Climate Sensitivity Poll

[BornFromTheVoid]

So I thought it might be interesting to see what members think climate sensitivity, or the temperature response to a doubling of CO2, is. 

The latest IPCC report suggests that equilibrium climate sensitivity is somewhere from 2.0C to 5.0C, but most likely close to 3.0C. (They rule out below 2.0C with high confidence, but only medium confidence about staying under 5.0C).

But what do you think, and why?

[SnowBear]

45 minutes ago, BornFromTheVoid said:

So I thought it might be interesting to see what members think climate sensitivity, or the temperature response to a doubling of CO2, is. 

The latest IPCC report suggests that equilibrium climate sensitivity is somewhere from 2.0C to 5.0C, but most likely close to 3.0C. (They rule out below 2.0C with high confidence, but only medium confidence about staying under 5.0C).

But what do you think, and why?

Can we really quantify any value on it? I would say it's a guess at best. 

With so many feedbacks, unknowns and cycles at play along with the huge overall complexity all we can probably say for definite is it will increase going by observations so far, by how much?... Who can really tell? 

[BornFromTheVoid]

1 hour ago, SnowBear said:

Can we really quantify any value on it? I would say it's a guess at best. 

With so many feedbacks, unknowns and cycles at play along with the huge overall complexity all we can probably say for definite is it will increase going by observations so far, by how much?... Who can really tell? 

This is part of what climate scientists do. It might seem overly complex (as most things are once you start to investigate) but there are ways of narrowing down the values. What do the basic laws of physics tell us? What does the geological record propose?  What does the warming so far suggest?

Going from the basic laws of physics, there's a 1.2C warming purely from just doubling CO2 before any other feedbacks are considered. This isn't even up for consideration anymore. But then you have the fact that warmer air holds more water vapour, which is another GhG. Warmer air melt snow and ice, reducing the planets reflectivity and allowing even more warming.
But then there are uncertainties too. Like, how will clouds react and how will that influence the planets reflectivity? 

Svante Arrhenius, a noble prize winning physicist, was the first to propose that CO2 was a key player in the ice ages back in the 1800s, and estimated that a doubling would produce 4 to 5C of warming. Over more than a century, with all the scientific advances made, the fact that we're still close to that figure suggest that we're probably near the true answer.

[SnowBear]

1 hour ago, BornFromTheVoid said:

This is part of what climate scientists do. It might seem overly complex (as most things are once you start to investigate) but there are ways of narrowing down the values. What do the basic laws of physics tell us? What does the geological record propose?  What does the warming so far suggest?

Going from the basic laws of physics, there's a 1.2C warming purely from just doubling CO2 before any other feedbacks are considered. This isn't even up for consideration anymore. But then you have the fact that warmer air holds more water vapour, which is another GhG. Warmer air melt snow and ice, reducing the planets reflectivity and allowing even more warming.
But then there are uncertainties too. Like, how will clouds react and how will that influence the planets reflectivity? 

Svante Arrhenius, a noble prize winning physicist, was the first to propose that CO2 was a key player in the ice ages back in the 1800s, and estimated that a doubling would produce 4 to 5C of warming. Over more than a century, with all the scientific advances made, the fact that we're still close to that figure suggest that we're probably near the true answer.

Lol not sure if you remember James Burkes Connections? 

Your post put me onto looking up the mini ice age links to the Great Die Off, then remembering recently the confirmation the Vikings landed in Newfoundland way before the 1492 landing by Columbus by near 500 years, and whether their interactions with the natives may have started that die off long before thought. Leading to reading up on so e of the sagas and Greenland Inuit accounts of the viking settlers... 

I need to read the Icelandic Sagas again, and more research too on those vikings who settled for a while in what they called Vinland, more for historical interest than any climate change link to the little and mini ice ages. 

In between was a few large volcanoes too! 

All because I wondered if Svante worked out certain natural events and disease timings similar to the recent UCL studies, or whether his calculation was purely from a chemist's point of view. I still haven't answered that, got side tracked

Edited January 7, 2022 by SnowBear

[SnowBear]

OK, as Born indicated above, physics says 1.2°c minimum. 

