Black carbon (BC) is one of the major aerosol components with relatively high implications on climatic patterns through its radiative forcing (RF). South Asia has recently experienced an increased concentration of pollution; however, relatively fewer studies have been carried out on long-term assessment of BC and its implications. The present study analyzed the long-term concentration of BC in selected urban locations over South Asia using the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2). The study employed statistical analysis, including linear regression techniques, to assess the long-term concentration of BC. The results show that a rapid increase of BC is observed over most urban locations of South Asia with the predominance in winter and hence requires strict regional control measures to reduce the excess concentration of BC in the atmosphere. High concentration of BC in winter is attributed to anthropogenic activities and changes in meteorological conditions that enhance the accumulation of pollutants in the atmosphere. The relationship of BC with cloud top temperature and cloud effective radius demonstrates the direct and indirect effect of BC on cloud properties in this region. The RF results reveal that aerosol optical depth has positive aerosol RF in the atmosphere and negative RF at the top of the atmosphere (TOA) as well as at the bottom of the atmosphere (BOA). Negative RF at the TOA indicates less forcing efficiency due to fewer BC aerosols. On the other hand, averaging aerosol RF within the atmosphere reveals positive forcing, which suggests the efficiency force exerted by BC aerosols after absorbing solar radiation.

Permafrost is a potential mercury (Hg) pool released by thawing, which can raise the risk of Hg pollution under global warming. Tree rings are useful archives of environment-specific Hg exposure over long periods. We determined Hg concentrations in tree rings of two dominant tree species (Larix gmelinii Rupr. and Pinus sylvestris var. mongolica) at permafrost sites in northeastern China. The biweighted mean Hg concentrations ranged from 0.36 to 3.96 ng g(-1) from 1840 to 2014. The tree-ring width had no significant influence on the Hg concentration. Larch Hg increased slightly before the 1970s and peaked in the 1990s. However, the pine Hg concentration increased continuously until the 1930s, decreased rapidly until the 1970s, then rose to a peak in the late 1980s. The change of Hg concentrations in larch and pine revealed a time offset of 4 to 5 years, which implied possibly high mobility of Hg in pine tree rings. Higher Hg concentrations from 1920 to 1960 and subsequent decreases in isolated permafrost forests revealed the local geographical Hg cycling history. Lower Hg concentrations and faster increases in larch suggest the role of additional winter Hg loading for the evergreen pine and species-specific differences in root absorption in response to melting permafrost. Our results highlight possible geographical impacts on tree-ring Hg records, improve understanding of Hg cycles in permafrost forest, and suggest a need to sample additional species in a range of permafrost environments.

Study region: Upper Heihe River Basin, Northwest China. Study focus: We investigated potential climate change under three Representative Concentration Pathways (RCP 2.6, 4.5, and 8.5) and their impacts on frozen ground in the upper Heihe River Basin using the ensemble climate data from eight general circulation models and the Soil and Water Assessment Tool (SWAT). New hydrological insights for the region: Air and ground freezing indices declined significantly during the baseline period (1976-2015), whereas the thawing indices increased, indicating the heat accumulation in study area. The frost depth, which refers to the potential frost depth of active layer in permafrost areas and the maximum frost depth in seasonally frozen areas, decreased significantly at the rate of 3 cm/10 yr. The SWAT-simulation and gray relational analysis revealed that soil water was controlled by precipitation and frost depth in spring and autumn. Compared to that of the baseline, the projected frost depth is projected to decline by 0.07-0.1 m during the near future (2020-2059) and 0.08-0.36 m for the far future (2060-2099). In addition, we developed a long-term warning system, which indicates that the degree of frozen ground degradation would be mild during the near future and would be severe for the far future under RCP 8.5. This study provides valuable insights into the protection of frozen-ground in the Upper Heihe River Basin.

Estimates of the effective radiative forcing from aerosol-radiation interaction (ERFari) of anthropogenic Black Carbon (BC) have been disputable and require better constraints. Here we find a substantial decline in atmospheric absorption of -5.79Wm(-2)decade(-1) over eastern central China (ECC) responding to recent anthropogenic BC emission reductions. By combining the observational finding with advances from Coupled Model Intercomparison Project phase6 (CMIP6), we identify an emergent constraint on the ERFari of anthropogenic BC. We show that across CMIP6 models the simulated trends correlate well with simulated annual mean shortwave atmospheric absorption by anthropogenic BC over China. Making use of this emergent relationship allows us to constrain the aerosol absorption optical depth of anthropogenic BC and further provide a constrained range of 2.4-3.0 Wm(-2) for its top-of-atmosphere ERFari over China, higher than existing estimates. Our work supports a strong warming effect of BC over China, and highlights the need to improve BC simulations over source regions.

