Permafrost soils contain enormous amounts of organic carbon, which could act as a positive feedback to global climate change due to enhanced respiration rates with warming. We have used a terrestrial ...ecosystem model that includes permafrost carbon dynamics, inhibition of respiration in frozen soil layers, vertical mixing of soil carbon from surface to permafrost layers, and CH4 emissions from flooded areas, and which better matches new circumpolar inventories of soil carbon stocks, to explore the potential for carbon-climate feedbacks at high latitudes. Contrary to model results for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), when permafrost processes are included, terrestrial ecosystems north of 60°N could shift from being a sink to a source of CO2 by the end of the 21st century when forced by a Special Report on Emissions Scenarios (SRES) A2 climate change scenario. Between 1860 and 2100, the model response to combined CO2 fertilization and climate change changes from a sink of 68 Pg to a 27 + -7 Pg sink to 4 + -18 Pg source, depending on the processes and parameter values used. The integrated change in carbon due to climate change shifts from near zero, which is within the range of previous model estimates, to a climate-induced loss of carbon by ecosystems in the range of 25 + -3 to 85 + -16 Pg C, depending on processes included in the model, with a best estimate of a 62 + -7 Pg C loss. Methane emissions from high-latitude regions are calculated to increase from 34 Tg CH4/y to 41–70 Tg CH4/y, with increases due to CO2 fertilization, permafrost thaw, and warming-induced increased CH4 flux densities partially offset by a reduction in wetland extent.
This paper presents an analysis of observed and simulated historical
snow cover extent and snow mass, along with future snow cover
projections from models participating in the World
Climate Research ...Programme Coupled Model Intercomparison Project
Phase 6 (CMIP6). Where appropriate, the CMIP6 output is compared to CMIP5
results in order to assess progress (or absence thereof) between
successive model generations. An ensemble of six observation-based
products is used to produce a new time series of historical Northern
Hemisphere snow extent anomalies and trends; a subset of four of these
products is used for snow mass. Trends in snow extent over 1981–2018
are negative in all months and exceed -50×103 km2 yr−1 during November, December, March, and May. Snow
mass trends are approximately −5 Gt yr−1 or more for all
months from December to May. Overall, the CMIP6 multi-model ensemble
better represents the snow extent climatology over the 1981–2014
period for all months, correcting a low bias in CMIP5. Simulated snow
extent and snow mass trends over the 1981–2014 period are stronger in
CMIP6 than in CMIP5, although large inter-model spread remains in the
simulated trends for both variables. There is a single linear
relationship between projected spring snow extent and global surface
air temperature (GSAT) changes, which is valid across all CMIP6 Shared
Socioeconomic Pathways. This finding suggests that Northern Hemisphere
spring snow extent will decrease by about 8 % relative to the
1995–2014 level per degree Celsius of GSAT increase. The
sensitivity of snow to temperature forcing largely explains the
absence of any climate change pathway dependency, similar to other
fast-response components of the cryosphere such as sea ice and near-surface permafrost extent.
We conducted a model-based assessment of changes in permafrost area and carbon storage for simulations driven by RCP4.5 and RCP8.5 projections between 2010 and 2299 for the northern permafrost ...region. All models simulating carbon represented soil with depth, a critical structural feature needed to represent the permafrost carbon–climate feedback, but that is not a universal feature of all climate models. Between 2010 and 2299, simulations indicated losses of permafrost between 3 and 5 million km² for the RCP4.5 climate and between 6 and 16 million km² for the RCP8.5 climate. For the RCP4.5 projection, cumulative change in soil carbon varied between 66-Pg C (1015-g carbon) loss to 70-Pg C gain. For the RCP8.5 projection, losses in soil carbon varied between 74 and 652 Pg C (mean loss, 341 Pg C). For the RCP4.5 projection, gains in vegetation carbon were largely responsible for the overall projected net gains in ecosystem carbon by 2299 (8- to 244-Pg C gains). In contrast, for the RCP8.5 projection, gains in vegetation carbon were not great enough to compensate for the losses of carbon projected by four of the five models; changes in ecosystem carbon ranged from a 641-Pg C loss to a 167-Pg C gain (mean, 208-Pg C loss). The models indicate that substantial net losses of ecosystem carbon would not occur until after 2100. This assessment suggests that effective mitigation efforts during the remainder of this century could attenuate the negative consequences of the permafrost carbon–climate feedback.
This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic ...forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone ITCZ, frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation ENSO dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed.
Plain Language Summary
Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5.
Key Points
The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persist
A long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6
The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR
Permafrost warming and potential soil carbon (SOC) release after thawing may amplify climate change, yet model estimates of present-day and future permafrost extent vary widely, partly due to ...uncertainties in simulated soil temperature. Here, we derive thermal diffusivity, a key parameter in the soil thermal regime, from depth-specific measurements of monthly soil temperature at about 200 sites in the high latitude regions. We find that, among the tested soil properties including SOC, soil texture, bulk density, and soil moisture, SOC is the dominant factor controlling the variability of diffusivity among sites. Analysis of the CMIP5 model outputs reveals that the parameterization of thermal diffusivity drives the differences in simulated present-day permafrost extent among these models. The strong SOC-thermics coupling is crucial for projecting future permafrost dynamics, since the response of soil temperature and permafrost area to a rising air temperature would be impacted by potential changes in SOC.
