While my last several posts have focused on ECS, I concur that in the 21st century that ESS is much more relevant, as what many consider to be "slow-response" feedback mechanisms may be initiated this century, as discussed in the following Previdi et al (2013) reference:
Previdi, M., B.G. Liepert, D. Peteet, J. Hansen, D.J. Beerling, A.J. Broccoli, S. Frolking, J.N. Galloway, M. Heimann, C. Le Quéré, S. Levitus, and V. Ramaswamy, 2013: Climate sensitivity in the Anthropocene. Q. J. R. Meteorol. Soc., 139, 1121-1131, doi:10.1002/qj.2165.
http://onlinelibrary.wiley.com/doi/10.1002/qj.2165/fullAbstract
"Climate sensitivity in its most basic form is defined as the equilibrium change in global surface temperature that occurs in response to a climate forcing, or externally imposed perturbation of the planetary energy balance. Within this general definition, several specific forms of climate sensitivity exist that differ in terms of the types of climate feedbacks they include. Based on evidence from Earth's history, we suggest here that the relevant form of climate sensitivity in the Anthropocene (e.g. from which to base future greenhouse gas (GHG) stabilization targets) is the Earth system sensitivity including fast feedbacks from changes in water vapour, natural aerosols, clouds and sea ice, slower surface albedo feedbacks from changes in continental ice sheets and vegetation, and climate-GHG feedbacks from changes in natural (land and ocean) carbon sinks. Traditionally, only fast feedbacks have been considered (with the other feedbacks either ignored or treated as forcing), which has led to estimates of the climate sensitivity for doubled CO2 concentrations of about 3°C. The 2×CO2 Earth system sensitivity is higher than this, being ∼4-6°C if the ice sheet/vegetation albedo feedback is included in addition to the fast feedbacks, and higher still if climate-GHG feedbacks are also included. The inclusion of climate-GHG feedbacks due to changes in the natural carbon sinks has the advantage of more directly linking anthropogenic GHG emissions with the ensuing global temperature increase, thus providing a truer indication of the climate sensitivity to human perturbations. The Earth system climate sensitivity is difficult to quantify due to the lack of palaeo-analogues for the present-day anthropogenic forcing, and the fact that ice sheet and climate-GHG feedbacks have yet to become globally significant in the Anthropocene. Furthermore, current models are unable to adequately simulate the physics of ice sheet decay and certain aspects of the natural carbon and nitrogen cycles. Obtaining quantitative estimates of the Earth system sensitivity is therefore a high priority for future work."
In general terms I concur with S.Pansa thinking that as society makes efforts to reduce GHG emissions, the previously un-considered "slow-response feedback mechanisms" will kick-in to maintain the world at, or above, the RCP 8.5 scenario. I would also like to emphasize that once these "slow-response positive feedbacks" are initiated they are extremely difficult (or impossible) to stop in less than several thousands of years. While S.Pansa mentioned such slow-response factors such as: permafrost degradation [both CO2 and CH4], Arctic Sea Ice albedo degradation, WAIS collapse, and smoke from wildfires and burning peat; I would like to mention the following that are at least as serious:
(a) GCM forecasts indicate that after about 2030 the Equatorial Pacific will remain permanently in an El Nino-like condition, and as we are now entering a positive PDO cycle that will likely last until 2030, we may never see a high degree of La Nina conditions again.
(b) The boreal forests are currently under stress, which could reach a tipping point in the next few decades (following a RCP 8.5 scenario) due to such factors as: insects, wildfires, droughts, changing seasons that effect reproductive cycles, and heat stress.
(c) The Antarctic Bottom Water, AABW, production currently rate is dropping, and the AMO should enter a warming phase within 10-years [which will limit sequestering ocean heat content into the deep ocean in the North Atlantic for at least 30-years]; which could slow the rate of heat storage in the ocean; which would leave more heat at the surface.
(d) The tropical rainforests will be subjected to accelerated degradation both during the coming positive PDO cycle and due to increasing population growth near the tropical rain forests; which will lead to both reduced CO2 absorption and increased CO2 and CH4 emissions, this century.
edit: (f) while somewhat less certain I believe that it is important to mention that if the ocean water at the seafloor of the Arctic Continental Shelves/Slopes increases [say due to better ocean water vertical mixing, or due to changes in bottom currents] then it is plausible that methane emissions from marine methane hydrates, could also have a serious impact this century.
Furthermore, Michael neglected to mention the temporary nature of many current negative feedback mechanisms as a reason way the effective TCR is apparently less than the average TCR [particularly during the faux hiatus period]. Such temporary negative feedback mechanisms [masking factors] include:
(a) Aerosols [particularly in China] that are likely to be cleaned-up somewhat, soon.
(b) The recent negative phase of the PDO that accelerated storage of heat in the ocean [particularly in the Southern Hemisphere].
(c) The accelerated growth of shrubs in the tundra that is absorbing CO2, but which will soon drastically reduce the land albedo in the northern latitudes that are normally covered with boreal winter snow; and which will contribute to wildfires in the boreal summer.
Again, if we deny the seriousness of our situation, why should we cut back on of anthropogenic radiative forcing? Therefore, OPEC may as well keep crude oil prices low to promote economic growth and to retard the growth of sustainable energy (and conservation). Obviously, we are currently above the median RCP 8.5 scenario, and it will take serious effort (will power) to get off this trend, such as passing legislation for carbon pricing (such as a carbon fee with a dividend plan).