The changes in near-surface soil freeze-thaw cycles (FTCs) are crucial to understanding the related hydrological and biological processes in terrestrial ecosystems under a changing climate. However, long-termdynamics of soil FTCs at the hemisphere scale and the underlying mechanisms are not well understood. In this study, the spatiotemporal patterns and main driving factors of soil FTCs across the Northern Hemisphere (NH) during 1979-2017 were analyzed using multisource data fusion and attribution approaches. Our results showed that the duration and the annual mean area of frozen soil in the NH decreased significantly at rates of 0.13 +/- 0.04 days/year and 4.9 x 104 km(2)/year, respectively, over the past 40 years. Theseweremainly because the date of frozen soil onset was significantly delayed by 0.1 +/- 0.02 days/year, while the end of freezing and onset of thawing were substantially advanced by 0.21 +/- 0.02 and 0.15 +/- 0.03 days/year, respectively. Moreover, the interannual FTC changes were more drastic in Eurasia than in North America, especially at mid-latitudes (30 degrees-45 degrees N) and in Arctic regions (>75 degrees N). More importantly, our results highlighted that near-surface air temperature (T-a) and snowpack are the main driving factors of the spatiotemporal variations in soil FTCs. Furthermore, our results suggested that the long-term dynamics of soil FTCs at the hemisphere scale should be considered in terrestrial biosphere models to reduce uncertainties in future simulations.

Accurate estimation of snow mass or snow water equivalent (SWE) over space and time is required for global and regional predictions of the effects of climate change. This work investigates whether integration of remotely sensed terrestrial water storage (TWS) information, which is derived from the Gravity Recovery and Climate Experiment (GRACE), can improve SWE and streamflow simulations within a semi-distributed hydrology land surface model. A data assimilation (DA) framework was developed to combine TWS observations with the MESH (Modelisation Environnementale Communautaire - Surface Hydrology) model using an ensemble Kalman smoother (EnKS). The snow-dominated Liard Basin was selected as a case study. The proposed assimilation methodology reduced bias of monthly SWE simulations at the basin scale by 17.5% and improved unbiased root-mean-square difference (ubRMSD) by 23%. At the grid scale, the DA method improved ubRMSD values and correlation coefficients for 85% and 97% of the grid cells, respectively. Effects of GRACE DA on streamflow simulations were evaluated against observations from three river gauges, where it effectively improved the simulation of high flows during snowmelt season from April to June. The influence of GRACE DA on the total flow volume and low flows was found to be variable. In general, the use of GRACE observations in the assimilation framework not only improved the simulation of SWE, but also effectively influenced streamflow simulations.

Black carbon (BC) deposited on snow lowers its albedo, potentially contributing to warming in the Arctic. Atmospheric distributions of BC and inorganic aerosols, which contribute directly and indirectly to radiative forcing, are also greatly influenced by depositions. To quantify these effects, accurate measurement of the spatial distributions of BC and ionic species representative of inorganic aerosols (ionic species hereafter) in snowpack in various regions of the Arctic is needed, but few such measurements are available. We measured mass concentrations of size-resolved BC (C-MBC) and ionic species in snowpack by using a single-particle soot photometer and ion chromatography, respectively, over Finland, Alaska, Siberia, Greenland, and Spitsbergen during early spring in 2012-2016. Total BC mass deposited per unit area (DEPMBC) during snow accumulation periods was derived from C-MBC and snow water equivalent (SWE). Our analyses showed that the spatial distributions of anthropogenic BC emission flux, total precipitable water, and topography strongly influenced latitudinal variations of C-MBC, BC size distributions, SWE, and DEPMBC. The average size distributions of BC in Arctic snowpack shifted to smaller sizes with decreasing C-MBC due to an increase in the removal efficiency of larger BC particles during transport from major sources. Our measurements of C-MBC were lower by a factor of 13 than previous measurements made with an Integrating Sphere/Integrating Sandwich spectrophotometer due mainly to interference from coexisting non-BC particles such as mineral dust. The SP2 data presented here will be useful for constraining climate models that estimate the effects of BC on the Arctic climate. Plain Language Summary Black carbon (BC) particles, commonly known as soot, are emitted from incomplete combustion of fossil fuels and biomass. They efficiently absorb solar radiation and thus heat the atmosphere. BC particles emitted at midlatitudes and in the Arctic are deposited onto snow in the Arctic, accelerating snowmelt in early spring by absorbing solar radiation. These processes contribute to warming in the Arctic. Calculations of this warming effect by using numerical models need to be validated by comparison with observed BC concentrations in snowpack. However, there are very few accurate records of concentrations of BC in snow because of technical difficulties in making these measurements. We developed a new laser-induced incandescence technique to measure BC concentrations in snowpack and applied it for the first time in six Arctic regions (Finland, Alaska, North and South Siberia, Greenland, and Spitsbergen). The BC concentrations we measured were highest in Finland and South Siberia, which are closer to large anthropogenic BC sources than the other regions, where our measured BC concentrations were much lower. On average, our BC concentrations were much lower than those previously measured by different techniques. Therefore, previous comparisons of modeled and observed BC concentrations need to be re-evaluated using the present data.

