My takeaway:
Fast feedback sensitivity is at least 3ºC per 2 x CO2 and likely closer to 4º.
Self - similarity might then tell you to divide the calculated temperature based on
the fast feedback sensitivity by about 2 as one would now.
Note also that the Earth System Sensitivity is deduced from various past climate change events like the Paleocene — Eocene Thermal Maximum (PETM), but the qualitative estimates of longer - term climate sensitivity are less precise than the HS12
fast feedback sensitivity estimates.
Not exact matches
The conclusion that limiting CO2 below 450 ppm will prevent warming beyond two degrees C is based on a conservative definition of climate
sensitivity that considers only the so - called
fast feedbacks in the climate system, such as changes in clouds, water vapor and melting sea ice.
The climate
sensitivity classically defined is the response of global mean temperature to a forcing once all the «
fast feedbacks» have occurred (atmospheric temperatures, clouds, water vapour, winds, snow, sea ice etc.), but before any of the «slow»
feedbacks have kicked in (ice sheets, vegetation, carbon cycle etc.).
Does it support the view that the slow -
feedback sensitivity is double that of the
fast feedback?
One issue that I have wondered about for some time is to what extent the paleoclimate record supports the distinction between slow -
feedback and
fast -
feedback climate
sensitivity.
I'm not even an amateur climate scientist, but my logic tells me that if clouds have a stronger negative
feedback in the Arctic, and I know (from news) the Arctic is warming
faster than other areas, then it seems «forcing GHGs» (CO2, etc) may have a strong
sensitivity than suggested, but this is suppressed by the cloud effect.
A 2008 study led by James Hansen found that climate
sensitivity to «
fast feedback processes» is 3 °C, but when accounting for longer - term
feedbacks (such as ice sheet disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the
sensitivity increases to 6 °C based on paleoclimatic (historical climate) data.
All this discussion of the Schmittner et al paper should not distract from the point that Hansen and others (including RichardC in # 40 and William P in # 24) try to make: that there seems to be a significant risk that climate
sensitivity could be on the higher end of the various ranges, especially if we include the slower
feedbacks and take into account that these could kick in
faster than generally assumed.
For instance, the
sensitivity only including the fast feedbacks (e.g. ignoring land ice and vegetation), or the sensitivity of a particular class of climate model (e.g. the «Charney sensitivity»), or the sensitivity of the whole system except the carbon cycle (the Earth System Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
sensitivity only including the
fast feedbacks (e.g. ignoring land ice and vegetation), or the
sensitivity of a particular class of climate model (e.g. the «Charney sensitivity»), or the sensitivity of the whole system except the carbon cycle (the Earth System Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
sensitivity of a particular class of climate model (e.g. the «Charney
sensitivity»), or the sensitivity of the whole system except the carbon cycle (the Earth System Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
sensitivity»), or the
sensitivity of the whole system except the carbon cycle (the Earth System Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
sensitivity of the whole system except the carbon cycle (the Earth System
Sensitivity), or the transient sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
Sensitivity), or the transient
sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after
sensitivity tied to a specific date or period of time (i.e. the Transient Climate Response (TCR) to 1 % increasing CO2 after 70 years).
At its present temperature Earth is on a flat portion of its
fast -
feedback climate
sensitivity curve.
This empirical
fast -
feedback climate
sensitivity allows water vapor, clouds, aerosols, sea ice, and all other
fast feedbacks that exist in the real world to respond naturally to global climate change.
It's also another piece of evidence that is consistent with
fast feedback climate
sensitivity of around 0.75 °C / W / m ².
Plotting GHG forcing (7) from ice core data (27) against temperature shows that global climate
sensitivity including the slow surface albedo
feedback is 1.5 °C per W / m2 or 6 °C for doubled CO2 (Fig. 2), twice as large as the Charney
fast -
feedback sensitivity.»
The long lifetime of the fossil fuel carbon in the climate system and the persistence of ocean warming for millennia [201] provide sufficient time for the climate system to achieve full response to the
fast feedback processes included in the 3 °C climate
sensitivity.
However, this climate
sensitivity includes only the effects of
fast feedbacks of the climate system, such as water vapor, clouds, aerosols, and sea ice.
The climate
sensitivity classically defined is the response of global mean temperature to a forcing once all the «
fast feedbacks» have occurred (atmospheric temperatures, clouds, water vapour, winds, snow, sea ice etc.), but before any of the «slow»
feedbacks have kicked in (ice sheets, vegetation, carbon cycle etc.).
http://arxiv.org/pdf/0804.1126 «Paleoclimate data show that climate
sensitivity is 3 °C for doubled CO2, including only
fast feedback processes.
This empirical climate
sensitivity corresponds to the Charney (1979) definition of climate
sensitivity, in which «
fast feedback» processes are allowed to operate, but long - lived atmospheric gases, ice sheet area, land area and vegetation cover are fixed forcings.
I am thinking that the permafrost
feedback article we were discussing was refering to a non-runaway
feedback, but rather a delayed
feedback, which is otherwise just like the
fast feedbacks except that it's slow response would make clear that it does
feedback on itself according to the climate
sensitivity from all other
feedbacks (it drives itself, via climate change, to go farther, but it approaches a limit asymptotically).
Global temperature change is about half that in Antarctica, so this equilibrium global climate
sensitivity is 1.5 C (Wm ^ -2) ^ -1, double the
fast -
feedback (Charney)
sensitivity.
