Climate sensitivity is hot these days. That is because ‘the lukewarmers’* have tried to suggest it is overestimated – and now real climate scientists are publishing studies showing the opposite: climate sensitivity may be underestimated.

Revising equilibrium climate sensitivity upwards, according to NASA GISS. Under ‘effective radiative forcing’ 20th century observational studies match complex models and paleoclimatology’s best estimates for CO2 climate sensitivity.

Today it’s the folks at NASA GISS, who recently published on the matter in Nature. Tomorrow it’s a Yale University-led research group, that just published about climate sensitivity in Science. You’ve guessed it – our quest to uncover the ‘Real’ Global Temperature Trend continues!

[*) 3rd generation climate deniers – ‘okay climate is warming, and yes that’s caused by us, now let’s try to still obstruct meaningful policy by denying the rest of the story’]

Quick conclusion: the lukewarmers are wrong – new research agrees climate sensitivity is at least not in the IPCC low range

These new studies still show different best-estimates for ‘Equilibrium Climate Sensitivity’ (ECS), the eventual atmospheric warming as a direct effect of doubling atmospheric CO2 concentrations (direct heat absorption + fast-acting atmospheric feedbacks, but excluding indirect carbon and albedo feedbacks, which are also net positive(!)). Some of the new studies place ECS values in the middle (around 3 degrees), some place it in the high end (3-4.5 degrees), some even place it in the ‘fat tail’ (beyond 4.5 degrees) of IPCC’s (2001, 2007, 2014) range.

So what is the main thing we can conclude thus far? None of these new studies place the climate sensitivity best estimate in the low range.

Yes, modelling the complexity of system Earth is difficult – but trying harder helps

Tomorrow we’ll pay attention to that very interesting new study about clouds – a bombshell we think – but today we have another one that should serve as a foundation to scientific thinking about climate forcing, namely the suggestion that ‘not all climate forcers are equal’ – equal in the way they act as a cooling or warming force, considering important factors like time scale and the geographical characteristics of a planet with a 3D atmosphere and a northern hemisphere with land masses and a southern hemisphere with just mainly a lot of oceans.

If your climate model calculations are still based on models ignoring one of these factors, the outcomes are probably less than perfect.

A 2014 publication in Geophysical Research Letters by two climate researchers (John Kummer and Andrew Dessler) of Texas A&M University (inspired by older work by James Hansen) was the first to show where most ‘simple planet model studies’ go wrong – assuming equal ‘efficacy’ (the amount of warming per unit global average forcing divided by the warming per unit forcing from CO2) for various climate forcers, like aerosols and ozone, which typically act at specific atmospheric altitudes, close to geographically defined emission sources and have far differing atmospheric lifespans. This study showed that attempting to better define efficacy for various forcers bridged the gap between low-outcome climate sensitivity model studies (the ones that are base on 20th century observed data) – and higher-outcome climate sensitivity complex-system model studies, paleoclimatology and research looking at interannual climate variations (with the latter three all agreeing on a higher climate sensitivity).

Now climate scientists of NASA’s Goddard Institute for Space Studies (Kate Marvel, Gavin Schmidt, Ron Miller and Larissa Nazarenko) have expanded on that work and recently performed an interesting study comparing the differences in the efficacy of the individually acting climate forcers. In their publication in Nature they write the following:

“The global mean climate responses to different forcings may differ because of the character of the forcings themselves (such as their geographical or vertical distribution) and because different forcings induce different patterns of surface warming or cooling, thereby affecting the net top-of-atmosphere radiation imbalance, and thus the ocean heat uptake rate.”

From that insight and their calculations they conclude the low-climate sensitivity model studies (suggesting climate sensitivity best estimate around 2 degrees Celsius) should be revised upwards to 2.6 degrees following differences in efficacies from ‘instantaneous radiative forcing’ – and further revised upwards to 3 degrees [the value most studies agree on] when climate forcing efficacy is calculated from ERF – ‘effective radiative forcing’.

This effective radiative forcing is the climate sensitivity calculation that incorporates temperature responses in the troposphere and land surface that are rapid compared to the ocean temperature response, using fixed-sea surface temperature experiments.

The ‘flaw’ of low-ECS climate model studies may not be so much in aerosols, the NASA study suggests, as the effective radiative forcing scenario (with high climate sensitivity) is accompanied with relatively low value for aerosol efficacy:

“Previous studies involving the GISS model found that rapid cloud changes in both hemispheres result from the rapid adjustment to aerosol forcing; effective radiative forcing isthus more hemispherically symmetric than instantaneous aerosol forcing. This increased symmetry may account for the reduced aerosol efficacy when calculated with ERF. However, further study in a multi-model context will be necessary to better constrain the efficacy associated with historical aerosol change,” the NASA researchers write on that matter.

Low bias in 20^th century climate sensitivity studies is ‘accident of history’

Rather they see a misrepresentation of ozone, volcanic and land use climate forcing in the models – as the end with the following conclusion:

“GISS ModelE2 is more sensitive to CO2 alone than it is to the sum of the forcings that were important over the past century. This is largely a result of the low efficacy of ozone and volcanic forcings and the high efficacy of aerosol and [land use] forcing (which have had a cooling effect over the historical period), although further study is needed to explore model differences in simulating efficacies and to enhance confidence in these estimates. Climate sensitivities estimated from recent observations will therefore be biased low in comparison with CO2-only simulations owing to an accident of history: when the efficacies of the forcings in the recent historical record are properly taken into account, estimates of [Transient Climate Respons – TCR] and [Equilibrium Climate Sensitivity – ECS] must be revised upwards. Accounting for this results in recent historical estimates for TCR and ECS that are more consistent with constraints based on palaeoclimate data and process-based constraints from modern climatology. Methodologies that attempt to combine independently derived constraints on sensitivity should ensure that such biases are corrected before any synthesis is performed.”

We think that last sentence should be seen as a ‘scientific warming’ to the IPCC process. Don’t let lukewarmers or other bad climate science infiltrate – again. Finetuning climate sensitivity estimates requires perfections of Earth system climate models. Leave that to the best scientists of the best scientific institutions, publishing in the best scientific journals – and next time leave out the politically motivated people that managed to lower the lower boundary climate sensitivity range* in the previous edition.

We’ll keep a sharp eye ourselves.

[From 2 degrees Celsius to 1.5. Funny coincidence how these same figures in completely different context was echoed by the UNFCCC process, just a year later, at COP21 Paris. Bet the lukewarmers didn’t see that one coming.]

© Rolf Schuttenhelm | www.bitsofscience.org