Long-term variations in aerosol optical properties, types, and radiative forcing over the Sichuan Basin (SCB) and surrounding regions in Southwest China were investigated based on two-decade data (2001-2020) from the Moderate Resolution Imaging Spectroradiometer, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, and the Santa Barbara DISORT Atmospheric Radiative Transfer model. The results showed that the aerosol optical depth (AOD550nm) in the SCB, a major polluted region in Southwest China, experienced an increasing tendency at a rate of +0.052 yr-1 during 2001-2006; thereafter, it decreased speedy up from -0.020 to -0.058 yr-1 over recent years, whereas the interannual variation in angstrom ngstrom exponent (AE470-660nm) presented a persistently increasing trend during 2001-2020, with a rate of +0.014 yr-1. An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB was observed. Over the polluted SCB region, the dominant aerosol types were biomass burning/urban industrial and mixedtype aerosols with the proportions of 80.7%-87.5% in regional aerosols, with a higher frequency of clean aerosols in recent years, reflecting an effect of controlling anthropogenic emission in the SCB owing to governmental regulation. By contrast, few changes were observed in the aerosol types and amounts in the eastern Tibetan Plateau (ETP), where clean continental aerosols dominate with high proportion of 93.7% in the clean atmospheric environment. A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB, resulting in the regional aerosol extinction coefficients at 532 nm (EC532nm) were declined by -0.22 km-1 from 2013 to 2020. Notably, the decreases in aerosol radiative forcing within the atmosphere were found in

Global warming has reduced the extent of permafrost, increased permafrost temperatures, and deepened the active layer across the Arctic. Permafrost degradation has detrimental effects on infrastructure and negative impacts on ecosystem services for many Arctic communities. This study examines the adaptive capacity for managing permafrost degradation in Northwest Greenland. The methods are based on questionnaire and interview data from fieldwork, frozen ground temperature records and published data forecasting the deepening of the active layer. Results illustrate the impact of permafrost degradation on the physical environment, hunting and harvesting, housing, and the economy in Northwest Greenland. House owners are mending damage caused by ground movement, and local institutions are concerned with the maintenance of roads and other public infrastructure impacted by permafrost. The scientific knowledge needed to inform decision-making is useful for identifying overall changes, but existing data sources are scarce, and more detailed permafrost maps are needed for long-term town planning. The study concludes that many individuals and institutions engage in autonomous adaptation on an ad hoc basis, rather than pursuing an overall strategy to increase the adaptive capacity in advance of future permafrost degradation in Northwest Greenland.

The Yangtze River and the Huang River are the two largest rivers in China. Annual runoff ratios (runoff/precipitation, denoted as RR) of the head regions of these two basins (HYR and HHR, respectively) have significantly decreased over the past several decades, closely related to changes in water storage capacity (WSC) and terrestrial water storage (TWS). However, such effects have rarely been quantified due to limitations associated with complicated arctic hydrological processes and the absence of long-term reliable TWS data. In this study, a TWS reconstruction dataset (TWSrec) was validated, and demonstrated good performance in capturing TWS variations derived from the Gravity Recovery and Climate Experiment (GRACE) and in the terrestrial water budget for these two head regions. Long-term (1980-2015) changes in TWS and WSC were then detected and their effects on RR were quantified through trend detection, change point analysis, and path analysis. Results showed that TWS increased significantly with a rate of 27.6 mm/10 yr and 19.8 mm/10 yr at HYR and HHR, respectively. These increases were mainly caused by wetting (increases in precipitation) or soil moisture increases from the TWS component perspective. WSC (represented as the ratio of TWS to precipitation) gradually enlarged in response to continuous climate warming. RR decreased significantly with rates of 2.0%/10 yr at HYR and 3.6%/10 yr at HHR, attributed to the increased evaporation ratio (similar to 80%) and increased WSC (similar to 20%) in both head regions. Further analysis suggested that permafrost degradation under climate warming could increase WSC. These results demonstrate that climate change has resulted in unstable terrestrial water storage at HYR and HHR, and that increases in WSC due to permafrost degradation play an important role in accurately simulating runoff in the Tibetan Plateau and other permafrost-degradation regions.

