I thought the recent discussions on climate sensitivity at
Ringberg were quite interesting. I have summarised each one to the best of my understanding. Note that these presentation are not peer reviewed science, but more the working hypothesis of researchers currently on the frontline, so a solid peer review may spot errors and issues with any of these claims. I have tried to represent each presenters view as faithfully as I can and noted as a comment anything that is my opinion about these presentations:
Andrews
- feedbacks within most models become stronger over time, from about year 21 onward
- different patterns of regional sea temperature change cause different feedbacks
- for the 'slow pattern' equilibrium climate sensitivity is 5 degrees,
- For the 1900-2012 observational pattern climate sensitivity is 2.3 degrees
Comment: The slow pattern looks similar to El Nino. The observed pattern looks more La Nina like. The question is whether the observed pattern is due to natural variability occurring on a century timeframe to slow down warming , or whether this pattern represents a significant negative feedback. If the pattern has been a response to Co2 it may continue and be a substantial negative feedback. However the pattern may change over time. In particular it could be a response to the rate of warming (eg related to the temperature difference between surface and deep ocean), in that case the patterns would induce a negative feedback while warming is fast, but as we approach equilibrium and the rate slows the feedback may disappear, with the net result that sensitivity is still high but it takes a lot longer to reach.
Annan I
- Compares model results with paleoclimate observations, particularly for the last glacial maximum.
- Earth System Sensitivity likely beteween 1.1 and 2 times Equilibrium Sensitivity, best estimate of 1.5. Most likely Equilibrium Sensitivity is 3, Earth System Sensitivity 4.5
- Modesl good at broad scale, poor at regional scale, no improvement from CMIP3 to CMIP5
- Doubts that sensitivity outside CMIP range (say 1.5 to 5) could reproduce changes observed in LGM and other paleo epochs
Annan II
- Discusses model independance and bayesian analysis.
- True independance would give a range of 2.9 to 3.5 for sensitivity (with a 95% confidence interval)
- No one thinks the models are independant.
- Although some have tried, no one has produced a useful definition of independance.
Armour
- models show feedbacks that increase over time. Energy balance models assume constant feedbacks.
- Therefore energy balance models may underestimate sensitivity should be considered a lower constraint
Armour Efficacy
- Discusses energy balance calculations
Bellouin
- Discusses 'fast adjustments'
Bengtsson
- discusses the usability of various sources of temperature data for estimating sensitivity
Bony
- Finds observational evidence for an IRIS effect.
- But finds no negative feedback associated with this effect
- Short wave effects from low clouds seem to oppose this effect
- Sensitivity unlikely to be below 3 degrees
Caballero
- Discusses paleo, primarily PETM. Sensitivity about 3K
- higher sensitivity at warmer temperature due to tropical cloud feedbacks
Comment: The model output looks to me like a stronger MJO, and an increased convective aggregation (IRIS effect)
- no compelling reason to think that modern ECS is outside "canonical" 2.5-4.5 K
Church
- Discusses heat uptake of oceans
Crucifix
- discusses energy balance modelling from paleo data
- finds effective (transient?) sensitivity of 2.33 K
Dessler
- equilibrium sensitivity is 1.8-2.2 without considering cloud effects (comment: and excluding earth system issues such as changes in Co2 via vegetation, permafrost etc)
- Once clouds are included it is 1.9 to 5.1
Edwards
- Based on paleo evidence equilibrium climate sensitivity is between 1.6 and 5.4
Fasullo
- As the energy balance was unchanged during the hiatus there is no evidence for weaker +ve feedbacks
- sea level rise during the "grand" hiatus 1945 - 1975 continued with no slowdown
- Therefore the slow down was not externally forced, implying a greater role for PDO, smaller role for aerosol cooling.
- Comment: less aerosol cooling tends to imply a lower climate sensitivity as less current warming has been offset by this cooling effect
Geoffroy
- Describes cloud changes for higher sensitivity models
Golaz
- to some extent climate sensitivity can be 'tuned'
comment: It is a favorite denier meme that the climate models can be tuned to get any result wanted. Note that Golaz says 'to some extent'. If they can be tuned to give any result wanted then why has no skeptical scientist tuned a climate model to produce a low sensitivity?
Gregory
- climate feedbacks increase over time.
- therefore energy balance estimates may be low
Hegerl
- Discusses diagnosis sensitivity from observations
- good lower bound, not so good upper bound due to potential non-linearities (comment: increasing over time is a non linearity)
Knutti
- Feedbacks change over time
- Comment: The presentation here makes it look more like a fast feedback vs slow feedback issue
Kosaka
- Energy accumulation during the hiatus has continued without slowdown, however energy balance model suggests that 'natural variability' would cause an increase in heat accumulation
- Therefore current hiatus is internal variability not externally forced
- Comment: while this would be the case if natural variability involves moving heat from the ocean surface to the depths, it would not be the case if natural variability causes a change in the radiative balance. Natural variability could affect the radiative balance through changes in clouds, or increasing warmth in snow covered areas resulting in an albedo shift.
Latif
- Southern ocean obs suggests a 100 year cycle, modelling can produce a 400 year cycle.
Lewis
- Many observational estimates of climate sensitivity are flawed (AMO, bad priors too high aerosol forcing). (comment: that is all the higher ones)
- Paleo estimates are generally uncertain
- models warm 3 times faster than obs for 1988 to 2012
- therefore the lower estimates of climate sensitivity from obs are the only good estimates and ECs < 2.2
Comment: while the criticisms he makes of other estimates may be (at least in part) genuine he ignores weaknesses in the estimates that he prefers. It is not very useful or even particularly clever in science to point out that particular methods have weaknesses or uncertainties (particularly in a field like climate science).
It is more interesting to point out how these weaknesses may be corrected and what happens when you do. As an example models overestimate recent warming. What happens if you try to correct for this by selecting model runs that match the recent slow warming? You get almost exactly the same warming rate.
Schmidt
- not all forcings have the same effect.
- adjusting for this effect on observational studies results in an increase in ECS as calculated from energy balance methods from 1.9 to 3.1
Sexton
- General discussion on issues around climate sensitivity
Sherwood
- Slower warming in SH since 1979 which is not explained by models
- Possible mechanism is that ozone reduction has caused an increase in Southern Ocean winds. This has increasing aerosols, leading to an increase in cloud that has caused regional cooling
Stephens
- discussion on cloud feedback issues (both +ve and -ve). Seems incomplete.
Stevens
- some (not yet entirely convincing) reasons why Equilibrium climate sensitivity is between 2.0 and 3.5
- cloud feedbacks are positive, but not strongly positive
- aerosol forcing is not likely to be high
- no strong negative feedbacks to push ECS under 2.0
Sutton Hawkins
- Constraining CMIP models based on observations results in a 10-20% reduction in 21 century projections
- using a different reference period for climate projections changes the result
Vial
- Discussion on detailed modelling of convection to investigate low cloud feedbacks. No results.
Webb
- Discussion on a variety of mechanisms for cloud feedback
Zelinka
- cloud feedbacks are not likely to be strongly negative.