2010-08-21 23:00:42Basic rebuttal 30: climate sensitivity is low DRAFT 2
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Risky Business: Gambling on Climate Sensitivity

Argument No. 30 – climate sensitivity is low

There are some things about our climate we are pretty certain about. Unfortunately, climate sensitivity isn’t one of them.  Climate sensitivity is the estimate of how much the earth's climate will warm if carbon dioxide equivalents are doubled.  This is very important because if it is low, as some sceptics argue, then the planet isn’t going to warm up very much. If sensitivity is high, then we could be in for a very bad time indeed.

There are two ways of working out what climate sensitivity is (a third way – waiting a century – isn’t an option, but we’ll come to that in a moment). The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence:

These calculations use data from sources like ice cores, paleoclimate records, ocean heat uptake and solar cycles, to work out how much additional heat the doubling of greenhouse gases will produce. The lowest estimate of warming is close to the models - 1.8°C (3.24°F ) on average - but the upper estimate is a little more consistent, at an average of around 3.5°C (6.3°F).

It’s all a matter of degree

To the lay person, the arguments are obscure and complicated by other factors, like the time the climate takes to respond. But climate sensitivity is not just an abstract exchange of statistics relevant only to scientists. It also tells us about the likely changes to the climate that today's children will inherit.

Consider a rise in sea levels, for example. Predictions range from centimetres to many metres, and the actual increase will be governed by climate sensitivity. The 2007 IPCC report proposed a range of sea level rises based on different increases in temperature, but we now know they underestimated sea level rise, perhaps by a factor of three, in part because of a lack of data about the behaviour of Greenland and Antarctic ice-sheets.

Current estimates of sea level rise alone, as a result of a two degree rise in temperature, are very worrying. More worrying is that the current projections do not account for recently accelerated melting of polar regions. There are also many other possible effects of a 2°C rise (3.6°F) that would be very disruptive.

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against mitigation because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

2010-08-22 02:42:56Little things
doug_bostrom

dbostrom@clearwire...
184.77.83.151

I'd break the first sentence up into two.

"...a third way – waiting a 100 years, isn’t..." --> "...a third way – waiting 100 years, isn’t...)

 

Pretty good treatment!

 

 

2010-08-22 03:35:15Cheers
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Nice one Doug - small but perfectly formed, although I found a slightly different solution. More's the point, I actually forgot to tidy that point up towards the end - now remedied with "Others suggest that because we can't tell for sure, we should wait and see".

2010-08-22 03:47:54
Ari Jokimäki

arijmaki@yahoo...
91.154.108.109

You have described climate sensitivity as a change to greenhouse gas doubling. I guess you can get climate sensitivity that way too, but usually it is said to be just CO2 doubling, not all GHG's.

There's something wrong in the second image; Gregory bars seem to be from Chylek numbers, Chylek bars seem to be from Tung numbers, Tung bars seem to be from Bender numbers, and Bender bars are from unknown numbers.

2010-08-22 06:36:33
doug_bostrom

dbostrom@clearwire...
184.77.83.151

Still stumbling over:

"Unfortunately, climate sensitivity isn’t one of them; it’s the estimate of how much the earth’s climate will warm if the proportion of greenhouse gases – principally water vapour, carbon dioxide, methane, nitrous oxide, and ozone – is doubled."

Taking into account Ari's point, maybe:

"There are some things about our climate we are pretty certain about but unfortunately climate sensitivity isn’t one of them. Climate sensitivity is the estimate of how much the earth’s climate will warm if the proportion of C02 in the atmosphere is doubled.  The total rise depends on additional heat trapped as other greenhouse gases increase as as a result of warming due to C02,  these additional gases being principally water vapor and methane."

Maybe a brief word on feedback is warranted? If you choose to go that route, consider inoculating the reader against runaway feedback, a favorite denier distraction...

 

2010-08-22 16:07:18Corrections
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Ari - well spotted. I got the labelling wrong, now fixed.

Ari and Doug on CO2e: I don't have a view on this at all. I found various definitions (see below) - confusingly - and stuck to the most 'accurate' i.e any forcing, not the more colloquial CO2. I could put in (or equivalent) or leave that out and just use CO2. Which is better, more accurate, more appropriate?