Due to an imbalance between incoming and outgoing radiation at the top of the atmosphere, excess heat has accumulated in Earth's climate system in recent decades, driving global warming and climatic ...changes. To date, it has not been quantified how much of this excess heat is used to melt ground ice in permafrost. Here, we diagnose changes in sensible and latent ground heat contents in the northern terrestrial permafrost region from ensemble‐simulations of a tailored land surface model. We find that between 1980 and 2018, about 3.9+1.4−1.6 $3.9\genfrac{}{}{0pt}{}{+1.4}{-1.6}$ ZJ of heat, of which 1.7+1.3−1.4 $1.7\genfrac{}{}{0pt}{}{+1.3}{-1.4}$ ZJ (44%) were used to melt ground ice, were absorbed by permafrost. Our estimate, which does not yet account for the potentially increased heat uptake due to thermokarst processes in ice‐rich terrain, suggests that permafrost is a persistent heat sink comparable in magnitude to other components of the cryosphere and must be explicitly considered when assessing Earth's energy imbalance.
Plain Language Summary
In recent decades, planet Earth has received more energy from the sun than it has radiated back into space. This has led to an excess of energy that is causing global warming and climate change. While most of this excess energy is absorbed by Earth's oceans, some of it is used to melt ice in perennially frozen ground called permafrost. However, we do not know how much. In this study, we use a computer model to calculate how much energy the permafrost in the Arctic has absorbed over the past four decades. We find that permafrost has absorbed about 3.9 sextillion Joules of energy between 1980 and 2018. About 44% of this energy was used to melt ice contained in the ground, while the remaining energy was used to warm the ground. Our results suggest that permafrost absorbs a similar amount of energy as other large bodies of ice on Earth, such as ice sheets, glaciers, or sea ice. Our study implies that the energy taken up by permafrost needs to be considered in global assessments of Earth's energy budget, which has not been the case in the past.
Key Points
We provide the first estimate of the heat uptake through both warming and thawing of Arctic terrestrial permafrost
From 1980 to 2018, the northern terrestrial permafrost region absorbed 3.9+1.4−1.6 $3.9\genfrac{}{}{0pt}{}{+1.4}{-1.6}$ ZJ of heat, about 44% of it from melting of ground ice
Thawing permafrost acts as a heat sink in the Earth's climate system, similar in magnitude to other components of the cryosphere
The examination of more than 1500 paleohydrological dated records collected between 10 and 28°N during the last 50 years have been used to improve our knowledge and understanding of the Sahara and ...Sahel vulnerability to the Atlantic monsoon changes in the long-term. We have analyzed the distribution of water bodies (mainly lakes and wetlands) over time and space: the central Saharan massifs played a major role in favoring water supply to the lowlands throughout the whole African Humid Period. In addition, distinct East–West dynamics is recorded with humidity starting – and stopping – several millennia earlier to the east than to the west of the Sahara.
A series of time lags are discussed: (1) between the maximum of deep (fresh water) lake formation during the early Holocene and the maximum of water body extensions during the mid-Holocene which highlight the primary role of aquifer water level in lake response to climate change (2) between the hydrological history of the Sahara and the Sahel and the forcings – mainly insolation changes – during the early and mid-Holocene which involves complex interactions between remnant ice sheet in the Northern Hemisphere, open water bodies in the Sahara and Sahel and the Atlantic monsoon system.
► A record of millennial scale hydrological changes in Sahara/Sahel during the Holocene. ► A time lag between the hydrological optimum and the maximum of NH insolation. ► Role of the groundwaters in the distribution and importance of water bodies. ► Role of the central massifs in supplying fresh waters to the lowlands. ► Evidence for a distinct E–W dynamics at the onset and the termination of the Holocene.
Abstract
Eurasian spring snow cover is widely considered as an important predictor of Asian summer monsoon rainfall, but its possible role in the formation of the north–south dipole structure of ...rainfall anomalies (NSDR)—a major mode of the eastern China summer rainfall variability—remains elusive. Here, we show that, there is a close connection between the western Eurasian spring snow cover (WESS) and NSDR during our research period 1967–2018, with less WESS tends to be accompanied by a wetter south-drier north pattern over eastern China, and vice versa. However, this relationship was not significant before the late 1990s, but has since become significant. Further analyses demonstrate that the shift in the WESS–NSDR relationship could be attributed to the modulation of summer North Atlantic Oscillation (SNAO). After the late 1990s, the WESS-related anomalous atmospheric circulations during summer are largely reinforced by the constructive superposition of those with same signs induced by SNAO, which in turn would intensify the impact of WESS and hence lead to a strong WESS–NSDR connection. In contrast, the influences of WESS are counteracted by those with opposite signs associated with SNAO before the late 1990s and thereby result in a weak snow–rainfall relationship. Our findings, along with the decline in Eurasian spring snow cover, provide a potential explanation for the recent ‘South Flood–North Drought’ pattern observed over eastern China.
CMIP5, CMIP6, and ERA5 Antarctic precipitation is evaluated against CloudSat data. At continental and regional scales, ERA5 and the median CMIP models are biased high, with insignificant improvement ...from CMIP5 to CMIP6. However, there are fewer positive outliers in CMIP6. AMIP configurations perform better than the coupled ones, and, surprisingly, relative errors in areas of complex topography are higher (up to 50 %) in the five higher-resolution models. The seasonal cycle is reproduced well by the median of the CMIP models, but not by ERA5. Progress from CMIP5 to CMIP6 being limited, there is still room for improvement.
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