These findings are incompatible with conventional Arrhenius radiative greenhouse/IPCC theory, which postulates radiative forcing from greenhouse gases has always caused a positive-feedback 'greenhouse' warming effect at all historical Earth surface temperatures and greenhouse gas concentrations.
However, the findings of this paper demonstrating that greenhouse gases have a negligible or cooling effect at present Earth surface temperatures are compatible with the Maxwell/Clausius/Carnot gravito-thermal greenhouse theory, the HS 'greenhouse equation,' Chilingar et al, Kimoto, Wilde, and others.
The authors find (excerpts),
"shortwave [solar] radiation is a strong positive feedback at low surface temperatures but weakens at higher temperatures, and longwave radiation [from greenhouse gases] is a negative feedback at low temperatures [295K or 15C], but becomes a positive feedback for temperatures greater than 295–300 K [current Earth temperature is 288K or 15C].
It has been recently suggested that some modes of convective organization may result from an instability of the background state of radiative-convective equilibrium, which results in separation of the atmosphere into moist regions with ascent and dry regions with subsidence (Emanuel et al. 2014). If such an instability indeed exists in the real [not modelled] atmosphere, it would reshape our understanding of tropical circulations, and could help to explain the growth and life cycle of large-scale organized convective systems such as tropical cyclones and the Madden-Julian Oscillation (e.g., Bretherton et al. 2005; Sobel and Maloney 2012). If this instability is temperature-dependent, as suggested by numerical modeling studies (Khairoutdinov and Emanuel 2010; Wing and Emanuel 2014; Emanuel et al. 2014), then the increasing tendency of convection to organize with warming could also alter the climate sensitivity significantly (Khairoutdinov and Emanuel 2010); it is unclear whether current global climate models capture this process adequately. [Described in a recent post by Stephen Wilde]
Is the phenomenon of self-aggregation in a 200 km × 200 km domain in a model with explicit convection and clouds possibly the same as that in a 20000 km × 20000 km domain in a model with parameterized [fudge-factored] convection and clouds? This question has largely gone unaddressed, but it is essential to answer if we want to understand the robustness of self-aggregation across our modeling hierarchy and its relevance to the real atmosphere.
In all simulations, the troposphere warms and dries relative to the initial condition (Figure 2(a)), though the stratosphere cools in simulations where Ts is lower than 300 K (Figure 2(b); Table 1). Tropospheric warming overall, and the increase in tropospheric warming with Ts, are consistent with the finding by Singh and O’Gorman (2013) that the lapse rate in RCE depends on entrainment and free-tropospheric relative humidity. In our simulations, aggregation decreases the free-tropospheric relative humidity in the domain mean, but increases the freetropospheric relative humidity in convectively active regions, plausibly reducing the influence of entrainment on the lapse rate and driving the thermal structure of the troposphere closer to a moist adiabat. Warming of the troposphere with aggregation can also be explained as a consequence of convective cores in moist regions drawing air with higher moist static energy from deeper within the boundary layer (Held et al. 1993).
The mean outgoing longwave radiation increases over the course of each simulation as a consequence of this drying, by an amount that increases with TS, ranging from ∼11 W m−2 at 280 K to ∼24 W m−2 at 310 K (Table 1).
The frozen moist static energy (hereafter referred to as h) is conserved in dry and moist adiabatic displacements, as well as freezing and melting of precipitation; h is given by the sum of the internal energy, cpT, the gravitational energy, gz, and the latent energy, Lvq − Lf qc,i (cp is the specific heat of dry air at constant pressure and g is the gravitational acceleration). In the latent energy term, Lv is the latent heat of vaporization, q is the water vapor mixing ratio, Lf is the latent heat of fusion, and qc,i is the condensed ice water mixing ratio:
h = cpT + gz + Lvq − Lf qc,i.
Atmospheric heating and cooling lead, respectively, to moistening and drying, because the weak temperature gradient approximation implies that anomalous heating is largely balanced by ascent, converging moisture into the column, while anomalous cooling is largely balanced by descent, diverging moisture out of the column.
In the four coldest simulations, (TS = 280K, 285K, 290K, 295K), the longwave radiation is at first a negative feedback, but in the warmer simulations [>295K], it is an important positive feedback. The magnitude of the shortwave feedback decreases by nearly a factor of 10 as the surface temperature increases from 280 K to 310 K, and the shortwave feedback also becomes much less important relative to the other feedbacks.
Although the behavior of the longwave radiation feedback term in our channel simulations appears to be consistent with the temperature-dependence suggested by Emanuel et al. (2014), cloud effects rather than clear-sky radiative transfer lead to our negative longwave feedback at low Ts (Figure 5). As predicted by Emanuel et al. (2014), the clear sky longwave feedback is weaker in the colder simulations – near zero or slightly negative – but this contributes only a small amount to the total longwave feedback. Aggregation occurs in spite of an initially negative longwave feedback at Ts <= 295 K, because this negative feedback is overridden by the combination of a positive surface flux and shortwave feedbacks; recall that the increasing strength of the shortwave feedback with decreasing temperature is largely due to clouds.
A negative longwave cloud feedback implies that the atmosphere itself is cooling more in the moist regions and cooling less in the dry regions, due to the presence of clouds. We speculate that this occurs because a low-temperature atmosphere is optically thin, so the addition of clouds can increase the atmospheric longwave cooling by increasing its emissivity. An increase in longwave cooling due to greater cloud fraction in moist regions (where bh 0 > 0) is then a negative feedback on aggregation.
A key result is that the behavior of the radiative feedbacks varies with temperature, primarily due to the contribution of clouds. The longwave radiative feedback at the beginning of the simulation becomes negative as Ts is decreased, which is compensated for by an increase in the magnitude of the shortwave radiative feedback."
Self-aggregation of convection in long channel geometry
Allison A. Wing1,* and Timothy W. Cronin2
Abstract: Cloud cover and relative humidity in the tropics are strongly influenced by organized atmospheric convection, which occurs across a range of spatial and temporal scales. One mode of organization that is found in idealized numerical modeling simulations is self-aggregation, a spontaneous transition from randomly distributed convection to organized convection despite homogeneous boundary conditions. We explore the influence of domain geometry on the mechanisms, growth rates, and length scales of self-aggregation of tropical convection. We simulate radiative-convective equilibrium with the System for Atmospheric Modeling (SAM), in a non-rotating, highly-elongated 3D channel domain of length > 104 km, with interactive radiation and surface fluxes and fixed sea-surface temperature varying from 280 K to 310 K. Convection self-aggregates into multiple moist and dry bands across this full range of temperatures. As convection aggregates, we find a decrease in upper-tropospheric cloud fraction, but an increase in lower-tropospheric cloud fraction; this sensitivity of clouds to aggregation agrees with observations in the upper troposphere, but not in the lower troposphere. An advantage of the channel geometry is that a separation distance between convectively active regions can be defined; we present a theory for this distance based on boundary layer remoistening. We find that surface fluxes and radiative heating act as positive feedbacks, favoring self-aggregation, but advection of moist static energy acts as a negative feedback, opposing self-aggregation, for nearly all temperatures and times. Early in the process of self-aggregation, surface fluxes are a positive feedback at all temperatures, shortwave [solar] radiation is a strong positive feedback at low surface temperatures but weakens at higher temperatures, and longwave radiation [from greenhouse gases] is a negative feedback at low temperatures but becomes a positive feedback for temperatures greater than 295–300 K [current Earth temperature is 288K]. Clouds contribute strongly to the radiative feedbacks, especially at low temperatures [ < 295 K].