Precipitation comes in various phases, including rainfall, snowfall, sleet, and hail. Shifts of precipitation phases, as well as changes in precipitation amount, intensity, and frequency, have significant impacts on regional climate, hydrology, ecology, and the energy balance of the land-atmosphere system. Over the past century, certain progress has been achieved in aspects such as the observation, discrimination, transformation, and impact of precipitation phases. Mainly including: since the 1980s, studies on the observation, formation mechanism, and prediction of precipitation phases have gradually received greater attention and reached a certain scale. The estimation of different precipitation phases using new detection theories and methods has become a research focus. A variety of discrimination methods or schemes, such as the potential thickness threshold method of the air layer, the temperature threshold method of the characteristic layer, and the near-surface air temperature threshold method, have emerged one after another. Meanwhile, comparative studies on the discrimination accuracy and applicability assessment of multiple methods or schemes have also been carried out simultaneously. In recent years, the shift of precipitation from solid to liquid (SPSL) in the mid-to-high latitudes of the Northern Hemisphere has become more pronounced due to global warming and human activities. It leads to an increase in rain-on-snow (ROS) events and avalanche disasters, affecting the speed, intensity, and duration of spring snow-melting, accelerating sea ice and glacier melting, releasing carbon from permafrost, altering soil moisture, productivity, and phenological characteristics of ecosystems, and thereby affecting their structures, processes, qualities, and service functions. Although some progress has been made in the study of precipitation phases, there remains considerable research potential in terms of completeness of basic data, reliability of discrimination schemes, and the mechanistic understanding of the interaction between SPSL and other elements or systems. The study on shifts of precipitation phases and their impacts will play an increasingly important role in assessing the impacts of global climate change, water cycle processes, water resources management, snow and ice processes, snow and ice-related disasters, carbon emissions from permafrost, and ecosystem safety.

Variations in the chemical composition of geofluids and of gas fluxes are significant parameters for understandingmud volcanismand correctly estimate their emissions in carbon species, particularly greenhouse gas, methane. In this study, muddy water and gas samples were collected from the Anjihai, Dushanzi, Aiqigou, and Baiyanggou mud volcanoes in the southern Junggar Basin during the four seasons, around a year. This region hosts the most active mud volcanism throughout China. Gas and water were analyzed for major molecular compositions, carbon and hydrogen isotopes of the gas phase, as well as cations and anions, hydrogen and oxygen isotopes of water. The emitted gases are dominated by CH4 with some C2H6, CO2, and N-2. The seasonal changes in the chemical composition and carbon isotopes of emitted gases are not significant, whereas clear variations in the amounts of cations and anions dissolved in the water are reported. These are higher in spring and summer than autumn and winter. The CH4, CO2, and C2H6 fluxes are 157.3-1108 kg/a, 1.8-390.1 kg/a, and 10.2-118.7 kg/a, respectively, and a clear seasonal trend of the gas seepage flux has been observed. In January, the macro-seepage flux of open vents is >= 65% higher than in April, whereas the micro-seepage flux significantly decreased, probably due to the frozen shallow ground and blockage of soil fractures around the vents by heavy snow and ice during January. This probably causes an extra gas pressure transferred to the major vents, resulting in higher flux of the macro-seepage in the cold season. However, the total flux of the whole mud volcano system is generally consistent around a year.

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO2), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO2 release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000-2015) to the future period (2061-2080) were estimated at 1.0 (95% confidence interval (CI): 0.8-1.2) and 1.3 (95% CI: 1.0-1.7) Pg (1 Pg = 10(15) g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061-2080, the regional priming potentials could be 8.8 (95% CI: 7.4-10.2) and 10.0 (95% CI: 8.3-11.6) Tg (1 Tg = 10(12) g) C year(-1) under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO2 emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback.