In Intergovernmental Panel on Climate Change (IPCC) reports, equilibrium climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) CO2 concentration (ΔTx2). (Wiki)

Climate sensitivity is the term used by the Intergovernmental Panel on Climate Change (IPCC) to express the relationship between the human-caused emissions that add to the Earth’s greenhouse effect — carbon dioxide and a variety of other greenhouse gases — and the temperature changes that will result from these emissions. (MIT)

Climate sensitivity is expressed as the global temperature change for a particular forcing (eg - °C change per W/m2 forcing). More commonly, it's given as the warming for doubled CO2 (i.e. from 280 ppm to 560 ppm). (John Cook)

On the subject of positive and negative feedbacks, I made the decision to leave both issues out, else I'm led immediately to certain qualifications (as Doug was just by suggesting it), it makes the rebuttal more complex - one that already suffers from irreducible complexity - and increases the length of the rebuttal. Now I'm happy to write more about this, but I'd be interested to know what others think before spending more time on it. It's also worth reviewing these two posts from JC:

http://www.skepticalscience.com/Working-out-climate-sensitivity.html

(I think this one is better than the intermediate rebuttal, actually)

http://www.skepticalscience.com/Working-out-climate-sensitivity-from-satellite-measurements.html

2010-08-23 01:49:18
doug_bostrom

dbostrom@clearwire...
184.77.83.151

Graham I see your point about complication but as it stands the description has a technical fault.

"[Climate sensitivity is]  the estimate of how much the earth’s climate will warm if the proportion of greenhouse gases – principally water vapour, carbon dioxide, methane, nitrous oxide, and ozone – is doubled."

It's not the collection of gases that is doubled, it's the C02 specifically, per your (good idea!) direct sourcing of IPCC:

...climate sensitivity refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) CO2 concentration...

Also, I'm still tripping over that semicolon followed by a pronoun!

I'm outta here for two days... bye all!

2010-08-23 02:19:19Misunderstanding
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Doug - "it's the C02 specifically"

Er...no, it isn't, not according to the texts I've quoted. It is CO2 or equivalent. Or am I misunderstanding the IPCC and the MIT explanation, which says GHGs, not exclusively CO2?

2010-08-23 22:05:44CO2 equivalence
nealjking

nealjking@gmail...
91.33.120.144

gpwayne,

A problem here is what is meant by "CO2 equivalence". It's not completely straightforward:

- If memory serves, a molecule of methane has much more impact on infrared absorption than a molecule of CO2. However, a molecule of methane survives in the atmosphere for a relatively short time, whereas CO2 hangs around in the system for about a thousand years. Overall, methane is given a factor of 8 for GHG effectiveness relative to CO2.

- The "greenhouse impact" of the gases don't add up linearly: the impact of the mixture (A + B) is not (the impact of A) + (the impact of B)

 

My impression is that a calculation of the enhanced greenhouse effect operates in these steps:

- Calculate the impact of the increased concentration of X on the infrared radiation fluxes

- Calculate the impact of the IR fluxes on the planet (radiative forcing); since the planet undergoes changes,

- Iterate.

 

To discuss a definite number, you probably should stick to CO2 (doubled): I guess that CO2 equivalence for anything else would be found by comparing the radiative forcing relative to that of CO2. Another point: We discuss radiative forcing due to additional CO2 as a logarithmic effect; but I think there are some GHGs for which the effect is not logarithmic.

I think that, if you look at your quotation from the MIT write-up, you will find that they have avoided being too specific about what they say in order to avoid getting pulled into these complexities. I don't recommend that in this case: I think we are better off talking specifically about climate sensitivity defined with respect to CO2 concentration, and then (if necessary) saying that the impact of other  GHGs can be evaluated by comparing their radiative forcing to that of CO2.

2010-08-25 18:08:38CO2?
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17
OK - I've changed the GHG reference to something less controversial. Some thumbs would be good at this point, if there are no further concerns about the content...
2010-08-25 18:27:52Time frame
nealjking

nealjking@gmail...
91.33.112.139

"Climate models have predicted the lowest temperature rise  would be on average 1.65 degrees C, but upper estimates vary a lot, averaging 5.2 degrees C."

