2.1 Model

The present investigation uses the MPI-ESM with the standard versions of the atmospheric component ECHAM6 and the ocean model MPIOM (Mauritsen et al., 2019) – more specifically the versions that are used in the sixth phase of the CMIP experiments (CMIP6; Eyring et al., 2016) – and the changes to the model are limited to the land component JSBACH (version 3.2; Reick et al., 2021). In JSBACH, we implemented a mask that made it possible to execute the modified code only in those grid cells in the Arctic and subarctic zone that – at present – are affected by permafrost (Fig. 1d), while the standard model code is run in the rest of the world. The mask is not based on the permafrost extent as simulated by JSBACH – as this differs between setups – but on the observed present-day extent using data from the Northern Circumpolar Soil Carbon Database (Hugelius et al., 2014). Note that throughout the paper, we refer to this region as the permafrost region, even though large parts of the respective areas do not feature permafrost for the entire duration of our simulations. The changes mainly pertain to the soil hydrology scheme, including the implementation of a module that simulates the dynamics of inundated areas due to ponding at the surface, but there are also important alterations to the parametrization of thermophysical processes. The representation of the soil physics in permafrost-affected regions is largely based on the implementation of Ekici et al. (2014), who introduced a five-layer snow scheme, the phase change of water within the soil and the effect of water on the soil's thermal properties. However, there are important differences between the implementation of Ekici et al. (2014) and the model used in the present study. These are described in more detail below. Please note that throughout the paper, “standard model” does not refer to the implementation of Ekici et al. (2014) but to the CMIP6 version of the model described in Reick et al. (2021).

2.1.4 Percolation, drainage and supercooled water

When pore water freezes, it blocks the pathways by which the remaining liquid water percolates into the deeper layers. As the standard JSBACH version does not include the phase change of water in the soil, the model does not account for the above effect on the vertical movement of water through the ground, and neither does the version of Ekici et al. (2014). In the present version we included an ice-impedance factor (F_perc,l) in the shape of a power law, which is common practice in present-day land surface models (Andresen et al., 2020). Here, we follow the approach of the Community Land Model (Swenson et al., 2012) and calculate F_perc,l for soil layer l as

$$\begin{array}{}\text{(9)}& F_{\mathrm{perc},l}={\mathrm{10}}^{-\mathrm{6}\frac{W_{\mathrm{ice},l}}{{\mathrm{\Theta }}_{\mathrm{fc},l}}}\text{,}\end{array}$$

with Θ_fc,l being the field capacity that constitutes the upper limit of the soil water content in JSBACH. In permafrost regions, the inclusion of F_perc,l effectively prohibits percolation to the bedrock boundary, which can result in the formation of a highly saturated zone perched on top of the perennially frozen fraction of the ground. This, however, also depends on the way drainage and supercooled water are handled by the model (see below).

JSBACH's soil hydrology scheme includes two drainage components. The majority of water is assumed to drain from the lowest hydrologically active layer at the border to the bedrock. However, in permafrost-affected regions the drainage via this pathway is strongly limited due to the impedance of percolation in the presence of ice. The second component is a lateral drainage one from all hydrologically active layers. Here, it is assumed that water flows horizontally through the soil until it reaches the river system or wider vertical channels – such as cracks, crevices and connected pathways in coarse material – that provide an additional pathway by which the water reaches the border between soil and bedrock where it runs off as base flow. Such vertical channels are assumed to be present in all grid cells at the coarse resolutions that JSBACH is typically run at. The present model version includes the option to apply an ice-impedance factor (F_drain,l) to the lateral drainage component, corresponding to the assumption that in permafrost-affected regions, the wider vertical channels would also be blocked by ice. Without an explicit treatment of the connected vertical channels or the excess ice in the ground, we approximate F_drain,l in a given soil layer l by a function of the pore ice content relative to the field capacity. However, in contrast to F_perc,l, we do not use the ice content and field capacity of layer l but that of all subjacent layers:

$$\begin{array}{}\text{(10)}& F_{\mathrm{drain},l}={\mathrm{10}}^{-\mathrm{6}\frac{W_{\mathrm{ice},\mathrm{sub}}}{{\mathrm{\Theta }}_{\mathrm{fc},\mathrm{sub}}}},\end{array}$$

with

$$\begin{array}{}\text{(11)}& W_{\mathrm{ice},\mathrm{sub}}=\sum _{i=l+\mathrm{1}}^n{W}_{\mathrm{ice},i}\end{array}$$

and

$$\begin{array}{}\text{(12)}& {\mathrm{\Theta }}_{\mathrm{fc},\mathrm{sub}}=\sum _{i=l+\mathrm{1}}^n{\mathrm{\Theta }}_{\mathrm{fc},i}\text{.}\end{array}$$

F_drain,l does not suppress the lateral drainage from layer l completely, even if the subjacent layers are fully (ice) saturated. However, in this case, only the water that exceeds the layer's field capacity is added to the drainage flux, accounting for the possibility that in fully saturated unfrozen soil layers, lateral subsurface flow allows direct drainage into the river system, even if the subjacent layers are frozen and the vertical channels are blocked by ice.

A given fraction of the water within the soil – the supercooled water – may remain liquid when temperatures drop below 0 ^∘C. In clay-rich soils, supercooled water constitutes up to a quarter of the total soil water content, as the absorptive and capillary forces that soil particles exert on the surrounding water inhibit the freezing process (Niu and Yang, 2006). Additionally, high salt concentrations lower the freezing point, and liquid brine lenses can even sustain microbial activity at temperatures substantially below 0 ^∘C (Jansson and Taş, 2014). Including the effects of supercooling in LSMs is difficult, not only because most models do not explicitly simulate the salt concentration within the soil, but also because it is not clear how to treat the mobility of liquid water at sub-zero temperatures. Water that remains liquid due to the salt content may still move through the surrounding soil–ice matrix, while the supercooled water that exists because of absorptive forces may be bound to the soil particles. In the implementation of Ekici et al. (2014), the supercooled water behaves similarly to water at temperatures above 0 ^∘C – that is, it can percolate through and drain from the soil. In the present version, the movement of supercooled water is diminished in the presence of ice, using the above-described ice-impedance factors. However, some water movement is still possible and especially clay-rich soils can lose a large fraction of the supercooled water over longer periods. To prevent this cold-season drying of the soils from happening, we additionally included the option to “immobilize” the supercooled water, essentially prohibiting all fluxes below the freezing point.