Purpose Warming-induced permafrost degradation is anticipated to change the global carbon cycle. We attempted to determine the effect of permafrost degradation on carbon emissions and carbon sequestration of seven wetlands in three zones of Northeast China, aiming to investigate the responses of carbon sources/sinks to permafrost degradation. Methods Three zones (permafrost zone, PZ; discontinuous permafrost zone, DPZ; and permafrost degradation zone, PDZ) were selected to represent permafrost degradation stages. In each zone, we selected seven wetlands along the moisture gradient, namely, marsh (M), thicket swamp (TS), forested swamps (alder swamp, FAS; birch swamp, FBS; and larch swamp, FLS), forested fen (larch fen, FLF), and forested bog (larch bog, FLB). We determined the annual carbon emissions of soil heterotrophic respiration from seven wetlands and the annual net carbon sequestration of vegetation, evaluated the net carbon balance by calculating the difference between annual net carbon sequestration and annual carbon emissions, and then determined the magnitude and direction of carbon-climate feedback. Results and discussion With permafrost degradation, most forested wetlands (excluding FAS in PDZ) still acted as carbon sinks in DPZ (0.30 - 1.88 t ha(-1) year(-1)) and PDZ (0.31 - 1.76 t ha(-1) year(-1)) in comparison to PZ (0.46 - 2.43 t ha(-1) year(-1)). In contrast, M and TS acted as carbon sources in DPZ (-1.72 and -0.82 t ha(-1) year(-1)) and PDZ (-2.66 and -0.98 t ha(-1) year(-1)) in comparison to PZ (-0.86 and 0.03 t ha(-1) year(-1)), this result could be attributed to the increased CO2 emissions (promoted by warmer soil temperatures) and CH4 emissions (promoted by warmer soil temperatures, higher water tables and greater thaw depths), the two significantly increased the annual carbon emissions (increased by 8.8 - 14.4% in DPZ and by 35.0 - 46.0% in PDZ), and the annual carbon emissions > the annual net carbon sequestration. Furthermore, in terms of net radiative forcing, five forested wetlands still showed negative net radiative forcing in DPZ (-6.90 to -1.10 t CO2-eq ha(-1) year(-1)) in comparison to PZ (-8.91 to -1.62 t CO2-eq ha(-1) year(-1)). In contrast, in PDZ, only FLB showed negative net radiative forcing (-6.29 t CO2-eq ha(-1) year(-1)) and significantly increased by 288.3% compared to PZ (P < 0.05), indicating an ever-increasing net cooling impact, while the other four forested wetlands all turned into positive net radiative forcing (0.84 - 53.56 t CO2-eq ha(-1) year(-1)) because of higher CH4 (CO2-eq) emissions, indicating net warming impacts. Conclusions Our results indicated that permafrost degradation affected the carbon sources/sinks of seven wetlands via different mechanisms. M and TS acted as carbon sources in both DPZ and PDZ, while permafrost degradation did not change the overall direction of the net carbon balance of five forested wetlands. Most forested wetlands (excluding FAS in PDZ) still acted as carbon sinks in both DPZ and PDZ, although there were fluctuations in carbon sink values. Moreover, despite being carbon sinks, most forested wetlands (excluding FLB) in PDZ showed positive net radiative forcing compared to DPZ and PZ (negative net radiative forcing) when using the methodology of CO2 equivalent, indicating climatic warming impacts, while FLB showed negative net radiative forcing, indicating a climatic cooling impact. Therefore, FLB should be protected as a priority in the subsequent carbon sink management practices in permafrost zones.

Permafrost peatlands, as large soil carbon pools, are sensitive to global warming. However, the effects of temperature, moisture, and their interactions on carbon emissions in the permafrost peatlands remain unclear, when considering the availability of soil matrixes. The permafrost peatland (0-50 cm soil) in the Great Xing'an Mountains was selected to explore the deficiency. The cumulative carbon dioxide (CO2) and methane (CH4) emissions from soil were measured under different temperatures (5 C, 10 C, and 15 C) and moisture content (130%, 100%, and 70%) treatments by the indoor incubation. The results showed that the soil carbon and nitrogen matrix determined soil carbon emissions. Warming affected the availability of soil carbon and nitrogen substrates, thus stimulating microbial activity and increasing soil carbon emissions. With soil temperature increasing by 10 C, soil CO2 and CH4 emission rates increased by 5.1-9.4 and 3.8-6.4 times respectively. Warming promoted soil carbon emissions, and the decrease of moisture content promoted CO2 emissions but inhibited CH4 emissions in the permafrost peatland. Soil moisture and the carbon and nitrogen matrix determined the intensity of CO2 and CH4 emissions. The results were important to assess soil carbon emissions from permafrost peatlands under the impact of future climate warming and to formulate carbon emission reduction policies.

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO2), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO2 release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000-2015) to the future period (2061-2080) were estimated at 1.0 (95% confidence interval (CI): 0.8-1.2) and 1.3 (95% CI: 1.0-1.7) Pg (1 Pg = 10(15) g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061-2080, the regional priming potentials could be 8.8 (95% CI: 7.4-10.2) and 10.0 (95% CI: 8.3-11.6) Tg (1 Tg = 10(12) g) C year(-1) under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO2 emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback.

Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.