 

You should indicate the time frame over which these estimates apply. Are we talking about the year 2100, 2200, or something else?

2010-08-25 19:41:09Time frame?
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Neal, we're talking about a doubling of CO2. There is no time frame in climate sensitivity calculations - that would be emissions rate/temp i.e. AR4 scenarios.

2010-08-26 03:28:58
nealjking

nealjking@gmail...
91.33.112.139
2010-08-27 20:56:36Bump
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17

Wot, no thumbs?

2010-08-27 21:06:13OK
nealjking

nealjking@gmail...
84.151.44.130
Neal
2010-08-28 11:43:35
doug_bostrom

dbostrom@clearwire...
184.77.83.151

Graham, thanks, I stand (somewhat) corrected on that! From what I read more carefully now than before it's C02 or emissions equivalents. I -think- emissions is supposed to imply something that is a direct cultural artifact. Water vapor is (again if I'm reading correctly) considered a feedback, according to the MIT def. anyway.

So much for my "expertise." :-) 

For what it's worth, a thumbs up though I'm -still- tripping over the pronoun after the semicolon. A quirk of mine, nobody else is objecting...

2010-08-28 16:24:25Pronouns
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17
Doug - since you're a nice chap, here's a special weekend semicolon purge just for you :)
2010-08-28 16:29:23
doug_bostrom

dbostrom@clearwire...
184.77.83.151
"Semicolon purge." Ouch. 8-P
2010-08-31 18:14:00Bump
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17
...c'mon folks, let's get this one out the door...
2010-09-01 01:38:16Thumbs
Nicholas Berini

nberini@gmail...
24.189.119.236

Thumbs up!

Ill only say that I think the first paragraph could be improved by a simple restructure; 

There are some things about our climate we are pretty certain about. Unfortunately, climate sensitivity isn’t one of them.  Climate sensitivity is the estimate of how much the earth's climate will warm if carbon dioxide equivalents are doubled.  This is very important....

Otherwise I really like the figures, gambling analogy, and conclusion. 


2010-09-01 21:09:59Good call
gpwayne
Graham Wayne
graham@gpwayne...
217.44.86.17
Nice one Nicholas - I've used your line. Thanks...
2010-09-02 09:23:37Something I didn't catch before:
nealjking

nealjking@gmail...
91.33.105.199

gpwayne:

"Probably the most significant, and equally distributed, environmental change will be a rise in sea levels."

I wouldn't say that: Sea-level rise will have the most obvious direct impact on humanity, but I personally think the impact of climate change on the ecology of the planet (region by region) and the devastation to biological diversity will be the most long-lasting impact. 

I suggest: "significant" => "obvious"

 

Neal

 

2010-09-02 23:43:56another good call
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172

I see what you mean Neal - changed accordingly.

2010-09-04 18:50:12Bump
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172

We're running out of thumbs, aren't we?

2010-09-05 00:57:14on important suggestion
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.140.3.99

One thing I think it would be important to clarify is that the best estimate of climate sensitivity puts the value at around 3°C.  It's good to talk about the minimum being 2°C and not lower, but I think it's also important to specify the most likely value.  It's also not likely to be higher than 4.5°C, which might be worth mentioning.

Coincidentally, I'm about to start working on the Advanced version of this rebuttal, and think this is really good for the basic level.

2010-09-05 15:29:39A few things
Jim Meador

jimm58@gmail...
67.101.213.221

First of all, I like this one quite a lot overall.

I agree with dana1981 about the best estimate.

Probably no one will agree, but I think it would make this more impactful to US readers if there is at least one reference to °F, maybe for the best estimate. Unfortunately that's what most people here understand intuitively.

I'd change: 

"tells us about the changes we are likely to see"

to 

"tells us about the likely changes to the world that todays children will inherit" 

 ...just to try to make it less abstract and more personal.