Having done a fair bit of reading overnight into a range of factors, which includes.. 

The Great Die back of the native Americans with the possible link to increase in vegetation in the Americas. 

Volcano record and the decrease in activity in recent years hence less SO². 

The clean air acts of the US and UK reducing pollution but also reducing reflective smogs and so on. 

The Little and Mini Ice Ages of the Atlantic, Greenland and Europe. 

Increase and decrease in ice flow during the Middle Ages from the Arctic.... 

I am going to put a wild guess at........ 2.5-3.0°c.

[Midlands Ice Age]

I have gone lower than most (possibly all) ...

Why?

 EPS is a figure output from the models after they still have to include around 20-30 'parameters' that  have to be included because of unknown and unclear climate science. They then have to use many iterations (and back dating)  to try and resolve the inbuilt 'latent errors' included in their assumptions. Most of these assumptions are in the very critical areas of humidity response and cloud feedbacks, and also the balance of assumptions and responses  across the critical  variable strato and tropospheric boundary feature.

Climate  modelling is still in a very early stage of understanding in these (probably critical) areas . 

I have been monitoring temperatures for about 15years, following a career in writing models, and seeing the impact of slightly incorrect assumptions. Experience teaches me that they can be  enormous.

I still cannot see any warming occuring and increasing beyond the rate of 1.2 -1.4C that the charts were showing up to about 2014, though there has been warming since mainly caused by El Nino in 2015,

We are seeing periods more dominated by La Nina types now,  and as they seem likely to continue for a while, aided  by the usual things not modeled (Like AMO, reducing solar cycles, and PDO ),  - things that are swinging now into the negative phases, I expect that future warming will be more limited than most expect. 

So my ideas feature more around the numbers at the very bottom of the range suggested by the IPCC.

MIA

Edited January 8, 2022 by Midlands Ice Age

[Gray-Wolf]

I've plumped for 3.5 to 4 but hold fears that I'll find myself on the low side?

I'd also think that any doubling would be merely a 'snapshot' as at such levels surely we see bits of the old carbon cycle resurrected & entering into the mix even if we have reduced our emissions to zero?

[BornFromTheVoid]

I've gone with 3.0C to 3.5C. It's simply what the vast majority of evidence suggests (even without climate models). Based on the fact that we've already had 1.1C of warming from less than a 50% increase in CO2, and the planetary energy imbalance (measures of energy coming in vs going out) is continuing to grow, means there's little to support anything under 2C imo.

 

[Dancerwithwings]

You haven’t stated when you think that’ll be, for me the problem is it’s how the worlds population growth is and it’s every demanding intake that it continues with its consumption for energy that’s the biggest factor, when that’ll happen and how big it’ll be, who knows !!! ......how long is a piece of string

[reef]

I went for 4.0 - 4.5C. I think a doubling of CO2 itself wouldn't lead to this increase, but the net effects of it such as more permafrost melting releasing methane, albedo change due to ice melting, more water vapour in the atmosphere etc would lend to a greater increase.

Edited January 8, 2022 by reef

[Dancerwithwings]

7 minutes ago, reef said:

I went for 4.0 - 4.5C. I think a doubling of CO2 itself wouldn't lead to this increase, but the net effects of it such as more permafrost melting releasing methane, albedo change due to ice melting, more water vapour in the atmosphere etc would lend to a greater increase.

You’ve mentioned many factors their that’ll come apparent over time, it’s new to all,  not just now but that’s on going....in my eyes we just don’t know what will happen....complicated it is for sure,  My thoughts are with MIA regards ones

[BornFromTheVoid]

16 hours ago, Dancerwithwings said:

You haven’t stated when you think that’ll be, for me the problem is it’s how the worlds population growth is and it’s every demanding intake that it continues with its consumption for energy that’s the biggest factor, when that’ll happen and how big it’ll be, who knows !!! ......how long is a piece of string

It's based on Equilibrium Climate Sensitivity (ECS) which is temperature response after CO2 has doubled and all the feedbacks have run their course, with a sort of balance again restored (takes 100s to 1000s of years).
The transient climate response is the more short term effects.