Temperature measurements in boreholes are the most common method allowing the quantitative and direct observation of permafrost evolution in the context of climate change. Existing boreholes and monitoring networks often emerged in a scientific context targeting different objectives and with different setups. A standardized, well-planned and robust instrumentation of boreholes for long-term operation is crucial to deliver comparable, high-quality data for scientific analyses and assessments. However, only a limited number of guidelines are available, particularly for mountain regions. In this paper, we discuss challenges and devise best practice recommendations for permafrost temperature measurements at single sites as well as in a network, based on two decades of experience gained in the framework of the Swiss Permafrost Monitoring Network PERMOS. These recommendations apply to permafrost observations in mountain regions, although many aspects also apply to polar lowlands. The main recommendations are (1) to thoroughly consider criteria for site selection based on the objective of the measurements as well as on preliminary studies and available data, (2) to define the sampling strategy during planification, (3) to engage experienced drilling teams who can cope with inhomogeneous and potentially unstable subsurface material, (4) to select standardized and robust instrumentation with high accuracy temperature sensors and excellent long-term stability when calibrated at 0 degrees C, ideally with double sensors at key depths for validation and substitution of questionable data, (5) to apply standardized maintenance procedures allowing maximum comparability and minimum data processing, (6) to implement regular data control procedures, and (7) to ensure remote data access allowing for rapid trouble shooting and timely reporting. Data gaps can be avoided by timely planning of replacement boreholes. Recommendations for standardized procedures regarding data quality documentation, processing and final publication will follow later.

Climate warming is causing permafrost thaw and there is an urgent need to understand the spatial distribution of permafrost and its potential changes with climate. This study developed a long-term (1901-2100), 1-km resolution daily meteorological dataset (Met1km) for modeling and mapping permafrost at high spatial resolutions in Canada. Met1km includes eight climate variables (daily minimum, maximum, and mean air temperatures, precipitation, vapor pressure, wind speed, solar radiation, and downward longwave radiation) and is suitable to drive process-based permafrost and other land-surface models. Met1km was developed based on four coarser gridded meteorological datasets for the historical period. Future values were developed using the output of a new Canadian regional climate model under medium-low and high emission scenarios. These datasets were downscaled to 1-km resolution using the re-baselining method based on the WorldClim2 dataset as spatial templates. We assessed Met1km by comparing it to climate station observations across Canada and a gridded monthly anomaly time-series dataset. The accuracy of Met1km is similar to or better than the four coarser gridded datasets. The errors in long-term averages and average seasonal patterns are small. The error occurs mainly in day-to-day fluctuations, thus the error decreases significantly when averaged over 5 to 10 days. Met1km, as a data generating system, is relatively small in data volume, flexible to use, and easy to update when new or improved source datasets are available. The method can also be used to generate similar datasets for other regions, even for the entire global landmass.

The Pan-Third Pole (PTP), stretching from Eastern Asia to Middle-central Europe, has experienced unprecedented accelerated warming and even retreat of glaciers. Absorbing aerosols reduce snow and ice albedo and radiative forcing, consequently enhancing a great melting of snow cover and ice sheet in the PTP. Employing the 10-year (2007-2016) space-based active and passive measurements, this study investigated the distribution, optical properties and decadal trends for dominating aerosols at a seasonal scale in the PTP divided into six subregions. Results showed that the sub-regions of PTP were mainly dominated by dust, polluted dust and elevated smoke. The Taklimakan Desert (TD) and the Iranian Plateau (IP) were dominated by mineral dust, accounting for 96% and 86% of the total aerosol extinction while the Central Europe (CE), Indo-China (IC) and Anatolia Plateau (AP) were dominated by the mixture of the dominating aerosol types. The mean aerosol extinction coefficient (MAEC) showed an obvious variability depending on the sub-regions and a tendency of decreasing with an increase in the topographic height. The strongest extinction layer (>0.1 km(-1)) mainly occurred below 4 km and the weak extinction layers (>0.001 km(-1)) were mainly distributed between 5 km and 8 km, indicating pronounced vertical transport in the region. The decadal trends of columnar aerosol optical depth (AOD) showed a relation with the contributions of the dominating aerosol types. For example, significant upward or downward trends of total aerosol loading in the IC region were driven by elevated smoke while the AOD trends of total aerosol loading for the CE, the AP and the IP were driven by the dominating aerosol types. The Tibetan Plateau (TP), the cleanest region in the PTP, has been regularly exposed to polluted air masses with significant amounts of absorbing aerosols. Therefore, understanding the dominating aerosol types, properties and decadal trends in the PTP region will contribute considerably to assessing their effects on radiative forcing, climate change, and even snowmelt and glacier retreat.