Figure 3JSBACH setups: qualitative comparison between the hydrological fluxes in the DRY and WET JSBACH setups. Shown are bare-soil evaporation (yellow), evaporation from wetlands (dark blue), transpiration (green), infiltration (white), surface runoff (grey) and drainage (light blue), with thick arrows indicating large fluxes and thin arrows small fluxes. The resistances symbols (red in the case of the DRY and blue for the WET setup) indicate whether the parameter settings in a setup facilitate a certain process – indicated by a small resistance symbol – or impede it – indicated by a large resistance symbol. Here, the resistance with respect to bare-soil evaporation and transpiration depends on the parametrization of the maximum water-holding capacity of the uppermost soil layer and of the root zone, respectively. The resistance with respect to evaporation from wetlands is modified via the maximum retention time, which determines the surface water storage and the fractional wetland cover. With respect to infiltration and surface runoff, the resistance depends on the scaling factor F_ARNO, which allows us to reduce the flux from the surface water storage to the ARNO scheme. The resistance with respect to drainage depends on the assumed mobility of supercooled water as well as on the ice-impedance factor (F_drain,l).

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Table 1Overview of the DRY and WET JSBACH setup.

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2.2 Setups

The present investigation is mainly based on two model setups that lead to different degrees of “wetness” in permafrost-affected grid cells – with the meaning of wetness not being limited to the soil water content. The “DRY” setup is characterized by low infiltration rates, poor soil water retention, and large runoff and drainage rates, while the “WET” setup assumes higher infiltration rates combined with high water retention and large evapotranspiration rates (Fig. 3). For the design of the simulations we made use of the optional formulations that were included in the soil hydrology scheme. In the DRY setup, poor soil water retention and high drainage rates are achieved by allowing the supercooled water to move through the ground and by not impeding the lateral drainage in the presence of ice in the subjacent soil layers (Table 1). Low infiltration rates were obtained by setting F_ARNO to 0.8, and, with respect to evapotranspiration, a high resistance was assumed by including ΔΦ_org in $W_{\mathrm{MAX},\mathrm{top}}^{*}$ and $W_{\mathrm{MAX},\mathrm{rz}}^{*}$. Finally we use a comparatively low maximum wetland lag (P_lag=100 d) which limits the extent of inundated areas and the corresponding infiltration and evaporation rates. For the WET setup, we assumed high infiltration rates, allowing all of the reservoir outflux to be separated into infiltration and surface runoff by the ARNO scheme (F_ARNO=1). Furthermore, we set a high water retention level in permafrost-affected soils, by assuming that supercooled water is stationary and that the lateral drainage flux is impeded by the ice content of the subjacent soil layers. With respect to evapotranspiration, we assume a low resistance by not accounting for ΔΦ_org in $W_{\mathrm{MAX},\mathrm{top}}^{*}$ and $W_{\mathrm{MAX},\mathrm{rz}}^{*}$. We also assumed a larger maximum lag for the surface waterbodies (P_lag=150 d), resulting in higher evaporative fluxes from inundated areas. Finally, the minimum root depth was increased from 10 to 30 cm, increasing the plant water availability mainly in the mountainous regions in eastern Siberia (not shown).

Figure 4Partitioning of hydrological surface fluxes: partitioning of the outgoing moisture fluxes amongst fluxes to the atmosphere and to the ocean (via the river discharge) and further subdivision into evaporation, transpiration, runoff and drainage. Panels show the average over the northern permafrost regions taken from standalone simulations with the atmospheric forcing corresponding to pre-industrial control conditions. The sum of all outgoing fluxes is equal to the precipitation rates, which is the same in all simulations (1.7 mm d⁻¹). Shown is the partitioning for the standard model (REF, a), the DRY setup (b) and the WET setup (c).

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As discussed in Sect. 2.1.1–2.1.4, many of the above parameters and optional formulations are a means to represent the uncertainty resulting from the complexity of interactions and processes or from structural shortcomings of the model rather than the uncertainty in the specific parameter values. Thus, they are used for model tuning, even if the specific parameter or formulation itself constitutes a measurable quantity. Furthermore, even if the parameter could at least in theory be better constrained, observation-based information suitable for the resolution of the model may not exist. A good example for this is the maximum wetland lag, which has been determined for specific lakes (Ambrosetti et al., 2003; Brooks et al., 2014), but upscaling the observation-based values to the model scale requires highly uncertain assumptions about the representative bathymetry of wetlands, ponds and lakes and their relative fraction in the overall inundated area. With the number of available tuning options, the design of the setups is – to a certain degree – arbitrary. Here, the differences between the WET and the DRY simulation do not cover the complete uncertainty range included in the parametrizations of JSBACH's soil hydrology scheme. For example, a much drier setup could have been obtained by keeping JSBACH's original formulation of prohibiting infiltration at sub-zero temperatures or maintaining a fixed maximum root-zone soil moisture level – that is, not reducing W_MAX,rz by the ice content. Furthermore, we focused on the representation of processes and, besides the minimum root depth, made no diverging assumptions with respect to the soil properties, which are particularly uncertain in regions with large soil organic matter concentrations. Our aim was not to compare the most extreme scenarios but to compare setups whose differences are in the range of “typical” inter-model differences. A measure for the latter was derived from the simulations of the Permafrost Carbon Network Model Intercomparison Project (PCN-MIP; McGuire et al., 2018), which targets the behaviour of state-of-the-art LSMs in permafrost-affected regions (a brief overview of the way the PCN-MIP participants represent key hydrological processes is included in the Supplement (Sect. S1, Table S1), while a detailed analysis can be found in Andresen et al., 2020). Here, our primary focus was the simulated partitioning of moisture fluxes between runoff (including drainage) and evapotranspiration, as this ratio can be expected to be the parameter most relevant for the climate feedback. Thus, we designed the setups in a way that the average differences in evapotranspiration in JSBACH standalone simulations with the WET and the DRY configuration do not (substantially) exceed the standard deviation of the ensemble of PCN-MIP participants (Fig. 4), which is roughly 0.3 mm d⁻¹ (Fig. 2b). A secondary goal was to maintain a similar plant water availability for the same atmospheric conditions so that differences in the vegetation cover in coupled simulations can be fully attributed to the feedback-mediated differences in climate and not to setup-induced soil moisture differences. The agreement with observations was not taken into account in the design of the setups. Nonetheless, we conducted a brief model evaluation – focused on the northern permafrost regions – which is included in the Supplement (Tables S3 and S4 and Figs. S1–S9).

