A new release of the Max Planck Institute for Meteorology Earth System Model version 1.2 (MPI‐ESM1.2) is presented. The development focused on correcting errors in and improving the physical processes representation, as well as improving the computational performance, versatility, and overall user friendliness. In addition to new radiation and aerosol parameterizations of the atmosphere, several relatively large, but partly compensating, coding errors in the model's cloud, convection, and turbulence parameterizations were corrected. The representation of land processes was refined by introducing a multilayer soil hydrology scheme, extending the land biogeochemistry to include the nitrogen cycle, replacing the soil and litter decomposition model and improving the representation of wildfires. The ocean biogeochemistry now represents cyanobacteria prognostically in order to capture the response of nitrogen fixation to changing climate conditions and further includes improved detritus settling and numerous other refinements. As something new, in addition to limiting drift and minimizing certain biases, the instrumental record warming was explicitly taken into account during the tuning process. To this end, a very high climate sensitivity of around 7 K caused by low‐level clouds in the tropics as found in an intermediate model version was addressed, as it was not deemed possible to match observed warming otherwise. As a result, the model has a climate sensitivity to a doubling of CO2 over preindustrial conditions of 2.77 K, maintaining the previously identified highly nonlinear global mean response to increasing CO2 forcing, which nonetheless can be represented by a simple two‐layer model.

General circulation models show that as the surface temperature increases, the convective anvil clouds shrink. By analyzing radiativeconvective equilibrium simulations, we show that this behavior is rooted in basic energetic and thermodynamic properties of the atmosphere: As the climate warms, the clouds rise and remain at nearly the same temperature, but find themselves in a more stable atmosphere; this enhanced stability reduces the convective outflow in the upper troposphere and decreases the anvil cloud fraction. By warming the troposphere and increasing the upper-tropospheric stability, the clustering of deep convection also reduces the convective outflow and the anvil cloud fraction. When clouds are radiatively active, this robust coupling between temperature, high clouds, and circulation exerts a positive feedback on convective aggregation and favors the maintenance of strongly aggregated atmospheric states at high temperatures. This stability iris mechanism likely contributes to the narrowing of rainy areas as the climate warms. Whether or not it influences climate sensitivity requires further investigation.anvil cloud | cloud feedback | convective aggregation | large-scale circulation | climate sensitivity H ow do clouds respond to a change in surface temperature? The answer is central to understanding how Earth's average surface temperature responds to external perturbations. But understanding how clouds change, particularly high clouds, is also crucial for understanding how regional patterns of temperature and rainfall may change with surface warming (1-5).Compelling physical arguments, with varying degrees of observational support, suggest that cloud changes with warming constitute a net positive feedback on radiative forcing (6). Two main contributors to this positive feedback are an expected reduction of low-level cloud amount (7-10) and a rise of high-level clouds (11,12). Some arguments have also been advanced for negative feedbacks that would reduce the sensitivity of Earth's temperature to perturbations, through for instance a greater preponderance of liquid in clouds at warmer temperatures (13) or, for reasons that are unclear, a reduction in the relative area of the wet, vs. dry, tropics with warming (14, 15). The wet tropics are very much associated with the occurrence of precipitating deep convection, whose detrained water condensate gives rise to the formation of high-level clouds referred to as anvils. A natural question thus arises: How does the area of the wet tropics, in particular their high anvil clouds, respond to warming?A seminal contribution to understanding controls on anvil clouds was the idea of Hartmann and Larson (11) that water vapor acts, through its control on clear-sky radiative cooling, as a thermostatic control of the height at which convective outflow occurs. According to this idea [known as the fixed anvil temperature (FAT) hypothesis] anvil clouds occur at the height where the convective detrainment maximizes. This height can be determined, via mass conservation, fr...

We review recent observations and models concerning the dynamics of Cryogenian global glaciation and their biological consequences.

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The Neoproterozoic is a time of transition between the ancient microbial world and the Phanerozoic, marked by a resumption of extreme carbon isotope fluctuations and glaciation after a billion-year absence. The carbon cycle disruptions are probably accompanied by changes in the stock of oxidants and connect to glaciations via changes in the atmospheric greenhouse gas content. Two of the glaciations reach low latitudes and may have been Snowball events with near-global ice cover. This review deals primarily with the Cryogenian portion of the Neoproterozoic, during which these glaciations occurred. The initiation and deglaciation of Snowball states are discussed in light of a suite of general circulation model simulations designed to facilitate intercomparison between different models. Snow cover and the nature of the frozen surface emerge as key factors governing initiation and deglaciation. The most comprehensive model discussed confirms the possibility of initiating a Snowball event with a plausible reduction of CO2. Deglaciation requires a combination of elevated CO2 and tropical dust accumulation, aided by some cloud warming. The cause of Neoproterozoic biogeochemical turbulence, and its precise connection with Snowball glaciations, remains obscure.

[1] Geological and geochemical evidence can be interpreted as indicating strong hysteresis in global climate during the Neoproterozoic glacial events (∼630 Ma and ∼715 Ma). Standard climate theory only allows such strong hysteresis if global climate enters a fully-glaciated "Snowball" state. However, the survival of photosynthetic, eukaryotic, marine species through these glaciations may indicate that there were large areas of open ocean. A previously-proposed "Slushball" model for Neoproterozoic glaciations could easily explain the survival of these organisms because it has open ocean throughout the tropics, but there is only a small amount of hysteresis associated with the Slushball state. In this paper a new state of global climate, the "Jormungand" state, is proposed. In this state the ocean is very nearly globally ice-covered, but a very small strip of the tropical ocean remains ice-free. The low ice latitude of the Jormungand state is a consequence of the low albedo of snow-free (bare) sea ice. If the ice latitude propagates into the subtropical desert zone, it can stabilize without collapsing to the equator because subtropical ice-covered regions have a relatively low top-of-atmosphere albedo as a result of the exposure of bare sea ice and relatively lower cloud cover. Moreover, there is strong hysteresis associated with the Jormungand state as greenhouse gas levels are varied because of the high albedo contrast between regions of bare and snow covered sea ice. The Jormungand state is illustrated here in two different atmospheric global climate models and in the Budyko-Sellers model. By offering a scenario that could explain both strong hysteresis in global climate and the survival of life, the Jormungand state represents a potential model for Neoproterozoic glaciations, although further study of this issue is needed.

Equilibrium climate sensitivity (ECS) is a widely accepted measure of Earth's susceptibility to radiative forcing. While ECS is often assumed to be constant to a first order of approximation, recent studies suggested that ECS might depend on the climate state. Here it is shown that the latest generation of climate models consistently exhibits an increasing ECS in warmer climates due to a strengthening of the water-vapor feedback with increasing surface temperatures. The increasing ECS is replicated by a one-dimensional radiative-convective equilibrium model, which further shows that the enhanced water-vapor feedback follows from the rising of the tropopause in a warming climate. This mechanism is potentially important for understanding both warm climates of Earth's past and projections of future high-emission scenarios

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