The Climate Feedback Debate

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From Watts Up With That?

Bob Irvine

The IPCC’s position, based on a doubling of CO2 concentration (CO2x2), is that an initial warming at equilibrium of 1.1°C without feedback is increased nearly 3-fold to about 3.0°C by strong net positive feedback.

This strong net positive feedback to small initial warming from increased CO2 concentration implies several improbable outcomes.

  • Firstly, that CO2 must be the dominant control knob of the climate. A corollary of this is that nearly the entire 33°C Greenhouse effect would disappear if the 8°C CO2 component was removed. This despite the sun still beating strongly into the tropical oceans creating large quantities of water vapour, a strong GHG. In Lacis et al, 2010, the CO2 GHE was zeroed and consequently the modelled high feedback reduced global mean surface temperature by 34.8°C within 50 years. They used GISS 2° x 2.5° Model E (AR5 Version) to achieve this unlikely result.
  • Secondly, all the models that include this high net feedback are running too hot and have been falsified.
  • And thirdly, that there would be little or no change in temperature over the last few millennia followed by a sharp up-tick as CO2 concentrations increased from about 1950 onwards. The hockey stick graphs produced by the IPCC to support this narrative are very obviously either very poor science or corrupt. Take your pick.

The problem and paradox here is that all the IPCC’s known feedbacks when counted together produce a strong positive temperature reinforcement while real world measurements point to benign or even negative feedback.


The IPCC AR4 (2007) defines its net climate feedback as follows.

In AOGCMs, the water vapour feedback constitutes by far the strongest feedback, with a multi-model mean and standard deviation for the MMD at PCMDI of 1.80 ± 0.18 W m–2 °C–1, followed by the (negative) lapse rate feedback (–0.84 ± 0.26 W m–2 °C–1) and the surface albedo feedback (0.26 ± 0.08 W m–2 °C-1). The cloud feedback mean is 0.69 W m–2 °C–1 with a very large inter-model spread of ±0.38 W m–2 °C–1 (Soden and Held, 2006).4

The IPCC AR6 (2019) changes some of these feedbacks significantly but ends up with approximately the same net feedback. The AR4 and AR6 feedbacks are compared in the table below. The lapse rate feedback (LRF) and the water vapour (WV) feedback have been combined in this table for comparison purposes.

FEEDBACKAR4 (2007)      (W/M-2 °C-1)AR6 (2019)      (W/M-2 °C-1)
Combined WV + LRF0.96 ± 0.081.30 (1.15 to 1.47)
Albedo0.26 ± 0.080.35 (0.1 to 0.6)
Cloud0.69 ± 0.380.42 (-0.1 to 0.94)
BioN/A-0.1 (-0.27 to 0.25)
Other (Approximate)0.190.04

Table 1 – Global temperature feedback for CO2 doubling in the IPCC’s AR4 and AR6.

You can see that if we multiply the AR4 and AR6 total (2.1) by the standard conversion factor (ƛ) 0.3 then plug the result into the standard feedback equation, we get the IPCC’s likely temperature at equilibrium after all feedback has acted.

Equation 1 – Equilibrium Temp. for CO2x2 (ECS) = 1.1/[1-(2.1×0.3)] = 3.0°C

The first thing to notice here is that the IPCC’s cloud feedback parameter is believed to have fallen significantly between 2007 and 2019 with this fall compensated for by a large rise in combined WV and LRF. One of the readers here may be able to offer a good explanation for this but it does seem unlikely considering the fall in Global Relative Humidity recorded by the UK Met Office, figure 2, and discussed below.

The other thing to notice is that the IPCC’s high feedback leads us very close to a crazy discontinuity as can be seen in figure 1, below. For example, if the LRF was not relevant or was removed then Equilibrium Climate Sensitivity (ECS) (CO2x2) using the AR4 figures would become an improbable 9.16°C as per equation 2.

Equation 2 – Equilibrium Temp. CO2x2 (ECS) (2007) = 1.1/[1-(2.94×0.3)] = 9.16°C

Equilibrium Climate Sensitivity (ECS) is increased by an improbable 6.16°C (9.16 – 3.0) simply by removing the LRF. This would occur, for example, if the hot spot above the tropics were less than expected or insignificant, something that appears to be the case. The atmosphere under this high feedback scenario would become very unstable and unlike the atmosphere we live with every day.

This may not be just wild conjecture as our understanding of average changes in emission at a warming average emission height are limited as can be seen by the IPCC’s large change in WV+LRF from 2007 to 2019. Although representing the same thing, they couldn’t even get their model spread (confidence limits) to overlap. What is going on here?