Although the paper focuses on the two setups described above, we performed a third, highly synthetic setup – the W2D setup – which exhibits increasingly dry conditions under future warming. This setup assumes that the characteristics of the soil hydrology are determined by the presence of near-surface permafrost and change when the latter is degraded. All grid cells start with the parametrizations of the WET setup, and the configuration is maintained as long as the model simulates permafrost in the upper 3 m of the soil. However, the parametrizations switch from WET to DRY (with the exception of the soil depths and the maximum wetland retention P_lag) whenever the annual maximum thaw depth in the grid cell extends beyond a depth of 3 m. For the high-emission scenario considered in this study, the majority of the grid cells in the northern permafrost regions transition from WET to DRY during the 21st century, with the W2D simulation becoming increasingly different from the WET simulation.

2.3 Simulations

The general setup of the simulations uses a 450 s time step for the atmosphere and land components, while the ocean model is run at a 2700 s time step. The horizontal resolution in the atmosphere and over land is T63 (1.9^∘ × 1.9^∘) – which corresponds to grid spacing of about 200 km in tropical latitudes – and GR15 (1.5^∘ × 1.5^∘) in the ocean – which corresponds to 160 km in the tropics. The atmosphere has a vertical resolution of 47 levels reaching up to 0.1 hPa, which is a height of about 80 km, while the ocean model uses 41 vertical layers reaching to a depth of up to 5000 m. The land is resolved by 18 subsurface layers that extend to a depth of about 160 m, 11 of which are used to represent the top 3 m of the soil column. This is very different to the standard vertical setup which represents the soil column by 5 layers reaching to a depth of less than 10 m. Imposing a deeper bottom boundary is important for a realistic representation of the soil thermodynamic regime, with implications for subsurface heat conduction and energy distribution (MacDougall et al., 2008; González-Rouco et al., 2009, 2021), as too shallow LSMs alter the distribution of temperatures in the subsurface (Alexeev et al., 2007; Smerdon and Stieglitz, 2006). As shown by Steinert et al. (2021a) a depth >150 m is required to resemble an infinitely deep soil in climate change simulations of centennial timescales. The improved vertical resolution and bottom boundary condition depth used herein produce changes in the subsurface thermal state that have been shown to interact with the phase changes and other hydrological features. In regions with active layer dynamics, the more realistic conduction of heat from the surface into the soil impacts the depth of the zero annual amplitude of ground temperatures, which has impacts on the simulation of near-surface permafrost extent in the Arctic and subarctic regions (Steinert et al., 2021b). Note that the downward extension of the soil column has no direct impact on the subsurface hydrology as we assume the ground to consist of impermeable bedrock below the soil depth of the standard model configuration.

Both simulations start in the year 1800, with the atmosphere and the ocean being initialized using pre-industrial control simulations with the standard MPI-ESM, which were performed for the CMIP6 DECK experiments. However, the land surface cannot be initialized in the same manner, as the CMIP6 DECK experiments use the standard setup. Thus, they do not include some of the essential variables in permafrost regions – such as the soil ice content – and have a different vertical discretization in the soil. Furthermore, the states that the standard model version simulates in the permafrost regions differ substantially from those simulated with either the WET or the DRY setup. Instead, we used a temperature approximation – based on the surface temperature – to initialize the soil temperature and assumed all soils layers to be close to saturation – that is, at 95 % of the field capacity. Starting from this state, we ran JSBACH in standalone mode for 200 years, with the atmospheric forcing data derived from the MPI-ESM pre-industrial control simulations. During this period, the soil temperatures and soil water and ice content adjust to the prescribed atmospheric conditions, allowing us to use the final states to initialize the coupled simulations.

In the simulations, the water and energy cycles of the MPI-ESM components are fully coupled. However, this is not the case for the biogeochemical cycle. Especially the magnitude of the permafrost carbon feedback is extremely difficult to estimate. Accounting for it in our investigations would have required not only extensive adaptations in JSBACH, but also the performance of a number of ensemble simulations to account for the uncertainty that is included in the formulations of the permafrost carbon cycle (de Vrese et al., 2021; de Vrese and Brovkin, 2021). The latter increases the computational demand of the experiment by an order of magnitude while not necessarily providing any additional insights into the physical land–atmosphere feedbacks in permafrost regions, which are the main focus of this investigation. Instead, we ran the model with prescribed atmospheric greenhouse gas concentrations, which corresponds to the assumption that present-day and future atmospheric CO₂ levels are largely determined by human activity and that all variations in the natural carbon fluxes are offset by corresponding adjustments in anthropogenic emissions. The simulations start with an atmospheric CO₂ concentration of about 280 ppmv, which increases to about 400 ppmv between the years 1800 and 2014. After 2014, the simulations follow a trajectory of high greenhouse gas (GHG) emissions based on Shared Socioeconomic Pathway 5 and Representative Concentration Pathway 8.5 (SSP5-8.5; van Vuuren et al., 2011). SSP5-8.5 targets a radiative forcing of 8.5 W m⁻² in the year 2100 and assumes the atmospheric CO₂ concentrations to increase to about 1100 ppmv. We chose to investigate SSP5-8.5, even though it may not be based on the most plausible assumptions (van Vuuren et al., 2011; Riahi et al., 2017; Hausfather and Peters, 2020), but investigating extreme scenarios often helps in highlighting impacts and understanding causal relationships.

Furthermore, we conducted an additional set of simulations in which the atmospheric greenhouse gas concentrations were prescribed in a way as to stabilize the global mean surface temperature at 1.5 ^∘C above the pre-industrial CMIP6 ensemble mean temperature. For the DRY setup this required lowering atmospheric CO₂ from 450 ppmv in the year 2030 to 400 ppmv in the year 2080. For the WET setup CO₂ levels were reduced from 530 to 465 ppmv between the years 2045 and 2095. It should be noted that the differences between these two simulations are a result not exclusively of the differences in model parametrizations but also of the differences in the strength of the CO₂ fertilization effect. Without dedicated simulations excluding the effect of rising atmospheric CO₂ on vegetation, it is not possible to determine how important the additional 68 ppmv in the WET simulation is for the simulated vegetation covers and the resulting feedbacks on the climate system. However, the WET simulation shows a slightly smaller overall vegetation cover at higher CO₂ and the same global mean temperature, indicating that the additional CO₂ fertilization is less important than the differences in the spatial distribution of temperatures and precipitation.

2.4 Uncertainty in Arctic and subarctic climate projections

With evapotranspiration rates being partly predetermined – as one of the target variables in the design of our setups (see “Setups” above) – we focus on the uncertainties in simulated surface temperatures and precipitation and how these relate to the evapotranspiration rates. In the northern permafrost regions, the simulations of the CMIP6 ensemble (Table S2) show a high inter-model correlation between precipitation and evapotranspiration – that is, a Spearman's ρ of 0.89 with a p value of $\mathrm{1.88}\times {\mathrm{10}}^{-\mathrm{15}}$ for the average over the permafrost-affected grid cells and over the period 2000–2100 (not shown). But, while there appears to be little uncertainty that the two processes are closely connected, it is not clear whether high (low) precipitation rates in a model are caused by high (low) evapotranspiration rates or vice versa or even whether there is little causal relation between the two but a similar dependency on a third factor. Here, our investigation aims to establish or eliminate soil-setup-induced differences in evapotranspiration rates as a potential cause of the variations in high-latitude precipitation between the CMIP6 participants. In contrast, the CMIP6 ensemble exhibits a low correlation between the average surface temperatures and evapotranspiration rates – that is, a ρ of 0.28 with a p value of 0.07. The poor correlation indicates that the surface temperatures are not exclusively determined by the evapotranspiration rates but by a number of factors – one of which may be evapotranspiration. Here, our simulations offer the chance to isolate the potential contribution of permafrost-hydrology-related evapotranspiration differences to the uncertainty in surface temperature in climate change projections.

As a measure of the uncertainty in these projections, we use the inter-model spread of the CMIP6 ensemble, which was derived from 44 simulations provided by 27 modelling groups (an overview table can be found in the Supplement, Table S2). Here, it should be noted that we use the term “inter-model” in a loose sense, as the ensemble does not consist of simulations with 44 different ESMs but includes multiple simulations with the same model being run with a different configuration – e.g. a high vs. a low resolution or with and without interactive vegetation. However, in the case that several simulations were available for the same model configuration but merely differ with respect to the initial conditions or minor parameter settings, we consider only one of the simulations to not underestimate the inter-model spread. To exclude outliers, we do not define the spread as the difference between ensemble minimum and maximum. Instead, we use two frequently applied measures, namely the interquartile range (IQR) – that is, the difference between the 25th and the 75th percentile – and the range contained within 1 ensemble standard deviation of the mean (±σ) – covering about two-thirds of the ensemble if the latter is normally distributed.

Figure 5Arctic futures: (a) 21st-century evapotranspiration trend in the DRY simulation. (b) Same as (a) but for precipitation. (c) Same as (a) but for the total soil water (liquid soil moisture and ice) content. (d–f) Same as (a)–(c) but for the WET simulation. Trends were estimated applying a simple linear regression to the values covering the period 2000–2099. Non-permafrost and glacier grid cells are hatched.

3.1 Land–atmosphere interactions

One of the questions that motivated this investigation is whether the Arctic and subarctic zone will become wetter or drier in response to future warming or rather if present-day ESMs can be used to predict this with some degree of confidence, given the uncertainties in the hydrology parametrizations of the land surface components. Both the WET and the DRY simulation predominantly show an increase in evapotranspiration and precipitation, even though the signal is not uniform across the northern permafrost areas (Fig. 5a, b, d, e). Especially in North America there are extensive regions in which the evapotranspiration rates decline during the 21st century. These regions enclose most of the areas that also exhibit a negative trend in precipitation, indicating that the latter is the result of reduced moisture recycling. Nonetheless, the general trend is an increase in the intensity of the hydrological cycle with the signal being more pronounced in WET than in DRY. In contrast, the two simulations show opposing trends in the total soil water content – that is, liquid water and ice (Fig. 5c, f). In WET, the soils in most grid cells lose water, with the trend being partly driven by a strong increase in the drainage rates (not shown). In this setup, lateral drainage is strongly inhibited in the presence of ice and the marked increase in these fluxes is the result of the warming-induced decrease in the soil ice concentration. In the DRY setup, the inhibition of the lateral subsurface flow in the presence of ice is less severe; hence, the increase in drainage due to permafrost degradation is less pronounced. As a result, the soils in the DRY simulation almost exclusively exhibit an increase in the water content, with the increase in precipitation (− evapotranspiration) being the dominant signal. It should be noted that JSBACH, as with most land surface models, does not include a representation of excess ice. Excess ice is the water that the soil can only hold when frozen but that exceeds the pore volume of the unfrozen ground. Thus, the thawing process effectively reduces the amount of water that the soils can hold, and it is plausible that the increase in the water content found in the DRY setup – and also found in other models (Andresen et al., 2020) – is only possible because the model neglects the feature of a higher soil water-holding capacity in the frozen state.

Figure 6Effects of soil hydrological conditions on near-surface climate: (a) latent heat flux in the northern permafrost regions in WET (blue) and DRY (red) MPI-ESM simulations for SSP5-8.5. Thin lines show the annual mean, averaged over the northern permafrost regions (note that grid cells covered by glaciers were excluded), while thick lines give the 10-year running mean. (b–i) Same as (a) but showing (b) the Bowen ratio, (c) precipitation, (d) surface runoff and drainage, (e) accumulated cloud cover, (f) solar radiation absorbed at the surface, (g) surface temperatures, (h) near-surface (top 3 m of the soil) permafrost volume, and (i) liquid soil water content.

