I've been looking at Carbon Brief's Guest post: Why the 1.5C warming limit is not yet a geophysical impossibility
It's by Dr Richard Millar who is is a postdoctoral research fellow at the Oxford Martin Net Zero Carbon Investment Initiative.
The Millar et al. paper is here.
I've only skimmed it so far and have not at all begun to really assimilate it. As I understand it (disclaimer!) it's based on assumptions about the distribution of values for TCR and ECS that ultimately come from the IPCC AR5, which they note is consistent with observations but lower than (some) models:
Our median estimate climate response (TCR = 1.6 °C, ECS = 2.6 °C) is defined as the median of log-normal distributions consistent with IPCC-AR5 likely bounds on the TCR and ECS (TCR: 1.0–2.5 °C; ECS: 1.5–4.5 °C). From this, the likely above/below values are found from the 33rd and 67th percentiles of the distribution (TCR: 1.3–1.9 °C; ECS: 2.0–3.3 °C). The median TCR of this log-normal distribution is significantly lower than in the IPCC-AR5 ESM ensemble but is more consistent with observed warming to date than many ensemble members (see Supplementary Methods), indicative of the multiple lines of evidence used to derive the IPCC-AR5 uncertainty ranges.
Those values of TCR and ECS are reasonably realistic IMVHO (though others here will vehemently disagree) but I'm still dubious about Millar et al's conclusions regarding avoidance of 1.5 C warming. I will wait to say more until I've had time to read further.
Ned,
You are correct that "... others here will vehemently disagree ..." that using AR5 median values for TCR and ECS is a good idea for calculating the remaining carbon budget, as Proistosescu & Huybers (2017), and others since AR5 was published, have clearly demonstrated that AR5 erred on the side of least drama w.r.t. these values:
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."
See also the linked article entitled: "Why the climate is more sensitive to carbon dioxide than weather records suggest"
https://phys.org/news/2017-07-climate-sensitive-carbon-dioxide-weather.htmlExtract: "According to their statistical analysis, historical weather observations reveal only a portion of the planet's full response to rising CO₂ levels. The true climate sensitivity will only become manifest on a time scale of centuries, due to effects that researchers call "slow climate feedbacks".
To understand this, it is important to know precisely what we mean when we talk about climate sensitivity. So-called "equilibrium climate sensitivity", or slow climate feedbacks, refers to the ultimate consequence of climate response – in other words, the final effects and environmental consequences that a given greenhouse gas concentration will deliver.
These can include long-term climate feedback processes such as ice sheet disintegration with consequent changes in Earth's surface reflection (albedo), changes to vegetation patterns, and the release of greenhouse gases such as methane from soils, tundra or ocean sediments. These processes can take place on time scales of centuries or more. As such they can only be predicted using climate models based on prehistoric data and paleoclimate evidence.
On the other hand, when greenhouse gas forcing rises at a rate as high as 2–3 parts per million (ppm) of CO₂ per year, as is the case during the past decade or so, the rate of slow feedback processes may be accelerated.
Measurements of atmosphere and marine changes made since the Industrial Revolution (when humans first began the mass release of greenhouse gases) capture mainly the direct warming effects of CO₂, as well as short-term feedbacks such as changes to water vapour and clouds.
A study led by climatologist James Hansen concluded that climate sensitivity is about 3℃ for a doubling of CO₂ when considering only short-term feedbacks. However, it's potentially as high as 6℃ when considering a final equilibrium involving much of the West and East Antarctic ice melting, if and when global greenhouse levels transcend the 500-700ppm CO₂ range."
Edit: Please note that in the first attached image that the bar for observational ICS values entitled "Marvel et al 2016; Otto et. al. accounting for efficacy" has a median value for ICS of about 3.25C (and an upper value close to 8C); which is close to the median value on the bar entitled "PH17 (w/Lewis Comments)" for the modeled value of ICS. However, the median value of the modeled ECS value for the bar entitled "PH 17" [Proistosescu & Huybers (2017)] is close to 3.7C (with an upper value close above 6C). This is all evidence that AR5 errs on the side of least drama.
Edit 2: The first attached image is from: "Sensible Questions on Climate Sensitivity"
http://www.realclimate.org/index.php/archives/2017/08/sensible-questions-on-climate-sensitivity/