Glacial retreat due to global warming is exposing large tracts of barren glacial sediments that are quickly colonized by CO2-fixing microbial communities that can constitute the climax community in many high-Arctic, alpine, and Antarctic environments. Despite the potential importance of these processes, little is known about microbial community successional dynamics and rates of carbon (C) sequestration in environments where higher plants are slow or unable to establish. We analyzed microbial community succession and C and N accumulation in newly exposed sediments along an Antarctic glacial chronosequence where moss and microbial autotrophs are the dominant primary producers. During the first 4 years of succession (0 to 40 m from the glacier) algae (including diatoms) were the most relatively abundant eukaryotes, but by the second phase studied (8 to 12 years) moss amplicon sequence variants (ASVs) dominated. The rise in moss coincided with a significant buildup of C and N in the sediments. The final two phases of the successional sequence (16 to 20 and 26 to 30 years) were marked by declines in microbial species richness and moss relative abundance, that coincided with significant decreases in both total C and N. These retrogressive declines coincided with a large increase in relative abundance of predatory Vampyrellidae suggesting a possible mechanism for retrogression in this and perhaps other terrestrial ecosystems at the edge of the cryosphere. These findings have implications for understanding CO2 sequestration and ecosystem succession in microbial-dominated regions of the cryobiosphere where large tracts of land are currently undergoing deglaciation.

We review recent climate changes over the Tibetan Plateau (TP) and associated responses of cryospheric, biospheric, and hydrological variables. We focused on surface air temperature, precipitation, seasonal snow cover, mountain glaciers, permafrost, freshwater ice cover, lakes, streamflow, and biological system changes. TP is getting warmer and wetter, and air temperature has increased significantly, particularly since the 1980s. Most significant warming trends have occurred in the northern TP. Slight increases in precipitation have occurred over the entire TP with clear spatial variability. Intensification of surface air temperature is associated with variation in precipitation and decreases in snow cover depth, spatial extent, and persistence. Rising surface temperatures have caused recession of glaciers, permafrost thawing, and thickening of the active layers over the permafrost. Changing temperatures, precipitation, and other climate system components have also affected the TP biological system. In addition, elevation-dependent changes in air temperature, wind speed, and summer precipitation have occurred in the TP and its surroundings in the past three decades. Before projecting multifaceted interactions and process responses to future climate change, further quantitative analysis and understanding of the change mechanisms is required.

Purpose of Review We assess the current understanding of the state and behaviour of aerosols under pre-industrial conditions and the importance for climate. Recent Findings Studies show that the magnitude of anthropogenic aerosol radiative forcing over the industrial period calculated by climate models is strongly affected by the abundance and properties of aerosols in the pre-industrial atmosphere. The low concentration of aerosol particles under relatively pristine conditions means that global mean cloud albedo may have been twice as sensitive to changes in natural aerosol emissions under preindustrial conditions compared to present-day conditions. Consequently, the discovery of new aerosol formation processes and revisions to aerosol emissions have large effects on simulated historical aerosol radiative forcing. Summary We review what is known about the microphysical, chemical, and radiative properties of aerosols in the pre-industrial atmosphere and the processes that control them. Aerosol properties were controlled by a combination of natural emissions, modification of the natural emissions by human activities such as land-use change, and anthropogenic emissions from biofuel combustion and early industrial processes. Although aerosol concentrations were lower in the pre-industrial atmosphere than today, model simulations show that relatively high aerosol concentrations could have been maintained over continental regions due to biogenically controlled new particle formation and wildfires. Despite the importance of preindustrial aerosols for historical climate change, the relevant processes and emissions are given relatively little consideration in climate models, and there have been very few attempts to evaluate them. Consequently, we have very low confidence in the ability of models to simulate the aerosol conditions that form the baseline for historical climate simulations. Nevertheless, it is clear that the 1850s should be regarded as an early industrial reference period, and the aerosol forcing calculated from this period is smaller than the forcing since 1750. Improvements in historical reconstructions of natural and early anthropogenic emissions, exploitation of new Earth system models, and a deeper understanding and evaluation of the controlling processes are key aspects to reducing uncertainties in future.