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Thus, when considering the total soil water content to be a key indicator, the uncertainty in the representation of the permafrost hydrology makes it impossible to provide an unambiguous answer to the question whether the Arctic and subarctic region will become wetter or drier in the future. Furthermore, the agreement on the direction of the trends in evapotranspiration and precipitation does not mean that the different soil hydrology setups entail similar conditions at and above the surface. On the contrary, the land–atmosphere interactions diverge substantially between WET and DRY, which has a distinct impact on the near-surface climate. In WET, the evapotranspiration rates are between 0.2 and 0.3 mm d⁻¹ larger than in DRY, which translates into a difference in latent heat flux of about 5 W m⁻² at the beginning and about 9 W m⁻² at the end of the 21st century (Fig. 6a). The additional evaporative cooling in WET constitutes 6 % to 10 % of the shortwave radiation that is absorbed by the surface and profoundly changes the partitioning of the surface energy budget. With the Bowen ratio decreasing to 0.3, the permafrost-affected areas are increasingly energy limited in WET, while in DRY, a Bowen ratio of almost 1 indicates that at least parts of the region experience some degree of water limitation (Fig. 6b).

With less sensible heat and more latent heat being transferred into the atmosphere, the boundary layer is initially cooler and moister in WET than in DRY (not shown). This leads to higher relative humidity and, consequently, to precipitation rates that are roughly 0.2 mm d⁻¹ larger in WET (Fig. 6c). The higher precipitation rates in turn increase the soil water availability, establishing a positive feedback in which the more intense moisture recycling is the main factor sustaining the higher evapotranspiration rates. More specifically, about 80 % of the additional evapotranspiration in WET is compensated for by higher precipitation rates, while merely 20 % is balanced by differences in runoff (Fig. 6d). Furthermore, the differences in relative humidity result in differences in the cloud cover, which constitute another important feedback on the surface energy balance (Fig. 6e). The increased cloudiness in WET occurs mainly during the snow-free period – spring to early autumn in the southern permafrost regions, with the length of the period decreasing in a northward direction – when the surface reflectivity is determined by a comparatively dark vegetation cover and similarly dark bare-soil areas. Thus, the more extensive cloud cover notably raises the planetary albedo (relative to DRY), reducing the surface incoming solar radiation by between 10 W m⁻² at the beginning and 13 W m⁻² at the end of the 21st century. When additionally taking into consideration the differences in the surface reflectivity – resulting from differences in the simulated snow and vegetation covers – the differences in absorbed shortwave radiation amount to roughly 12 W m⁻² (Fig. 6f). The reduction in the available energy cools the surface further, which leads to less sensible heat being transferred into the boundary layer, contributing to the higher relative humidity in the atmosphere. Here, it should be noted that the differences in evaporative cooling and those in the planetary albedo affect the available energy very differently because the former mainly redistribute while the latter provide a net change to the energy content of the system. However, even when focusing exclusively on the surface latent heat flux and the incoming shortwave radiation, the cloud effect (10–13 W m⁻²) has a larger impact on the surface energy balance than the evaporative cooling that initially caused it (5–9 W m⁻²).

The energy balance determines the temperatures at and below the surface, which in turn are highly relevant for the question whether the high latitudes will become wetter or drier in the future. It can be argued that a more suitable measure for the wetness of the permafrost areas is the liquid – rather than the total – water content of the soils, with the former controlling the majority of the physical and biophysical land processes. Here, the trend in liquid soil moisture depends not only on the development of the total water content, but also on the temperature evolution, as these determine the ratio of liquid and frozen water in the soil (Fig. 6g). In both simulations, the 21st-century warming results in a substantial decline in the extent and thickness of the near-surface permafrost (Fig. 6h; note that the near-surface permafrost volume corresponds to an initial permafrost area of about 22×10⁶ km² in WET and about 16×10⁶ km² in DRY; at the end of the 21st century the values are 11×10⁶ and 4×10⁶ km², respectively), leading to a marked decrease in the soil ice content and a corresponding increase in liquid water (Fig. 6i). Thus, when basing the question of future wetness on the liquid soil water content, our simulations provide a clear answer: the trends in evapotranspiration, precipitation and soil moisture all suggest that, on average, the Arctic and subarctic region will become wetter in the future. This general trend appears to be a robust feature, the direction of which is independent of the representation of the permafrost hydrology in the model. However, the magnitude of the (liquid) soil moisture trend is very sensitive to the parametrization of the model, mainly to the resulting differences in the simulated surface temperatures. Here, the WET simulation is consistently colder, which results in a higher near-surface permafrost volume at the beginning of the 21st century and a lower liquid soil moisture content. With the higher initial permafrost volume, the 21st-century warming has a more pronounced impact on the thaw rates, and the resulting trend in the liquid soil moisture is much larger in WET than in DRY, despite WET exhibiting a negative trend in the total soil moisture (Figs. 5c and f and 6i).

Figure 7Comparison to CMIP6 ensemble – annual means: (a) simulated differences in evapotranspiration in permafrost regions and the respective CMIP6 ensemble spread. The black line shows the differences between WET and the DRY (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{evap}}$), the green area gives the interquartile range (IQR^evap) – that is, the difference between the 75th and the 25th percentile – of the CMIP6 ensemble, and the grey area provides 2× the ensemble standard deviation (±σ^evap). (b, c) Same as (a) but for (b) precipitation (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{pr}}$, IQR^pr, ±σ^pr) and (c) surface temperatures in permafrost grid cells (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$, IQR^ts, ±σ^ts). Shown are annual means.

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3.2 Differences in climate compared to the CMIP6 spread

Another question motivating our study was to which extent differences in the parametrizations of the soil hydrology could help explain the large inter-model spread that the present generation of ESMs exhibits in the Arctic and the subarctic zone. In the northern permafrost regions, the absolute value of the differences in evapotranspiration between DRY and WET (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{evap}}$) captures the range of typical inter-model differences comparatively well, and at the beginning of the 21st century they match the respective interquartile range of the CMIP6 ensemble (IQR^evap) almost perfectly (Fig. 7a). Subsequently, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{evap}}$ increases considerably, exceeding IQR^evap by 2030, but remains well within the range of 2 (CMIP6) ensemble standard deviations (±σ^evap). It should be noted that this good agreement was to be expected as the setups were designed to produce differences that do not exceed the range of typical evapotranspiration differences between commonly used LSMs (although these typical differences were not determined based on fully coupled CMIP6 simulations but on the ensemble of standalone simulations that were performed for the PCN-MIP). Thus, the more interesting question is how sensitive the simulated climate is to these differences in evapotranspiration, more specifically whether the latter lead to differences in other key variables that are similarly consistent with the respective CMIP6 spreads.