A consequence of this is that small changes in our understanding of the WV and LRF have a disproportionate effect on the estimated ECS.

This high net positive feedback to CO2x2 could, of course, be possible but to my mind is very unlikely, particularly given the temperature variation over recent millennia and the recent failure of the climate models that incorporate this high net feedback.

Figure 1 – Primary feedback compared to final equilibrium temperature, °C. Graph- Thanks to “The GH Defect…saving the planet from idiocy”.

To counter the IPCC’s high feedback, sceptical types like myself need to come up with a feasible alternate scenario. This is what I have tried to do in the next section.


There are several possible reasons why net feedback could be significantly lower than the IPCC’s published range as outlined in table 1 including various negative cloud feedbacks that have not to date been included by the climate models.  In addition to these the possibility that decreasing relative humidity would significantly lower feedback response in a warming world is discussed below.

Global Specific Humidity is following global temperature very closely, so the atmosphere is accumulating moisture as it warms. The problem for the climate models is that they assume that Global Relative Humidity will remain steady or increase slightly over the oceans as the planet warms. This is simply not happening according to the UK Met Office with significant implications for the WV feedback and its partner the LRF.

How much this would affect ECS and our understanding of the WV/LRF feedback is hard to say. What we can say is that it should increase the negative LRF feedback significantly compared to the positive WV feedback. The modeled signature of this WV feedback is a distinct warming high above the tropics. This has not occurred as expected and has caused many to doubt the models in this area. The falling Relative Humidity over the oceans could be related to and possibly help to explain the failure of this hot spot to materialize.

Figure 2 – Global time series of annual average relative humidity for the land (green line), ocean (blue) and global average (dark blue), relative to 1981-2010. The two-standard deviation ranges for uncertainty are shown combining the observation, sampling and coverage uncertainty.  Credit:  Met Office Climate Dashboard

Figure 3 – Global time series of annual average specific humidity for the land (green line), ocean (blue) and global average (dark blue), relative to 1981-2010. The two-standard deviation ranges for uncertainty are shown combining the observation, sampling and coverage uncertainty. Credit: Met Office Climate Dashboard.

Note: The Specific Humidity as seen in Figure 3 bears a striking resemblance to the UAH satellite temperature series while not matching the NASA GISS series well at all.

Development of these graphs is discussed here.

ESSD – Development of the HadISDH.marine humidity climate monitoring dataset (

Here is a quote from Dr. Kate Willett that indicates the problem this falling Relative Humidity could have for the climate models.

“This decrease is difficult to explain given our current physical understanding of humidity and evaporation. For example, the expectation from climate models is that ocean relative humidity should remain fairly constant or increase slightly.”

Dr Kate Willett, a climate scientist at the Met Office Hadley Centre

Guest post: Investigating climate change’s ‘humidity paradox’ – Carbon Brief

Bony et al. discuss the relationship between Relative Humidity and feedback in the climate system below.

“As illustrated in Fig. 12, the free troposphere is particularly critical for the water vapor feedback, because humidity changes higher up have more radiative effect (Shine and Sinha 1991Spencer and Braswell 1997Held and Soden 2000Marsden and Valero 2004Forster and Collins 2004Inamdar et al. 2004). In the Tropics, the upper troposphere is also where the temperature change associated with a given surface warming is the largest, owing to the dependence of moist adiabats on temperature. If relative humidity changes little, a warming of the tropical troposphere is thus associated with a negative lapse rate feedback and a positive upper-tropospheric water vapor feedback. As explained by Cess (1975), this explains a large part of the anticorrelation discussed in the introduction between the water vapor and lapse rate feedbacks of climate models (Fig. 1). It explains also why the magnitude of relative humidity changes matters so much for the magnitude of the combined water vapor–lapse rate feedbacks: a change in relative humidity alters the radiative compensation between the water vapor and lapse rate variations, so that an increase (decrease) in relative humidity will enhance (lessen) the water vapor feedback relative to the lapse rate feedback.”

How Well Do We Understand and Evaluate Climate Change Feedback Processes? in: Journal of Climate Volume 19 Issue 15 (2006) (


The IPCC has indicated many times that they believe the science around global climate feedback is settled. How can this be reconciled with the published WV/LRF increasing significantly between their 2007 report and their 2019 report (Table 1). The published confidence limits don’t even overlap.

The climate model expectation is that relative humidity should remain steady or rise slightly with increased temperature. This is not happening (Figure 2) and cannot be reconciled with the large rise in WV/LRF between the 2007 and 2019 IPCC reports.

Unless somebody has a better explanation, it seems likely that the IPCC needed to keep the 3.0°C ECS for political reasons and simply altered the various feedback parameters to suit.