 

 

 

2010-09-05 17:34:49Good suggestions
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172

Dana and Jim - thanks for both comments, all of which I agree with (I'm slightly concerned about 'save our kids' but hey, this is the basic version, right? :)

All suggestions now accomodated (or will be in just a moment...tea is brewing...)

...tea is brewed, changes completed...

2010-09-05 22:10:22"Lowest temperature rise"
James Wight

jameswight@southernphone.com...
58.105.164.221
“Climate models have predicted the lowest temperature rise  would be on average 1.65 degrees C”
I’d try to reword this. I know that you’re talking about the lowest estimate for climate sensitivity, but readers might be confused by “the lowest temperature rise”. Maybe “the lowest possible temperature rise”?

And I agree with Dana and Jim about the best estimate. I think laypeople would tend to be more interested in the best estimate than the upper and lower estimates.
2010-09-05 22:14:07Least instead of lowest
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172
Hi James - actually, how about 'the least temperature rise...' which I think is more accurate. As for the best estimates, I already put them in...
2010-09-05 22:21:32Temperature unit conversions
James Wight

jameswight@southernphone.com...
58.105.164.221
Graham, you’ve done the conversions incorrectly. Yes, a temperature of 3°C is 37.4°F. But the two scales have different zero points, so a temperature change of 3°C = 5.4°F.
2010-09-05 23:23:24James - thanks
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172
I was looking at those numbers thinking, I don't know - they look odd to me! Got up too early, I think - anyway, sorry about that. I'll fix it straight away.
2010-09-06 09:18:35better
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.140.3.99

That's an improvement.  But toward then end when you say

"All the models and evidence confirms a minimum warming close to 2°C for a doubling of greenhouse gases."

Again I'd suggest talking about the most likely value, and maybe the upper limit too.

"All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2, with a most likely value of 3°C and the potential to warm 4.5°C or even more."

Note that I changed "greenhouse gases" to "CO2" in this sentence, because the 3°C climate sensitivity is for a doubling of CO2 only.  If you doubled all atmospheric greenhouse gases, the would be even more warming.

2010-09-06 14:21:54Fifth thumb
James Wight

jameswight@southernphone.com...
58.105.164.221
Don’t worry, Graham, you’re not the first to make that mistake. I’ve seen it in published news reports.

Hmmm, I don’t really like “the least temperature rise” either. But maybe it’s just me. Okay, I’ll give you your fifth thumb now.
2010-09-06 16:36:29Invisible thumbs
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172

Ah James - your thumb is like my email attachments, there in principle but not so much in practice when I forgot to acually attach them.

Dana: OK, no problem, I've used your line instead.

2010-09-06 17:46:50Fifth thumb, take 2
James Wight

jameswight@southernphone.com...
58.105.164.221
Oops. Here is the fifth thumb.
2010-09-06 22:49:01Nitpick
John Cook

john@skepticalscience...
121.222.93.62
Maybe it's my illiterate colonial convict background showing but does the word "abstruse" qualify as plain English? We don't want people reaching for their dictionaries when they read our basic rebuttals.
2010-09-06 23:22:14Insurance
gpwayne
Graham Wayne
graham@gpwayne...
81.152.234.172
OK - I've changed it to 'obscure' on the basis that I don't want to experience a rash of break-ins or have a few inexpicable 'accidents', know what I mean?
2010-09-07 01:40:07Climate rebuttal 30: thumbs up
jimalakirti

jimalakirti@gmail...
70.58.169.156

I think changing "melting of both regions", being a paragraph away from the antecedent,  might be clearer and have more visceral impact of it were changed to "melting of the polar regions."

 

But this is clearly worth a thumb anyway. 

2010-09-07 02:44:18looks good
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.140.3.99
Looks good to me.  Thumbs galore!
2010-09-10 22:49:56Minor edit
gpwayne
Graham Wayne
graham@gpwayne...
86.158.204.104
jimalakirti - agreed, and thanks.
2010-09-21 14:28:59Published
John Cook

john@skepticalscience...
121.222.93.62
Sorry you had to wait for this one so long, Graham, after Dana's advanced rebuttal on climate sensitivity, thought I should give it some room to breath before I revisted the subject.