The effects of precipitation (Pr) and evapotranspiration (ET) on soil moisture play an essential role in the land-atmosphere system. Here we evaluate multimodel differences of these effects within the Coupled Model Intercomparison Project Phase 5 (CMIP5) compared to Soil Moisture Active Passive (SMAP) products in the frequency domain. The variability of surface soil moisture (SSM), Pr, and ET within three frequency bands (7 ~ 30 days, 30 ~ 90 days, and 90 ~ 365 days) after normalization is quantified using Fourier transform. We then analyze the impact of ET and Pr on SSM variability based on a transfer function assuming these variables with a linear time-invariant (LTI) system. For the simulated effects of ET and Pr on SSM variability, models underestimate them in the two higher frequency bands and overestimate them in the lowest frequency band but show better estimates in transitional zones between dry and wet climates. Besides, the effects on SSM by Pr and ET are found to be different across the three frequency bands, and models underestimate the one of Pr and ET as the dominant factor controlling SSM variability in each frequency band. This study identifies the spatiotemporal distribution of the CMIP5 model deficiencies in simulating ET and Pr effects on SSM. Overcoming these deficiencies could improve the interpretability and predictability of Earth system models in simulating interactions among the three variables.

Effects of permafrost degradation on carbon (C) and nitrogen (N) cycling on the Qinghai-Tibetan Plateau (QTP) have rarely been analyzed. This study used a revised process-based biogeochemical model to quantify the effects in the region during the 21st century. We found that permafrost degradation would expose 0.98±0.49 (mean±SD) and 2.17±0.38 Pg C of soil organic carbon under the representative concentration pathway (RCP) 4.5 and the RCP 8.5, respectively. Among them about 60% will be decomposed, enhancing heterotrophic respiration by 9.54±5.20 (RCP 4.5) and 38.72±17.49 (RCP 8.5) Tg C/yr in 2099. Deep soil N supply due to thawing permafrost is not accessible to plants, providing limited benefits to plant growth and only stimulating net primary production by 6.95±5.28 (RCP 4.5) and 27.97±12.82 (RCP 8.5) Tg C/yr in 2099. As a result, permafrost degradation would weaken the regional C sink (net ecosystem production) by 303.55±254.80 (RCP 4.5) and 518.43±234.04 (RCP 8.5) Tg C cumulatively during 2020–2099. Permafrost degradation has a higher influence on C balance of alpine meadow than alpine steppe ecosystems on the QTP. The shallower active layer, higher soil C and N stocks, and wetter environment in alpine meadow are responsible for its stronger response of C balance to permafrost thaw. This study highlights that permafrost degradation could continue to release large amounts of C to the atmosphere irrespective of potentially more nitrogen available from deep soils.

Lakes account for about 10% of the boreal landscape and are responsible for approximately 30% of biogenic methane emissions. However, its quantification is still of large uncertainty under changing climate conditions. Finland has the densest lake system in the world with most lakes situated in the boreal zone. This study uses a large observational dataset of lake methane concentrations to constrain its methane emissions with an extant process-based lake biogeochemical model. We found that the total current diffusive emission from Finnish lakes is 0.12±0.03 Tg CH yr and will increase by 26-59% by the end of this century. We discovered that while warming lake water and sediment temperature played an important role, the climate impact on ice-on periods was a key indicator to the degree of emission increase in the future. We concluded that these boreal lakes remain as a significant methane source under warming climate in this century.

Northern peatlands are a large C stock and often act as a C sink, but are susceptible to climate warming. To understand the role of peatlands in the global carbon-climate feedback, it is necessary to accurately quantify their C stock changes and decomposition. In this study, a process-based model, the Peatland Terrestrial Ecosystem Model, is used to simulate pan-Arctic peatland C dynamics from 15ka BP to 1990. To improve the accuracy of the simulation, spatially-explicit water run-on and runoff processes were considered, four different pan-Arctic peatland distribution datasets were used, and a spatially-explicit peat basal date dataset was developed using a neural network approach. The model was calibrated against 2055 peat thickness observations and the parameters were interpolated to the pan-Arctic region. Using the model, we estimate that, in 1990, the pan-Arctic peatlands soil C stock is 396-421 Pg C, and the Holocene average C accumulation rate was 22.9 g C•m-2 yr-1. Our estimated peat permafrost development history generally agrees with multi-proxy-based paleo-climate datasets and core-derived permafrost areal dynamics. During 500 BP to 1990, the pan-Arctic region went through the Little Ice Age and Anthropocene warming. Under Anthropocene warming, in the freeze-thaw and permafrost-free regions, the peat C accumulation rate decreased, but it increased in permafrost regions. Our study suggests that if current permafrost regions switch to permafrost-free conditions in a warming future, the peat C accumulation rate of the entire pan-Arctic region will decrease, but the sink and source activities of these peatlands are still uncertain. permafrost. Under Anthropocene warming, in the freeze-thaw and permafrost-free regions, the peat C accumulation rate decreased, but it increased in permafrost regions. This result suggests if permafrost regions switch to permafrost-free conditions, the peat C accumulation rate of the entire pan-Arctic region will decrease.