If what Alley says, is correct, does this mean that it isn't possible (or very difficult) to constrain sensitivity using observations?
I have written about this topic in many different posts scattered across numerous different threads, so it is understandable that with a topic as complex as climate change that confusion remains on this topic; which few scientists and zero policymakers will address honestly/openly. Effective climate sensitivity is a catch-all term that relates the change in global mean surface temperatures to radiative forcing and adjusted for thermal inertial.
Thus whether a given perturbation in radiative forcing (say modern society rushing up to the IPCC's "Carbon Budget" as fast as it can, and then slamming on the socio-economic breaks as hard as it can); might still increase the effective climate sensitivity through the rest of this century depends on a large number of factors including:
(a) resonance around strange attractors [like a strong El Nino acting to: (1) increase deep atmospheric convection in the Equatorial Pacific conveying water vapor high into the troposphere thus creating high clouds that act as a positive feedback by trapping long-wave radiation trying to escape into space; while at the same time pushing dry air from the troposphere towards the surface where it desiccates low cloud cover, thereby reducing an otherwise negative feedback as low clouds tend to reflect incoming solar radiation back into space; or (2) increasing the advection of water vapor from lower latitudes to both of the polar regions thus increasing the rate of Polar Amplification; or (3) accelerate ice mass loss from the WAIS thus contributing to Hansen et al. (2015) positive feedback where the freshening of the surface waters in the Southern Ocean both increase sea ice extent thus reducing the radiation of long-wave radiation to space and also by decreasing the rate of the MOC thus creating greater temperature gradients between the tropics and the polar regions thus contributing to extreme weather events and a possible flip to an equable climate say by the end of this century.]
(b) The rate of activation of non-linear feedbacks (both positive and negative) [like increasing wind velocities from increasing storm activity accelerating the rate of trans-evaporation resulting in higher than expected amounts of water vapor in the atmosphere; and/or a temporary surge in vegetation growth triggered by increasing atmospheric CO2 concentrations, followed in several decades by a surge in the release of CO2 & CH4 temporarily sequestered back into the atmosphere by heat and drought/flood stress.]
(c) The rate at which Earth Systems state-dependent tipping points on individual feedback mechanisms are reached [like the accumulation of sufficient heat content in the ocean so that changes in the paths of warm ocean currents trigger Clathrate Gun scenarios along continental slopes in various portions of the ocean such as the Arctic Ocean, the Southern Ocean, etc.]
(d) The rate at which the negative forcing of aerosols is eliminated [like a rapid switch to renewables drastically reduces anthropogenic aerosol emissions to this negative forcing is reduced faster than previously expected].
Edit: I add that the US DOE is so concerned that the current generations of ESMs do not adequately bound climate sensitivity in future projections that they are currently spending hundreds of millions of dollars to create the state-of-the-art ACME ESM in order to try to get a better handle on this difficult issue.
