Guest commentary by Drew Shindell
The unique chemistry that causes dramatic ozone depletion in the polar springtime lower stratosphere has been studied intensely for the past 2-3 decades and much that was speculated about 30 years ago when the problem first emerged has been verified and made more coherent. However, a new report concerning laboratory measurements of a key molecule involved in this chemistry have raised questions about current understanding. The results (Pope et al., J. Phys. Chem., 2007) suggest a reduced ability for sunlight to break apart the chlorine monoxide dimer (Cl2O2) and have already led to a great deal of debate about their implications. I’ll try here to help assess what these new measurements really mean.
The past decades of study have developed a comprehensive understanding of how polar ozone depletion (“Ozone Holes”) takes place. In brief, human-produced halocarbons (chlorofluorocarbons (CFCs) and a few other molecules like methyl bromide) are broken down by sunlight in the stratosphere, releasing chlorine and bromine. These highly reactive atoms mostly go into fairly long-lived molecules that are not very reactive and therefore act as ‘reservoirs’. There are two situations in which a substantial amount of chlorine, the more important of the two, can come out of the reservoirs in large enough amounts to destroy a substantial amount of ozone. One is in the upper stratosphere around 40-50 km altitude, where strong sunlight forms reactive molecules that frees the chlorine. The other is the polar springtime lower stratosphere, where extremely cold temperatures lead to unique chemistry on the surface of ice particles that again transforms chlorine from its reservoirs into more reactive forms.
Atmospheric observations show that in both these situations, there is indeed enhanced reactive chlorine and simultaneous depletion of ozone. Measurements from satellites, aircraft, and ground-based instruments all give independent, consistent information verifying the links between cold temperatures in the polar springtime lower stratosphere and chlorine, and between chlorine and ozone. It’s important to note that none of the laboratory data on the direct chemical reactions that destroy ozone have been questioned. What has now been questioned is not the link between the chlorine released from CFCs and ozone loss, but rather the rate at which the chlorine atoms can destroy ozone via a particular cycle involving the Cl2O2 molecule.
Measurements of this molecule are exceedingly difficult to make in the laboratory as it is highly unstable. Several earlier measurements of the relevant rate have shown variations of a factor of 3 or so, so that the uncertainty in the rate is not new. However, we have substantial auxiliary evidence for what the rates must be i.e. observations of chlorine in the atmosphere provide independent constraints on Cl2O2. Limited direct observations of Cl2O2, as well as many measurements of total chlorine and of chlorine monoxide (ClO), constrain the amount of Cl2O2 (which can’t be greater than the total minus the amount in the ClO molecule). These observations are inconsistent with both the new measurements and earlier reports of a reduced ability of sunlight to break up Cl2O2 (Shindell and de Zafra, GRL, 1995, 1996; Stachnik et al., GRL, 1999; Stimpfle et al., JGR, 2004). Thus although the current state of knowledge is that the laboratory measurements on the stability of the Cl2O2 molecule vary by roughly a factor of 10 (including the newly reported values), the independent measurements suggest strongly that the upper half of that range is more likely to be correct, not the lower.
Given the difficulty in making the laboratory measurements, it is quite possible that these are wrong, and confirmation of the new results is certainly needed. Should the results hold up, the chemistry involved in polar ozone loss may need to be re-evaluated. As there are other cycles that do not involve the Cl2O2 molecule but cause similar dramatic ozone depletion, such as cycles including both ClO and BrO (its bromine-containing analogue), any revision to current understanding would most likely simply shift the relative importance of the various ozone-destroying cycles. However, as noted, it is not clear how one would reconcile these measurements with actual atmospheric observations, which are not consistent with a more stable Cl2O2 molecule.
A wealth of observational data supports the role of chlorine and bromine in polar ozone loss, and uncertainty in a single step of the relevant chemistry does not undermine the Montreal Protocol controlling substances that release these atoms into the stratosphere. It is important, however, that the new results be tested so that we can be confident we understand the potential effects of future changes in temperature on polar ozone loss (as different chemical reactions have different sensitivities to temperature). This will allow us to better understand the effects of climate change on the stratospheric ozone layer, and to verify the effectiveness of the Montreal Protocol, which has already shown signs of success in reducing the growth of atmospheric concentrations of CFCs, and seems to have lead to at least a leveling off of ozone depletion over most of the planet. Full recovery is not expected for a few decades though.
Drew, I have two questions for you: Last month your paper suggested that ozone variability played a much stronger role in local forcing of temperature at certain times of year, compared to background radiative forcing; would a finding such as this, if it were confirmed, have any effect on the results of your work? Secondly, if we are seeing Polar amplification of global warming, will there come a point when temperatures do not get low enough to affect the chemistry in the lower atmosphere?
Regards,
[Response: Tis the other way round – GW cools the stratosphere, remember – so the worry is that temperatures might decrease enough for enhanced OD – William]
Is this lab result meant to simulate what happens in either of the two situations? ((1) upper stratosphere plus strong sunlight; (2) polar springtime lower stratosphere cold enough for ice crystals.)
I remember the latter was a recent surprise, as the stratosphere got cold enough often enough, some winters, for it to develop.
Or — is this lab work to understand the reaction in very pure conditions — eliminating anything else to make sure nothing could be present that, like those ice crystals, would affect the rate?
One looks forward to the quantification of how the emerging Cl2O2 rate data will effect model estimates of the trade off between reduced energy efficiency ,and hence increased CO2 RF from thermal station powered air conditioning, and demands for the accelerated phase out of HCFC’s in the name of limiting their climate forcing. Since AGW correlates to increased air conditioner use , the rate shift may give rise to an interesting policy conundrum.
Understanding an intermediate step doesn’t change the outcome.
You can still fry an egg after realizing you don’t fully understand how and why protein denatures.
> Indian chemical companies are happy to ship as much
> chlorofluorocarbons as needed, Mr. Bothelo said. When
> asked what the chemical looks like, he abruptly had a
> mechanic pour a little out of a battered metal tank onto
> the oil-stained ground. The milky gas flowed toward the
> dirt, bounced and then faded away, vanishing into the air.
>
> “If it were something so bad,” Mr. Bothelo said, “they
> would not legally sell it.”
http://www.nytimes.com/2007/02/23/business/23cool.html?_r=1&pagewanted=all
It seems to me that the Sander lab’s new result is difficult to reconcile with a lot of good ab initio theory on ClOOCl out there as well as results of molecular beam photolysis experiments.
Minor quibble Cl2O2 is the empirical formula that fits three isomers. The one that is important is ClOOCl. There is some interesting matrix isolation work from the early 90s that managed to interconvert all the isomers.
A thought that just hit, is that it is a lot easier to photodissociate the other two isomers, and the interconversion might happen on the PSC particles (this is a real WAGNER – wild assed guess, no explanation required)
Eli:
Point well taken- molecular beam and mass spec vacuum systems are designed to eliminate pump aerosols, and keep surface interactions out of play and the line of flight.
If the chemistry of ozone deletion is so well understood, why have the predictions of healing of the ozone “hole” not come to pass?
[Response: The predictions of recovery have all suggested that it will take decades for the Cl/Br burden to decrease, and there is the added complication of increasing CO2 making the stratosphere colder and favoring ozone destruction (see Shindell et al 1999 for instance). However, we should be near the minimum now. – gavin]
A bit OT, but: is the recent agreement to speed up timelines on eliminating HCFCs a “win” for
cutting back on greenhouse gas emissions, as advertised, or is it sidestepping the issue, as some have
argued?
The elementary questions are collected here with links to answers
at the top of page and in comments here:
http://www.theozonehole.com/askthescientist.htm
Current condition reports here:
http://www.theozonehole.com/ozonehole2007.htm
Hank: Thanks for the links. Now here’s an even more elementary question: How does GW lead to stratopspheric cooling? I live w/ an unrepentent denier.
