late-glacial climate conditions from two case-study glaciers at 18° and 22° S in the arid (sub)tropical western Andes of northern Chile. The model uses (i) the geometry of the Late-glacial maximum glaciation, (ii) modern diurnal and annual cycles, amplitudes and lapse rates of the climate, (iii) empirical–statistical sublimation, melt and accumulation models developed for this area, and (iv) dynamic ice flow through two known cross-sections for steady-state conditions. The model is validated with modern conditions and compares favorably with the glaciological features of today. The mass-balance model calculates the modern equilibrium-line altitude at 18° S as high as 5850 m (field data 5800 m), whereas no glaciers exist in the fully arid southern area at 22° S despite altitudes above 6000 m and continuous permafrost. For Late-glacial times, the model results suggest a substantial increase in tropical summer precipitation (ΔP = +840 (− 50/+ 10) mm a−1 for the northern test area; +1000 (− 10/+ 30) mm a−1 for the southern test area) and a moderate temperature depression (ΔT = −4.4 (− 0.1/+ 0.2) °C at 18° S; −3.2 (±0.1) °C at 22° S). Extratropical frontal winter precipitation (June–September) was <15% of the total annual precipitation. A scenario with higher winter precipitation from the westerlies circulation belt does not yield a numerical solution which matches the observed geometry of the glaciers. Therefore, we conclude that an equatorward displacement of the westerlies must be discarded as a possible explanation for the late Pleistocene glaciation in the Andes of northern Chile.

Abstract. In order to study the effects of postdepositional processes on snow chemistry a surface snow experiment was performed on the Cerro Tapado summit glacier in northern Chile, a subtropical glacier site presumably influenced by strong sublimation. Chemical species irreversibly trapped in the snow were significantly enriched in the surface layer by

The Central Andes play a pivotal role for glacier and climate reconstruction along the American Pole-Equator-Pole transect. Nevertheless, knowledge about late Quaternary palaeoenvironmental changes in this region is extremely limited. With the advent and application of surface exposure dating during the last few years, the establishment of more detailed glacial chronologies could provide important insights into forcings and mechanisms of glaciation and climate change. This paper reviews previously published exposure ages and compares them with independent age control on glacial chronologies and with information about the palaeohydrological conditions. Although available data are still very limited and there are remaining systematic uncertainties related to surface exposure dating, the following simplified palaeoglacial/palaeoclimate model is presented to serve as a testable hypothesis for future studies. (i) Glaciers in the humid tropical Andes were mainly temperature sensitive and therefore advanced during temperature minima. Advances are dated to 20-25 ka, $15 ka and 11-13 ka, i.e. synchronous to the global Last Glacial Maximum (LGM), the Heinrich event 1 and the Younger Dryas/Antarctic Cold Reversal. (ii) Further south and west, precipitation decreases and glaciers therefore become more precipitation sensitive. Maximum or at least prominent glacial advances as far south as 308 S are dated into the Lateglacial, i.e. synchronous with lake transgression phases (Tauca: 14-18 ka, and Coipasa: 11-13 ka). (iii) Between $30 and 408 S, glaciers reached their maximum extent much earlier during the pre-LGM ($35-40 ka). This is attributed to a northward shift and/or intensification of the westerlies, whereas conditions during the global LGM were too dry to allow for significant glacial advances. (iv) South of 408, glaciers become temperature sensitive again and reached their maximum accordingly synchronous to the global LGM.