Figure 8Comparison to CMIP6 ensemble – seasonality: (a) simulated differences in evapotranspiration in permafrost regions and the respective CMIP6 ensemble spread. The black line shows the differences between WET and the DRY (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{evap}}$), the green area gives the interquartile range (IQR^evap) – that is, the difference between the 75th and the 25th percentile – of the CMIP6 ensemble, and the grey area provides 2× the ensemble standard deviation (±σ^evap). (b, c) Same as (a) but for (b) precipitation (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{pr}}$, IQR^pr, ±σ^pr) and (c) surface temperatures in permafrost grid cells (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$, IQR^ts, ±σ^ts). Shown is the seasonality averaged over the 21st century.

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In the case of precipitation, the differences in the soil hydrology parametrizations appear to offer a large explanatory potential (Fig. 7b). On average ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{pr}}$ amounts to about 0.16 mm d⁻¹, IQR^pr to about 0.19 mm d⁻¹ and ±σ^pr to 0.32 mm d⁻¹. Thus, even if considering ±σ^pr to be the more appropriate measure, about half of the inter-model spread of the CMIP6 ensemble may be explainable by diverging evapotranspiration rates resulting from differences in the parametrizations of the permafrost hydrology. Here, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{pr}}$ exhibits a marked peak in the summer months, when the causative differences in evapotranspiration are largest ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{evap}}$ (Fig. 8a, b). A similar behaviour can be seen for the ensemble spread, even though the (relative) seasonal variations are less pronounced, especially in the case of IQR^pr. With regards to the surface temperatures, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ matches the overall magnitude of the CMIP6 ensemble spread similarly well (Fig. 7c), with ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ being equal to IQR^ts (2.6 ^∘C) and representing about two-thirds of ±σ^ts (3.7 ^∘C). However, as with the differences in precipitation, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ peaks – at 4.2 ^∘C – when the differences in evapotranspiration are largest (Fig. 8c), which is not the case for IQR^ts and ±σ^ts. While the latter exhibit a notable increase in summer, their annual maximum occurs during winter – with 3.8 and 7.2 ^∘C, respectively – when ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ is the lowest. As described above (Sect. 3.1), the differences between WET and DRY mainly originate from a divergence of the cloud cover and the resulting differences in the planetary albedo. But as there are only minor differences in cloudiness – and very little solar radiation – during the snow cover season, this feedback is not present during the winter months. More importantly, the cloud radiative effect differs between the snow-covered and the snow-free period. The albedo of clouds is similar to those of snow- and ice-covered surfaces, and an increase in cloudiness does not lower the planetary albedo in winter. Instead, (low) clouds are more likely to increase the surface temperatures as they raise the surface net radiation by reflecting the longwave radiation emitted by the surface (Vihma et al., 2016). Thus, it is mainly the differences in soil heat content – resulting from differences in the energy uptake during the snow-free period – and differences in latitudinal heat transport (not shown) that sustain ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ during winter, while it is most likely differences in the parametrization of the snow and ice albedo that determine the large ensemble spread (Menard et al., 2021). Consequently, the explanatory power of the differences in the soil hydrology parametrizations appears to be limited to the snow-free period.

Furthermore, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ shows a good agreement with the inter-model spread especially during the first half of the century, ranging between IQR^ts and ±σ^ts. During the second half of the century, both IQR^ts and ±σ^ts increase by more than 1 ^∘C, while ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ shows no significant increase. Thus, ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ matches the magnitude of the CMIP6 spread well but appears to lack an important dynamical component. It may appear counterintuitive that the temperature differences between WET and DRY remain fairly constant during the 21st century despite the difference in evapotranspiration and evaporative cooling increasing over time. The reason for this is that evapotranspiration initially lowers the temperatures at the surface, but it eventually increases the air column temperature by the heat release during condensation. Thus, the evaporative cooling redistributes energy between latent and sensible heat and between surface and atmosphere but does not change the energy content of the coupled land–atmosphere system. The combination of lower surface temperatures and higher atmospheric temperatures, in turn, increases the downward fluxes of sensible heat and longwave radiation or decreases the upward fluxes, which largely balances the effect of the evaporative cooling at the surface. Over longer periods, local surface temperatures only change significantly if the evaporated or transpired water is advected out of the region and the net effect can be approximated by the latent heat included in the precipitation − evapotranspiration difference (P−E). Here, WET and DRY exhibit similar trends – as indicated by the trends in surface runoff and drainage (Fig. 6d; note that over longer periods P−E ∼ runoff and drainage) – and the difference in P−E remains constant over time. Furthermore, the divergence of the evapotranspiration rates results in increasingly large differences in the cloud cover, affecting the planetary albedo. In contrast to the evaporative cooling, the albedo differences change the amount of energy reflected back to space, hence the total energy content of the system. However, the albedo effects resulting from the divergence in the cloud cover are compensated for by decreasing differences in the surface reflectivity, mainly resulting from a convergence of the simulated vegetation covers. As a result, the differences in absorbed shortwave radiation do not change over time (Fig. 6f) and, with no significant trends in the differences in either P−E or the planetary albedo, the temperature differences between WET and DRY remain almost constant during the 21st century.

Figure 9Comparison to CMIP6 ensemble – 21st-century trends: (a) trends in evapotranspiration during the 21st century with 2× the CMIP6 ensemble standard deviation, the interquartile range of the CMIP6 ensemble, differences between WET and DRY, and differences between W2D and WET – averaged over the northern permafrost regions. (b, c) Same as (a) but for (b) precipitation and (c) surface temperatures. Trends were estimated applying a simple linear regression to the values covering the period 2000–2099.

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This raises the question whether the uncertainty in the permafrost hydrology is insufficient to explain the increase in the inter-model spread, especially during the second half of the century, or whether the issue of constant temperature differences is specific to our setups. Per design, the simulations with the WET and the DRY setups become more similar over time as many of the distinctions depend on the soil ice content, which decreases throughout the simulation. To test how far the lack of trends is related to this design feature, we compared the WET simulation to a simulation with the W2D setup. In the W2D simulation, the parametrizations switch from WET to DRY when the near-surface permafrost disappears in a given grid cell, with the W2D simulation becoming increasingly different from the WET simulation. With respect to precipitation and surface temperature, the differences between W2D and WET exhibit trends that are substantially larger than those in the differences between DRY and WET (Fig. 9b, c). The trend in ${\mathrm{\Delta }}_{|\mathrm{W}\mathrm{2}\mathrm{D}-\mathrm{WET}|}^{\mathrm{pr}}$ is also larger than the trend in and IQR^pr, closely matching the trend in ±σ^pr, while the trend in the temperature differences, ${\mathrm{\Delta }}_{|\mathrm{W}\mathrm{2}\mathrm{D}-\mathrm{WET}|}^{\mathrm{ts}}$, is very similar to the trend in IQR^ts. This indicates that differences in soil hydrology parametrizations may, in principle, contribute to the trend of the inter-model spread, given that the parametrizations do not become more similar with the advancing permafrost degradation. Here, however, the trends in ${\mathrm{\Delta }}_{|\mathrm{W}\mathrm{2}\mathrm{D}-\mathrm{WET}|}^{\mathrm{pr}}$ and ${\mathrm{\Delta }}_{|\mathrm{W}\mathrm{2}\mathrm{D}-\mathrm{WET}|}^{\mathrm{ts}}$ stem from a much larger causative trend in the evapotranspiration differences, ${\mathrm{\Delta }}_{|\mathrm{W}\mathrm{2}\mathrm{D}-\mathrm{WET}|}^{\mathrm{evap}}$ (Fig. 9a). The latter is almost twice as large as the trend in ±σ^evap and almost 20 times larger than the trend in IQR^evap, strongly suggesting that the trends in the temperature spread of the CMPI6 ensemble are not caused exclusively by the divergence of evapotranspiration rates (Hahn et al., 2021).

3.3 Relevance for global climate and the state of tipping elements

While the above results show that the differences in the parametrization of the permafrost hydrology may not fully explain the spread of the CMIP6 ensemble – especially the latter's seasonality and 21st-century trend – the differences in simulated climate of the continental permafrost areas are substantial. This raises the question whether the respective effects are confined to this region or whether they are relevant for the climate on larger scales. Non-glaciated, permafrost-affected areas only cover about a third of the Arctic and subarctic zone. Yet averaged over the planet's surface north of 50^∘ N, the temperature differences between WET and DRY (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts};+\mathrm{50}\mathrm{N}}$) amount to about 2.0 ^∘C (Fig. 10). This is notably less than ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ but still more than twice as much as would have been the case if the temperature effects were limited to the land areas affected by permafrost, indicating that the permafrost hydrology is indeed relevant for the entire region, including glaciers and the ocean.

Figure 10Comparison to CMIP6 ensemble – surface temperature north of 50^∘ N: (a) simulated differences in evapotranspiration in regions (land and ocean) north of 50^∘ N and the respective CMIP6 ensemble spread. The black line shows the differences between WET and the DRY (${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts};+\mathrm{50}\mathrm{N}}$), the red area gives the interquartile range (IQR${}^{\mathrm{ts};+\mathrm{50}\mathrm{N}}$) – that is, the difference between the 75th and the 25th percentile – of the CMIP6 ensemble, and the grey area provides 2× the ensemble standard deviation ($\pm {\mathit{\sigma }}^{\mathrm{ts};+\mathrm{50}\mathrm{N}}$).

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Figure 11Effect on global climate: simulated global mean surface temperatures for the DRY (red) and the WET (blue) setups and range of the CMIP6 ensemble mean ± 1 standard deviation for the SSP5-8.5 scenario. Shown are the points in time at which the tree cover in permafrost-affected regions exceeds one-third of the surface area, the simulated global mean surface temperature reaches 1.5 ^∘C above pre-industrial levels (with the latter being based on the CMIP6 ensemble mean temperature) and the near-surface permafrost volume decreases below 10 %.

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Arctic and subarctic temperatures constitute the main drivers of a number of important processes, some of which have implications for the global climate. For example, the magnitude of the permafrost carbon feedback is largely determined by the speed with which the vast pools of soil organic matter in the Arctic and subarctic zone will become exposed to conditions that are required for microbial decomposition and hence by the rate of permafrost thaw. As shown above, the simulated degradation of the terrestrial permafrost is very different in WET and DRY and the point in time when the near-surface permafrost (almost) disappears from the northern high latitudes differs by about 50 years (Fig. 11). The terrestrial net carbon flux in the Arctic and subarctic region also depends on the trend in Arctic greening, as the expanding vegetation takes up increasingly large amounts of atmospheric CO₂ (Qian et al., 2010; Keenan and Riley, 2018; Pearson et al., 2013; McGuire et al., 2018; Zhang et al., 2018). Here, the surface temperatures determine the length of the growing season, with ${\mathrm{\Delta }}_{|\mathrm{DRY}-\mathrm{WET}|}^{\mathrm{ts}}$ leading to substantial differences between the simulated vegetation dynamics in WET and DRY. For example, the moment after which the tree cover exceeds a third of the land surface in the permafrost-affected regions differs by more than 60 years between the two simulations. Both processes, permafrost degradation and Arctic greening, are highly relevant for the atmospheric greenhouse gas concentration but are not the only ways for permafrost hydrology to affect the global climate.

Temperature differences in the Arctic and subarctic zone modulate the latitudinal temperature gradient, causing a change in the meridional heat transport which leads to a southward propagation of the temperature signal. In fact, the differences in the permafrost hydrology impact the total energy content of the Earth system to such an extent that surface temperatures across the entire Northern Hemisphere are significantly affected. Thus, the global mean temperature in WET and DRY differs by about 0.5 and 0.6 ^∘C at the beginning and the end of the 21st century, respectively (Fig. 11), despite the non-glaciated, permafrost-affected areas in the Arctic and subarctic region covering merely 5 % of Earth's surface. Here, 0.5–0.6 ^∘C constitutes substantial differences, not only in comparison to the CMIP6 ensemble spread, but also relative to the temperature increase during the 21st century. In the case of the high-emission scenario SSP5-8.5, the point in time when the simulations reach the same global mean temperature differs by about 15 years (Fig. 11). And because the albedo differences affect primarily the Arctic and subarctic region, this global mean temperature is reached with a substantially different latitudinal distribution – with the DRY simulation exhibiting predominantly higher temperatures in the northern high and middle latitudes, while the temperatures are significantly lower throughout the tropics (Fig. 12a).

