As a follow-on to my last post, the first linked reference runs two modeled radiative forcing scenarios for the Eocene, one driven by high atmospheric CO₂ concentrations and one driven by reduced cloud albedo (and moderate CO₂ concentrations). "The two simulations have an almost identical global-mean surface temperature and equator-to-pole temperature difference …" but different regional signatures that could be checked using both local paleodata and/or other models run with same forcing scenarios. It goes without saying that the reduced cloud albedo scenario indicates higher climate sensitivity, and also closely matches the paleodata findings of moderate atmospheric CO₂ concentrations during the Eocene shown in the first attached image
Carlson, H. and Caballero, R.: Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene, Clim. Past, 13, 1037-1048,
https://doi.org/10.5194/cp-13-1037-2017, 2017.
https://www.clim-past.net/13/1037/2017/Abstract. Recent work in modelling the warm climates of the early Eocene shows that it is possible to obtain a reasonable global match between model surface temperature and proxy reconstructions, but only by using extremely high atmospheric CO2 concentrations or more modest CO2 levels complemented by a reduction in global cloud albedo. Understanding the mix of radiative forcing that gave rise to Eocene warmth has important implications for constraining Earth's climate sensitivity, but progress in this direction is hampered by the lack of direct proxy constraints on cloud properties. Here, we explore the potential for distinguishing among different radiative forcing scenarios via their impact on regional climate changes. We do this by comparing climate model simulations of two end-member scenarios: one in which the climate is warmed entirely by CO2 (which we refer to as the greenhouse gas (GHG) scenario) and another in which it is warmed entirely by reduced cloud albedo (which we refer to as the low CO2–thin clouds or LCTC scenario) . The two simulations have an almost identical global-mean surface temperature and equator-to-pole temperature difference, but the LCTC scenario has ∼ 11 % greater global-mean precipitation than the GHG scenario. The LCTC scenario also has cooler midlatitude continents and warmer oceans than the GHG scenario and a tropical climate which is significantly more El Niño-like. Extremely high warm-season temperatures in the subtropics are mitigated in the LCTC scenario, while cool-season temperatures are lower at all latitudes. These changes appear large enough to motivate further, more detailed study using other climate models and a more realistic set of modelling assumptions.
The second linked Harvard Gazette article is entitled: "Reconciling predictions of climate change", & I provide the attached second associated image of a panel of the first supplemental figure from Proistosescu & Huybers (2017) that at for at least the climate model HadGEM2-ES the posterior pdf for the near future ECS has a median value of 6C with right-tailed values in the range of 8C, which is very similar to Eocene temperatures (see the third image) While it is very difficult to say at the moment what model projections are correct, I provide the following general comments:
1. The vast majority of climate models have trouble matching the observed relatively high Arctic Amplification values from paleo periods that are a little bit warmer than today (e.g. see fourth attached image).
2. The world is currently warming at a rate that is several time faster than during the PETM.
3. The 100-year CO₂-eq value at the end of 2016 was about 521ppm which is already at Eocene levels.
http://news.harvard.edu/gazette/story/2017/07/conflicting-estimates-of-rise-in-global-temperature-resolved/Extract: "“The historical pattern of warming is that most of the warming has occurred over land, in particular over the northern hemisphere,” said Cristian Proistosescu, Ph.D ’17, the first author of the paper. “This pattern of warming is known as the fast mode — you put CO2 in the atmosphere and very quickly after that, the land in the northern hemisphere is going to warm.”
But there is also a slow mode of warming, which can take centuries to realize. That warming, which is most associated with the Southern Ocean and the Eastern Equatorial Pacific, comes with positive feedback loops that amplify the process. For example, as the oceans warm, cloud cover decreases, and a white reflecting surface is replaced with a dark absorbent surface.
The researchers developed a mathematical model to parse the two modes within different climate models.
“The models simulate a warming pattern like today’s, but indicate that strong feedbacks kick in when the Southern Ocean and Eastern Equatorial Pacific eventually warm, leading to higher overall temperatures than would simply be extrapolated from the warming seen to date,” said Peter Huybers, professor of Earth and planetary sciences in the Department of Earth and Planetary Science, and of environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the co-author of the paper.
Huybers and Proistosescu found that while the slow mode of warming contributes a great deal to the ultimate amount of global warming, it is barely present in present-day warming patterns. “Historical observations give us a lot of insight into how climate changes and are an important test of our climate models,” said Huybers, “but there is no perfect analogue for the changes that are coming.”"
See also:
Cristian Proistosescu and Peter J. Huybers (05 Jul 2017), "Slow climate mode reconciles historical and model-based estimates of climate sensitivity", Science Advances, Vol. 3, no. 7, e1602821, DOI: 10.1126/sciadv.1602821
http://advances.sciencemag.org/content/3/7/e1602821Extract: "The latest Intergovernmental Panel on Climate Change Assessment Report widened the equilibrium climate sensitivity (ECS) range from 2° to 4.5°C to an updated range of 1.5° to 4.5°C in order to account for the lack of consensus between estimates based on models and historical observations. The historical ECS estimates range from 1.5° to 3°C and are derived assuming a linear radiative response to warming. A Bayesian methodology applied to 24 models, however, documents curvature in the radiative response to warming from an evolving contribution of interannual to centennial modes of radiative response. Centennial modes display stronger amplifying feedbacks and ultimately contribute 28 to 68% (90% credible interval) of equilibrium warming, yet they comprise only 1 to 7% of current warming. Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates."