2012-02-13 15:29:08Tropical Thermostat Post
Chris Colose

colose@wisc...
96.249.11.201

Just worked on this in response to a latest WUWT article, but also an issue that can be tracked back to the literature as well. 

I'd like to get this out quickly (hopefully tomorrow), since WUWT manufactures articles quite rapidly and it will be quickly forgotten.

http://www.skepticalscience.com/tropical_thermostat.html

2012-02-13 15:56:02
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.108.231

"Unstoppable Global Warming: Every 1500 years: book" => 'book' isn't in the title, right?

"WUWT articles (whose sole purpose is to provide a cheerleading forum for "anything except CO2" arguments, no matter how bad)." => while certainly true, I don't know that the bit in parentheses needs to be there.  Kind of a cheap shot.  I'd just stick to the science.

"First, it is worth reviewing some basic tropical meteorology.   First, a few quick bullet points about the tropics:" => awkward starting 2 consecutive sentences with the same word.

2012-02-13 15:56:56Quick fly-by comment
John Cook

john@skepticalscience...
130.102.158.12

Chris, I hope you don't mind I made some minor formating changes - added a rel="nofollow" to your link to WUWT, indented the quote for some visual structure and converted your subheadings to Heading 3 style. Nothing drastic, just channelling my inner-anal-retentive-designer.

2012-02-13 16:17:42
Chris Colose

colose@wisc...
72.226.121.32

Dana, I made all those changes (it gets difficult to hide my hostility for what Watts does).

Thanks, John.

2012-02-13 16:39:07
Rob Painting
Rob
paintingskeri@vodafone.co...
118.92.230.194

Not that I'm suggesting you add it to the post, but the elimination of ancient coral from the tropics during greenhouse climate episodes indicates the tropical ocean got mighty hot too.

I discussed this in the advanced version on coral atoll drowning, and will link to it in the comments threads if anyone starts quibbling about the paleo evidence.

2012-02-14 03:12:29
thingsbreak

things.break@gmail...
66.7.151.194

There are a number of paleo proxies that suggest tropical ocean temps much higher than present. There's a recent paper in P^3 reconstructing brachipod paleo environments that finds tropical SSTs far in excess of today's during the Paleozoic.

2012-02-14 03:30:53
MarkR
Mark Richardson
m.t.richardson2@gmail...
192.171.166.133

I think it's too wordy to be published right now and I'm not comfortable with some of the phrasing. I'll try to make time later to expand on this!

2012-02-14 09:48:01
Chris Colose

colose@wisc...
169.226.41.99

Guys, I don't want to rush my own post, but with this being a response to another article it would be good to get it out soon.  It won't make much sense 5 days later, especially with the short timeframe of a WUWT article being active (it's already the third or fourth article on the blog now).

2012-02-14 09:50:52
dana1981
Dana Nuccitelli
dana1981@yahoo...
64.129.227.4

I don't think many WUWT readers also read SkS, and we've got a pretty packed schedule the next couple of days.  We'll get it out soon though, no worries.  I'd like to see Mark's comments.

2012-02-14 11:04:27
Rob Painting
Rob
paintingskeri@vodafone.co...
203.96.201.24

I agree with Chris, although the level of writing is pitched a tad higher than we prefer, this is time sensitive.

2012-02-14 14:42:14
logicman

logicman_alf@yahoo.co...
109.156.34.130

suggestion:

Despite all of this, some recent papers have shown that the apparent tropical sea surface temperature (SST) threshold is an artifact with no basis in physics.

Despite all of this, some recent papers have shown that the apparent threshold to tropical sea surface temperature (SST) is an artifact with no basis in physics.

I think the 2nd version is easier to grasp for EFL - English as a Foreign Language - readers.

 

suggestion:

I thought it would be worth elaborating that point in this post.

 

The Tropics, loosely ~30 N-30 S latitude (though that is just arbitrary and a number of definitions exist) receive the substantial bulk of Earth's annual incoming solar radiation,

You need to clarify this: during their respective summers, the poles receive the most heat.

See: George Best - Elizabethan climate scientist.

and

http://earthobservatory.nasa.gov/.../EnergyBalance/...

 

The tropics are dynamically distinct than the mid-latitude regions that (I assume) many of us are more familiar with. 

The tropics are distinctly more dynamic than the mid-latitude regions that (I assume) many of us are more familiar with. 

 

There is a very weak seasonal cycle in the tropics,

A very good point which cannot be emphasised too strongly.  People who have only ever experienced the classic four seasons have no intuitive grasp of tropical climate.  It's not all shorts, sandals and pina colada: there's also the awesome spectacle of fast-moving monsoon floods.  (But I must say that in England I would get very strange looks if I stepped out into the rain with nothing but shorts and a bar of soap. ;-)

 

minor typo:

Note that I have said little of the cloud feedback issue relevant for climate sensitivity; rather, there is no compelling physical jusitification to suggest

 

I agree that any response to the WUWT bowl of glistening cherries should be posted before the rottenest ones get hidden under a new layer.

2012-02-14 18:49:33
Chris Colose

colose@wisc...
96.249.11.201

logicman,

 

I will correct the gramatical issues in the post. 

 

As far as the reference to dynamics in the tropics, actually I was not suggesting that they are more "variable" (or 'dynamic' in that sense), but that the fluid dynamical mechanisms (including the dominant circulation regime, Hadley cell vs. eddies, or the relevant triggers for convection) are different than in the higher latitudes.

As far as your reference to the poles getting sunlight, I specifically mentioned annual mean values, and it's certainly much more intuitive to people that the equator gets more sunlight (which is why the dominant heat transport mechanisms are poleward rather than equatorward, and why the equator is hotter).  The quotes in the article you cite simply refer to the solar radiation when integrated over a day.  And actually, they're wrong about the sunlight being more direct than the equator (unless they meant more direct relative to the winter months?).  The sunlight is obviously more direct at the equator (you gets more Watts for every square meter), and the high albedo in the polar regions makes that even more true.  The tilt of the Earth doesn't change that argument in the annual mean, although in principle it could if the Earth had a larger tilt in excess of 50 degrees or so. 

However, when you sum up all the contribution over the day, you get more intense sunlight during 12 hours at the equator, and 12 hours of darkness, so in the average you're adding up 0's for half the day and still dividing by 24 hours.  In the poles, there is no nighttime in the summer, so even when you add up 24 hours worth of smaller numbers, when you divide by 24 you get a bigger average result.  I'm not incredibly sure getting into this is relevant for the post (or relevant for anything physical actually), and is mostly a consequence of how you average sunlight over a certain timeframe on a tilted planet.  That doesn't mean the poles get warmer in the summer than at the equator.

2012-02-14 19:32:33
MarkR
Mark Richardson
m.t.richardson2@gmail...
134.225.187.225

Second paragraph is too 'snide' towards WUWT I think. I happen to agree with you, but changing it would still be better IMO. I'd also go through and cut down the 'fat' to make it shorter. It's obviously up to you, but I think shorter might just encourage more readers (ever tried to read a Bob Tisdale post?). My example:

 

A recent post at Watts Up With That by Willis Eschenbach goes into detail concerning a purported "tropical thermostat" that sets an upper limit on the ocean temperature, and presumably could help regulate the response to radiative forcing in a higher CO2 world. Indeed, Singer and Avery also made reference to such a thermostat in their "Unstoppable Global Warming: Every 1500 years" book

In this case the WUWT hypothesis is actually based on some scientific literature.  WUWT presents a histograms of ocean temperatures that have a sharp cut off at some threshold temperature (at ~31 °C, diagram reproduced below).  Both WUWT and some old scientific papers suggest this is a 'maximum ocean temperature' independent of solar or greenhouse forcing.

 

Figure 1. A “histogram” shows how many data points fall in each of the 1°C intervals shown along the bottom axis. The maximum is in the interval 28°-29°C. Figure and Caption reproduced from WUWT article.

Newer scientific papers show that many of the hypothesized mechanisms to explain this 'tropical thermostat' are based on misconceptions and observations of an apparent limit of ~31°C are not evidence of an upper bound on ocean temperature.

The explanation begins with a few basics from tropical meteorology:

  • The Tropics, loosely ~30 N-30 S latitude (though a number of definitions exist) receive most of Earth's incoming sunlight, and in fact receive more incoming energy than outgoing infrared radiation to space.  This implies that there is a substantial loss of energy by the tropics toward the poles by non-radiative means (atmospheric and ocean transport).
  • The tropics are dynamically distinct from the mid-latitude regions that many of us are more familiar with.  Instead of heat transport being manifest in 'eddies' (cyclones and anticyclones, and associated warm/cold fronts), the tropics instead fall under a giant overturning circulation called the Hadley cell.  Due to the weak Coriolis effect there, temperatures in the tropics are rather uniform horizontally.  In the vertical (from the surface upward) we typically think of the temperature structure in the tropics as being very close to moist adiabatic, especially over the ocean.
  • Instead of big changes in temperature between seasons, there are typically big changes in moisture. Typically the monsoon system is strongest over many regions in the summer, creating distinct wet and dry seasons.

Some have proposed 'thermostat' mechanisms which might keep SST's from ever exceeding ~31°C.  Proposals involve negative feedback cloud responses (ex. Ramanathan and Collins 1991) or enhanced evaporation that keeps the SSTs down (ex. Newell, 1979 or Hartmann and Michelsen, 1993).  In a paper on mass extinctions, Veron, 2008 mentioned that:

"...the surface temperature of the largest oceans would have been limited by the Thermal Cap of ~31C, widely believed to be the highest temperature large oceans can reach.’’

Is this "widely believed"? The answer is no.  Several papers specifically rebutted the cloud thermostat hypothesis of Ramanathan and Collins (ex. Fu et al., 1992 and later observational papers) although the thermostat arguments have been refuted a number of times (see also Wallace, 1992); Pierrehumbert, 1995 for instance, discussed the regulation of tropical SSTs in general and showed that there is no physical basis for an upper temperature bound.  More recent papers (Sud et al., 2008; Williams et al., 2009) came to similar conclusions.  So, how can we think about the problem physically?

The Tropical Energy Budget

In order to understand why neither clouds nor evaporation act as a significant buffer in the modern tropical climate, it's worth considering a few more bits of physics:

  • In the modern tropical climate the cloud shortwave (albedo) and cloud longwave (greenhouse) effects nearly cancel each other at the top of the atmosphere. The reduction in solar radiation absorbed is nearly balanced by a reduction in the outgoing infrared radiation that cools the planet - their net effect is close to zero. Even if you removed clouds altogether from the tropics, you could introduce many subtle impacts on the atmospheric heating distribution, circulation, and perhaps differences in SST across the tropics.  This doesn't imply that clouds are unimportant, just that there are other factors involved and there's no explanation yet of why clouds would have some special place is controlling the absolute tropical SST.
  • In a new climate the albedo effect of clouds might beat the greenhouse effect.  This is a separate argument about climate sensitivity.  Most studies show that the longwave feedback effect is likely to be robustly positive (see ex. Zelinka and Hartmann, 2010; the IPCC Fifth assessment report reserves a whole chapter for these cloud and aerosol issues). The albedo side of the equation is more uncertain but there's no convincing argument as to why this should provide a "thermal cap" on the oceans independent of any large forcing.
  • The tropics are partly stabilized heat transport towards the poles and also by dry regions where infrared radiation more easily escapes to space. Near the equator the large moisture content acts as an infrared 'insulator', but dry regions have a weaker greenhouse effect. Transport of heat into these dry regions lets them act like "radiator fins" (Pierrehumbert, 1995) where energy can more readily leak out into space.  If it weren't for this heat escape then the bulk of the tropics would, in isolation, collapse into a runaway greenhouse state.  This, however, doesn't mean that tropical SSTs cannot increase at all!
  • Evaporation does not regulate the absolute value of SST, an argument taken up by Pierrehumbert (1995, cited previously) and shown by a number of papers such as Miller, 2011. Instead, evaporation wipes out the differences between the SST and the overlying air temperature.  Changes in top-of-atmosphere heating impact SST more than changes in the surface effects, since the whole atmosphere has to regulate its outgoing longwave radiation in response to changes.  Most longwave radiation escapes in the high atmosphere, and the troposphere is well-mixed by convection such that it warms and cools as unit in order to balance ingoing and outgoing energy.  Evaporation is a buffer that takes up the slack between radiation absorbed at the surface and the flux of energy required to keep the SST close to air temperature.

What about the Histogram of SSTs and the trigger for Deep Convection?

The arguments for clouds regulating the tropical ocean temperature boil down the question of what SST is required for the onset of deep convection (and thus deep cloud formation).  In the modern climate, this is 25-30 °C, but depends on when air near the surface becomes buoyant relative to air in the upper atmosphere. If the troposphere warms then the SST threshold to kick in convection must also increase. In other words, deep convection forms the 'temperature cliff' seen in the figure, but there is no magical reason that it has to be 31 °C, it can change. The figure below shows a particular result for the longwave and shortwave fluxes.  The threshold for deep convection is readily seen in figure 2a (the LW flux): when convection starts lots of clouds form and the longwave radiation spikes upwards due to the greenhouse effect. As temperatures rise (going from blue to red line), the temperature threshold rises (moves to the right).

 

Figure 2. a) TOA cloud LW flux as a function of SST, b) TOA cloud SW flux as a function of SST; Solid blue and dashed red lines correspond to the ensemble median over years 0–20 and 60–80, respectively, from 15 IPCC AR4 coupled ocean-atmosphere models for the 1% per year scenario. Vertical lines indicate the interquartile range.

The Past Climate Record 

There is abundant palaeoclimate evidence that tropical sea temperatures can rise well above present values.  Improved understanding of oxygen proxies and the development of new proxies ('thermometers of the past') such as TEX86 and Mg/Ca have shown that Eocene SST's in the tropics could have been even hotter than 35 °C (see ex, Huber, 2008).

Conclusion

There have been a number of "false thermostats" and incorrect assumptions about a universal and unchanging "convective threshold" that kicks in heavy cloud formation. There's been a long history of refuting thermostats of this sort, but apparently this isn't universally appreciated. This isn't a discussion of the cloud feedback effect but it shows there is no compelling physical justification for tropical sea surface temperatures to be pegged at some magical maximum value independent of the forcing, or that clouds/evaporation must act as some sort of tropical regulation mechanism.  The figure below shows a physics based simulation by Sud et al (2008) simulating 10N-10S latitude tropical SSTs for present day heating versus the extra heating that would happen if CO2 were doubled:

 

This reads more easily to my eyes, and it shortens your length by 500 words without losing anything. Another thought is; people spend a limited time on here, wouldn't you rather they read 4 articles than 3? ;)

2012-02-15 07:27:37
Chris Colose

colose@wisc...
169.226.41.99

Thanks Mark, appreciate the feedback.  I didn't completely change it to your style, and I felt a couple points needed another sentence or two of emphasis, but I did shorten up several segments in accord with what you did. 

I also cited one more study that supports the post with modern observations (I combined this with the past climate record section and changed the section title).  I only found about that paper yesterday in a seminar at our department, which coincidentally was on a very related subject.

2012-02-15 08:01:10
dana1981
Dana Nuccitelli
dana1981@yahoo...
64.129.227.4

We'll aim to publish this tomorrow, Chris.

2012-02-15 08:01:16
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.243.213

I'd suggest that your post become a rebuttal too Chris, because we can guarantee this zombie will be re-animated at some point in the future.

2012-02-15 16:37:33
thingsbreak

things.break@gmail...
98.204.66.145

On Self-Regulation Of The Climate System – A New Analysis By Willis Eschenbach

2012-02-15 16:51:35
Chris Colose

colose@wisc...
72.226.121.32

Thanks! I will be sure to include Pielke Sr. as someone who referenced that post.  It's just really irresponsible on his part.  It's something any climate scientist should be able to research within 10 minutes after tracking some of the early publications like Ramanathan and Collins.

Then again, a number of recent studies (like the one on mass extinctions or corals mentioned in my post, not necessarily written by atmospheric specialists, apparently still think it's a valid theoretical basis)

2012-02-15 17:15:25
Alex C

coultera@umich...
67.194.47.80

>>>In a new climate it is possible that the albedo effect of clouds could win...

2012-02-15 17:34:44
Chris Colose

colose@wisc...
72.226.121.32

thanks

2012-02-15 17:41:47
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.236.74

"like the one on mass extinctions or corals mentioned in my post, not necessarily written by atmospheric specialists, apparently still think it's a valid theoretical basis"

Hard to believe they can write such rubbish, especially as their colleagues have published work indicating the tropical oceans were lethal to coral during the Cretaceous, and that the PETM and EECO warming shifted coral distributions poleward too.

Mind you, John Bruno, himself an expert on coral, has said that others in the field have some pretty strange ideas about other branches of climate science.  

2012-02-15 17:51:49
Chris Colose

colose@wisc...
72.226.121.32

Rob,

I'm not an expert in corals or their use as a proxy, but I'm not sure they are as useful a proxy for this question as you think.  Corals can respond largely to temperature anomalies (not just absolute temperature thresholds), and also to a range of geochemical factors (e.g., ocean acidification) or sedimention load changes.  And as living creatures that evolve and with a number of diverse species, any sort of "threshold" would not apply universally.

2012-02-15 18:41:13
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.236.74

That scleractinian (symbiotic) coral are eliminated from the tropical regions when the paleothermometers indicate extremely hot sea surface temperatures, strongly suggests hot water exerts some kind of high-temperature threshold. If it was a one-off it would be difficult to make the distinction, but this is a recurring theme. And it's difficult to see how sediment changes could have affected coral atolls in the middle of the Pacific as the atolls slowly transited the equatorial regions through plate tectonic movement. As a number of papers have suggested death by coral bleaching seems to be the culprit.

Inevitably virtually all species come up against thresholds which are inimicable to them. My point is that this supports the notion of a very hot tropical ocean - it fits into the general broad framework which considers the physiology of symbiotic coral and it's suspectibilty to excessive heat. And none of which precludes the possibility that other factors, such as anoxia, played a part either. 

2012-02-15 19:19:33
MarkR
Mark Richardson
m.t.richardson2@gmail...
134.225.187.225

I think it reads a bit better now Chris.

I'd consider changing the title to something more general: 'tropical thermostat won't stop global warming' or something similar. It would make it sound more generally applicable and it gets the point across to someone who's just skim reading!

2012-02-16 03:48:18
Chris Colose

colose@wisc...
169.226.41.99

I agree...title changed

2012-02-16 04:01:14
MarkR
Mark Richardson
m.t.richardson2@gmail...
192.171.166.133

One more technical point: I think it looks more professional if you change formatting slightly. Justify the text and centre the figures! (or 'align full' the text and 'align centre' the pics, according to SkS formatting)

2012-02-16 10:03:26formatting
dana1981
Dana Nuccitelli
dana1981@yahoo...
64.129.227.4

I always clean up the formatting and make posts look purty before publishing.

I agree we should make this into a rebuttal.  I can handle that - won't take me but a few minutes.  Just need to remember to do it.

2012-02-16 19:52:01
MarkR
Mark Richardson
m.t.richardson2@gmail...
134.225.187.225

I know it's already published, but just wanted to give the thumbs up :p