Figure 12State of tipping elements and other core climate elements at 1.5 ^∘C above the pre-industrial mean: (a) differences in the annual mean surface temperatures between the DRY and the WET setup for a global mean surface temperature of 1.5 ^∘C above the pre-industrial CMIP6 ensemble mean. Shown is the difference between simulations in which the atmospheric greenhouse gas concentrations were modified to stabilize the climate for a 50-year period. (b–d) Same as (a) but for (b) glacier ablation rates, (c) annual average Arctic sea-ice concentration and (d) relative difference in subsea permafrost volume within the top 10 m of the subsurface. Shown is the Laptev Sea and the East Siberian Sea. Panel (d) is not based on simulations with the MPI-ESM but on simulations with an adapted JSBACH model that represents the permafrost dynamics on the Arctic shelves using the bottom temperatures from DRY and WET. A detailed description of this model version is given in Wilkenskjeld et al. (2022). (e–i) Same as (a) but showing the (e) (annually) burned area in boreal regions, (f) precipitation during the West African monsoon, (g) minimum 3-year mean precipitation in the Amazon Basin, (h) vegetation cover in the region of the West African monsoon and (i) strength of the Atlantic Meridional Overturning Circulation. Shown is the 50-year mean. In panels (a)–(f) and (h), areas where differences are not significant (p value >0.05) are hatched, while panel (g) shows the minimum 3-year mean precipitation over land during the 50-year period without a test of significance. Dark grey areas in panel (i) show the bathymetry of the Atlantic Ocean.

The sustainability of a given climate trajectory depends on the associated risks for natural and human systems (IPCC, 2018), some of which stem from regional tipping elements (Lenton et al., 2019), such as the West African monsoon, reaching a critical threshold. How close these elements are to a tipping point at a given global mean temperature depends on the latitudinal temperature gradient which determines the local temperature change. For a climate stabilization at a desirable level – e.g. 1.5 ^∘C above the pre-industrial mean – the state of many tipping elements in the northern cryosphere is distinctly different between DRY and WET. The near-surface permafrost volume in the northern high latitudes is 15 %–30 % lower in DRY than in WET, and the ablation rates of the Greenland ice sheet differ by up to 1 mm d⁻¹. In addition, the annual mean Arctic sea-ice concentration is reduced by up to 15 % (Fig. 12b, c) in the DRY simulations. The difference in the simulated ice coverage is particularly prominent during the summer months with the DRY simulation featuring an almost ice-free Arctic Ocean, while around 2×10⁶ km² remains ice-covered in the WET simulation (not shown). In the SSP5-8.5 simulations, the differences during summer are even more pronounced, with the DRY simulations reaching an ice-free state several decades before the WET simulations. These findings, again, shed interesting light on the regional differences in the temperature response to the two scenarios. For a given model, as well as for the observational record, a clear, linear relationship between global mean temperature and Arctic sea-ice coverage has long been identified across all months (Gregory et al., 2002; Mahlstein and Knutti, 2012; Niederdrenk and Notz, 2018). However, our investigation now shows for the first time that for a given model the same global mean temperature, in our case a warming of +1.5 ^∘C, can result in differences in the simulated sea-ice coverage owing to differences in the regional amplification of the global temperature signal. Our simulations also show that these differences are not necessarily equally pronounced across all months, as the sea-ice concentrations in March in the SSP5-8.5 simulations are barely distinguishable between the WET and the DRY simulations, while they are clearly different in September.

The sea-ice cover has a strong effect on the benthic temperatures, which, in turn, determine the state of the roughly 3.5×10⁶ km² of permafrost soils that has been submerged since the Last Glacial Maximum and forms a large part of the Arctic Shelf (Sayedi et al., 2020; Steinbach et al., 2021). The permafrost-affected sediments hold about 500 Gt of organic carbon and methane gas, with continuous thawing from the surface increasing the vulnerability of the carbon pools (Schuur et al., 2015). Here, the subsea permafrost extent near the sea bottom is strongly affected by the temperature differences between DRY and WET, in particular in the Laptev Sea and the East Siberian Sea where the frozen fraction in the top 10 m of the subsurface differs by up to 50 % (Fig. 12d; note that the subsea permafrost was diagnosed with a different model version, which is described in Wilkenskjeld et al., 2022).

In the boreal zone, the differences in the simulated temperatures and vegetation covers have a strong impact on the frequency and extent of wildfires. In DRY, the burned grid-cell fraction is up to 2 % yr⁻¹ larger, reaching up to 10 times the area burned in WET (Fig. 12e). Furthermore, the strength of the Atlantic Meridional Overturning Circulation (AMOC) is partly determined by the rate with which the surface currents cool in the North Atlantic. The lower latitudinal temperature gradient in DRY reduces this part of the thermohaline circulation, weakening the AMOC by up to 1.5 Sv (10⁶ m³ s⁻¹) in both the North and the South Atlantic Ocean (Fig. 12i). However, not all tipping elements are closer to the critical threshold due to the smaller temperature gradient in DRY. Most prominently, the position and latitudinal oscillation of the Intertropical Convergence Zone is shifted, which results in a higher intensity of the West African monsoon in DRY. Precipitation rates during the period June–September increase by up to 1 mm d⁻¹ relative to WET, which constitutes up to 70 % of the precipitation in the Sahel zone (Fig. 12f). This difference in the monsoon precipitation increases the plant-available water, and the simulated vegetation cover is up to 15 % larger in DRY than in WET (Fig. 12h). In the Amazon Basin, the precipitation rates during multi-annual periods of low precipitation (here 3 years) are up to 1 mm d⁻¹ larger in DRY (Fig. 12g). This corresponds to a relative difference of up to 40 % of the drought precipitation and may have implications for a potential dieback of the Amazon rainforest.

The above examples are by no means a complete list of the remote effects resulting from the differences in the permafrost hydrology. However, they clearly show how important the respective parametrizations are for the global climate, even though the northern permafrost regions make up only a small part of the land surface.