1) "If climate sensitivity is high, then we can expect substantial warming in the coming century as emissions continue to increase. If climate sensitivity is low, then future warming will be substantially lower."
2) "In fact, it seems that uncertainty about values of ECS has been increasing. The bottom of the ‘likely’ range has been lowered from 2 to 1.5oC in the AR5, whereas the AR4 stated that ECS is very unlikely to be less than 1.5oC. It is also significant that the AR5 does not cite a best estimate, whereas the AR4 cites a best estimate of 3oC. The stated reason for not citing a best estimate in the AR5 is the substantial discrepancy between observation-based estimates of ECS (lower), versus estimates from climate models (higher).
3) "If the climate sensitivity is on the low end of the range of estimates, and natural internal variability is on the strong side of the distribution of climate models, different conclusions are drawn about the relative importance of human causes to the 20th century warming."
4) "Whether or not human caused global warming is dangerous or not depends critically on whether the ECS value is closer to 1.5oC or 4.5oC."
5) "Given the uncertainties in equilibrium climate sensitivity and the magnitude and phasing of natural internal variability on decadal to century timescales, combined with the failure of climate models to explain the early 20th century warming and the mid-century cooling, I conclude that the climate models are not fit for the purpose of identifying with high confidence the proportional amount of natural versus human causes to the 20th century warming."
Is this an accurate summary of facts? Do we have good evidence about the true value of ECS that she does not mention? Is there evidence for scenarios where a 'low end' Equilibrium Climate Sensitivity still results in a dangerous warming? Or are there indeed values for ECS that would be considered 'likely' but do not result in dangerous warming?
Lack of knowledge about the true value of ECS is not an excuse to avoid precautionary measures, but it would seem to raise questions about the suitability of current climate models as predictors for future climate. At the least, it would imply that determining the true ECS should be a very high priority while other mitigation measures are taken.