Climate change is an acknowledged phenomenon. Even so, its consequences are not easily predictable. Lakes in Lakes Region of southwestern Turkey have been shrinking. Aksehir Lake, located on important bird migration routes, is one of those aforementioned lakes that has continually shrunk until completely drying up in 2008. This study aims to investigate the variation of meteorological and hydrological parameters during the shrinking and drying up of Lake Aksehir. Previous studies were mainly related with coastline changes of Aksehir Lake and attributed the changes to increased air temperatures and evaporation in conjunction with reduced precipitation and decreased surface flow. In this study, snow dynamics, both snow cover (SC) extent and duration besides snow water equivalent (SWE) are also investigated. Moreover, the inclusion of soil moisture (SM) data is additions to the current literature. SC, SWE and SM data obtained from satellite images recorded over the study area indicated that SC both in extent and duration was smallest during the 2008 winter-the same year in which the lake totally dried. SWE and SM values were also lowest during the study period. These were in agreement with the highest recorded air temperatures and reduced precipitation with respect to long-term averages over the study period. Recorded high evaporations above the long-term averages might have intensified recession which eventually resulted in drying of lake in 2008.

In this study, the period that corresponds to the threshold of a 1.5 degrees C rise (relative to 1861-1880) in surface temperature is validated using a multi-model ensemble mean from 17 global climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). On this basis, the changes in permafrost and snow cover in the Northern Hemisphere are investigated under a scenario in which the global surface temperature has risen by 1.5 degrees C, and the uncertainties of the results are further discussed. The results show that the threshold of 1.5 degrees C warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively. When the global average surface temperature rises by 1.5 degrees C, the southern boundary of the permafrost will move 1-3.5 degrees northward (relative to 1986-2005), particularly in the southern Central Siberian Plateau. The permafrost area will be reduced by 3.43 x 10(6) km(2) (21.12%), 3.91 x 10(6) km(2) (24.1%) and 4.15 x 10(6) km(2) (25.55%) relative to 1986-2005 in RCP2.6, RCP4.5 and RCP8.5, respectively. The snow water equivalent will decrease in over half of the regions in the Northern Hemisphere but increase only slightly in the Central Siberian Plateau. The snow water equivalent will decrease significantly (more than 40% relative to 1986-2005) in central North America, western Europe, and northwestern Russia. The permafrost area in the QinghaieTibet Plateau will decrease by 0.15 x 10(6) km(2) (7.28%), 0.18 x 10(6) km(2) (8.74%), and 0.17 x 10(6) km(2) (8.25%), respectively, in RCP2.6, RCP4.5, RCP8.5. The snow water equivalent in winter (DJF) and spring (MAM) over the QinghaieTibet Plateau will decrease by 14.9% and 13.8%, respectively.