Seasonal distinctiveness in the microphysical and optical properties of columnar and near-surface (in the well mixed region) aerosols, associated with changes in the prevailing synoptic conditions, were delineated based on extensive (spread over 4 years) and collocated measurements at the tropical coastal location, Trivandrum (8.55 degrees N; 76.97 degrees E, 3 m a.m.s.l.), and the results were summarized in Part 1 of this two-part paper. In Part 2, we use these properties to develop empirical seasonal aerosol models, which represent the observed features fairly accurately, separately for winter monsoon season (WMS, December through March), inter-monsoon season (IMS, April and May), summer monsoon season (SMS, June through September) and post monsoon season (PMS, October and November). The models indicate a significant transformation in the aerosol environment from an anthropogenic-dominance in WMS to a natural-dominance in SMS. The modeled aerosol properties are used for estimating the direct, short wave aerosol radiative forcing, under clear-sky conditions. Our estimates show large seasonal changes. Under clear sky conditions, the daily averaged short-wave TOA forcing changes from its highest values during WMS, to the lowest values in SMS; this seasonal change being brought-in mainly by the reduction in the abundance and the mass fraction (to the composite) of black carbon aerosols and of accumulation mode aerosols. The resulting atmospheric forcing varies from the highest, (47 to 53 W m(-2)) in WMS to the lowest (22 to 26 W m(-2)) in SMS.

In Part 1 of this two-part paper, we present the results of extensive and collocated measurements of the columnar and near-surface (in the well mixed region) properties of atmospheric aerosol particles at a tropical coastal location, Trivandrum (8.55 degrees N; 76.97 degrees E), located close to the southwest tip of Indian peninsula. These are used to evolve average, climatological pictures of the optical and microphysical properties and to delineate the distinct changes associated with the contrasting monsoon seasons as well as the transition from one season to the other. Our observations show a dramatic change in the columnar aerosol optical depth (AOD) spectra, being steep during winter monsoon season (WMS, months of December through March) and becoming quite flat during summer monsoon season (SMS, June through September). The derived angstrom angstrom exponent (alpha) decreases from a mean value of 1.1 +/- 0.03 in WMS to 0.32 +/- 0.02 in SMS, signifying a change in columnar aerosol size spectrum from an accumulation mode dominance in WMS to a coarse mode dominance in SMS. The composite aerosols near the surface follow suit with the share of the accumulation mode to the total mass concentration decreasing from similar to 70% to 34% from WMS to SMS. The overall mass burden also decreases in tandem. The changes in alpha are well correlated to those in the accumulation fraction of the mass concentration. Long-term measurements of the concentration of aerosol black carbon (BC), show prominent annual variations, with its mean value decreasing from as high as 6 mu gm(-3) in WMS to 2 mu gm(-3) in SMS. Correspondingly, its mass mixing ratio to the composite aerosols comes down from 11% to 4%. The changes in AOD and alpha are significantly positively correlated to those of BC concentration. The columnar properties are, in general well associated with the features near the surface. The implications of these changes to the optical properties and single scattering albedo and the resulting impact on direct radiative forcing are examined in the companion paper (Part 2).

ALBIOC (ALbedo- BIOsphere- Carbon) is an integrated terrestrial biosphere model designed as a too] to explore the effects of climate and atmospheric CO, concentration on vegetation, land-surface characteristics and carbon storage. The model is based, although designed to be simple in structure and computationally fast, on biophysical and ecophysiological principles and simulates in a fully interactive manner the potential distribution of vegetation, terrestrial carbon storage and physical land-surface properties. Testing was extensive and focused on broad spatial patterns (5 degrees resolution) of biome distribution, and variables important for the surface energy balance and hydrological cycle (seasonal snow cover, surface albedo, runoff and evaporation) and for the global carbon cycle (seasonal canopy cover, primary production and carbon storage). Because ALBIOC simulates a range of physical and biogeochemical variables in an integrated way, it was possible to test the model against a more comprehensive range of indicators than has normally been the case for terrestrial biosphere models. The simulated vegetation distribution is as accurate as more specialised biogeography models taking into account the coarse resolution of the model. ALBIOC simulates a global NPP of 57 PgC/year, which is in the range of the values found in the literature and other model estimates. Land-surface albedo. snow depth, runoff, and FPAR showed a generally good agreement with observations within the known limits of available data sets of these variables. The model's mechanistic basis would allow extension to simulate, e.g. transient response to rapid climate change (vegetation dynamics) and carbon isotopic balances. while its computational efficiency renders it suitable for inclusion in Earth system models of intermediate complexity. (C) 2001 Elsevier Science B.V. All rights reserved.