The «Charney»
sensitivity is generally thought of as the medium - term response of the system, including all the
fast feedbacks and some of the longer term ones (like the ocean).
Charney
sensitivity refers to the climate
sensitivity when
fast - reacting
feedbacks (Planck response is a given — also, water vapor, clouds,... I think sea ice, seasonal snow) occur but with other things (land - based ice sheets,... vegetation -LRB-?)-RRB-
And presumably it shows that the «slow»
feedbacks which are not included in the Charney climate
sensitivity roughly doubles the effect of the «
fast»
feedbacks which are.
Essentially Charney climate
sensitivity is calculated only with the
fast feedbacks: water vapor, sea ice, etc..
The carbon cycle
feedback is potentially important to 21st century climate projections, but is not conventionally included in the climate
sensitivity as it is not a
fast feedback.
It is standard practice to include only the
fast feedback processes, including changes in water vapour, in the calculation of climate
sensitivity, but to exclude possible induced changes in the concentrations of other greenhouse gases (as well as other slow
feedback processes).
It specifically states that climate
sensitivity does not conventionally include carbon cycle
feedback as it is «not a
fast feedback.»
As such, even in the case of the carbon cycle, it would appear that WG1 AR4 deviated very little if at all from
fast feedback Charney climate
sensitivity.
So using the lower
sensitivity of the current model of 2.7 C as I understand it (at least for «
fast feedbacks») his predictions match observed temperatures more closely.
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.
Hansen et al. 2013 (a) CO2 amount required to yield a global temperature, if
fast -
feedback climate
sensitivity is 0.75 °C per W m − 2 and non-CO2 GHGs contribute 25 % of the GHG forcing.
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 empirical
fast -
feedback climate
sensitivity that we infer from the LGM - Holocene comparison is thus 5 °C / 6.5 W / m2 ~ 3/4 ± 1/4 °C per W / m2 or 3 ± 1 °C for doubled CO2.
The conclusion that limiting CO2 below 450 ppm will prevent warming beyond two degrees C is based on a conservative definition of climate
sensitivity that considers only the so - called
fast feedbacks in the climate system, such as changes in clouds, water vapor and melting sea ice.
The long lifetime of the fossil fuel carbon in the climate system and the persistence of ocean warming for millennia [201] provide sufficient time for the climate system to achieve full response to the
fast feedback processes included in the 3 °C climate
sensitivity.
The Equilibrium Climate
Sensitivity (ECS) The Economist refers to is how much Earth temperatures are expected to rise when one includes
fast feedbacks such as atmospheric water vapor increase and the initial greenhouse gas forcing provided by CO2.
Is there a commonly used term for what James Hansen is referring to when he says Equilibrium
Sensitivity [including slow
feedbacks] is 6 C for doubled Co2 compared with 3 C for ECS only considering
fast feedbacks.
Third, our calculations are for a single
fast -
feedback equilibrium climate
sensitivity, 3 °C for doubled CO2, which we infer from paleoclimate data.
Do many people make a specific distinction between
fast and slow
feedbacks as well as between transient and equilibrium
sensitivity?
However, this climate
sensitivity includes only the effects of
fast feedbacks of the climate system, such as water vapor, clouds, aerosols, and sea ice.
Moreover the recent decline of the yearly increments d (CO2) / dt acknowledged by Francey et al (2013)(figure 17 - F) and even by James Hansen who say that the Chinese coal emissions have been immensely beneficial to the plants that are now bigger grow
faster and eat more CO2 due to the fertilisation of the air (references in note 19) cast some doubts on those compartment models with many adjustable parameters, models proved to be blatantly wrong by observations as said very politely by Wang et al.: (Xuhui Wang et al: A two-fold increase of carbon cycle
sensitivity to tropical temperature variations, Nature, 2014) «Thus, the problems present models have in reproducing the observed response of the carbon cycle to climate variability on interannual timescales may call into question their ability to predict the future evolution of the carbon cycle and its
feedbacks to climate»
Second, the abstract admits that, «Pleistocene climate oscillations yield a
fast -
feedback climate
sensitivity of 3 ± 1 °C for a 4 W m − 2 CO2 forcing if Holocene warming relative to the Last Glacial Maximum (LGM) is used as calibration, but the error (uncertainty) is substantial and partly subjective» and also «Ice sheet response time is poorly defined».
The low estimates of climate
sensitivity by Chylek and Lohmann (2008) and Schmittner et al. (2011), ~ 2 °C for doubled CO2, are due in part to their inclusion of natural aerosol change as a climate forcing rather than as a
fast feedback (as well as the small LGM - Holocene temperature change employed by Schmittner et al., 2011).»
The
fast -
feedback climate
sensitivity is a reasonably smooth curve, because the principal
fast -
feedback mechanisms (water vapor, clouds, aerosols, sea ice) do not have sharp threshold changes.
Figure 1: Schematic diagram of the equilibrium
fast -
feedback climate
sensitivity and Earth system
sensitivity that includes surface albedo slow
feedbacks.
Hansen and Sato also differentiate between
fast feedback and longer - term climate
sensitivity, as illustrated in Figure 1 above.
Yesterday we saw that combining ocean thermal inertia, ocean carbon cycle inertia and climate
sensitivity fast feedback inertia, there may still be a warming time lag of up to 10 years (the first years of which show rapid warming, beyond which we see progression to asymptote).
The average
fast -
feedback climate
sensitivity over the LGM — Holocene range of climate states can be assessed by comparing estimated global temperature change and climate forcing change between those two climate states [3,86].