James, putting your question into Google leads to this:
RealClimate » Why does the stratosphere cool when the troposphere …
[edit]
[Response: Actually, that was probably the worst post I ever wrote, and it’s not correct. I should delete it, but it stays around for historical reasons I guess. A better explanation is available through this link: https://www.realclimate.org/index.php/archives/2006/11/the-sky-is-falling/ . The basic explanation involves the fact that you have multiple lines where IR is absorbed in the troposphere. If you increase CO2, you get a suppression of upward LW into the stratosphere in the CO2 bands at the same time you get increased outward radiation from the stratosphere, producing a cooling. The total upward LW is roughly the same because the other bands (from water vapour mainly) make up for the deficit at the CO2 bands. – gavin]
Thank you Gavin. (Blush. No ‘wisdom’ option on search.) You could edit the beginning of that ‘worst post’ so Google will go to the edit.
Gavin (inline to #12) wrote:
First me and now Hank.
Gavin,
People are going to continue to refer to the post – particularly since it is dealing with a topic that is at first counterintuitive for the layman. Likewise people are going to refer to it at other websites.
Personally I am glad you keep it around. But what I would recommend is a disclaimer at the top to the effect that it contains mistakes, its being kept around for historical reasons, that you think the discussion itself is valuable (as I would assume you do), and then a link to the post “The Sky is Falling” as what you would recommend in its place.
This will make things easier for you (since you wouldn’t even have to do the inline when people refer to it in discussions – in essence “the inline” would already be there at the top of the old post) and at the same time help those who are coming in from other websites – who otherwise won’t know that there are problems with the piece itself until they get to the discussion.
PS to #14
Nevermind…
I see that you already did what I suggested.
Does RC concur, and approve of the EPA’s recent decision to allow use of methyl iodide as a soil fumigant (with subsequent release of MeI to the atmosphere) as protective of atmospheric chemistry?
Removing sources of atmospheric bromine is good.
Replacing methyl bromide, long known as an immediate hazard to public health, with “methyl iodide, marketed by Tokyo-based Arysta LifeScience Corp” — also a known hazard to public health — isn’t.
http://scienceblogs.com/effectmeasure/2007/09/methyl_iodide_strawberry_field.php
“… Cornell’s nobel laureate (1981, chemistry) Roald Hoffman made his own assessment: “I wouldn’t like to live near a field where it’s applied.” I guess I can infer that also means he wouldn’t like to work in the field.”
James Cottingham (#11) wrote:
A quick note….
Statospheric cooling is indicative of an increased greenhouse effect. One of the fingerprints, if you will. If global warming were the result of increased solar radiation, then one would expect both the troposphere and the stratosphere to become warmer, but the stratosphere has cooled – just as one would expect given an increased greenhouse effect due to the reduction in the amount CO2-band longwave radiation – and as has been predicted.
Re 17
Thanks Hank for your every vigilant help.
I know that MeI is more toxic than MeBr, and I particularly hate the thought of exposing of innocent populations to vapor drift as agricultural towns like Salinas, Stockton, and Fresno develop other industries. (Some days I am less concerned about the health of non-agricultural workers in Sacramento.)
However, I see stuff like
Photochemical aspects of tropospheric iodine behavior by Jenkin, et al, in Journal of Atmospheric Chemistry, 1985
and I wonder if the current lack of concern by the EPA for effects of Iodine on O3 is because of a lack of effect, or because we just have not (yet) released enough Iodine into the troposphere to see an effect.
That is; Is it no effect, or just no data?
Does Realclimate have any connection with any other environmental groups such as greenpeace?
[Response: No, but see our earlier statements. -gavin]
Although referenced in the article Gavin points to, one can simply go to Dr. Elmar Uherek’s explanation, which wins the gold leaf award on stratospheric cooling
One perhaps nitpicky point… The article says “chlorine, the more important of the two [relative to Br]”– but I’m pretty sure that, atom for atom, Br is stronger. Cl is just “more important” because there’s more of it.
Real Climate is not an “environmental group”. The authors here are all working climate scientists. It’s a science education and information effort.
Of course, the respondents here are just random occupants of cyberspace.
Good question. Looks likely it’s equally bad and that would make the E”P”A’s move even dumber than it appears for immediate risk.
All I know is what Google Scholar finds, for example
http://www.atmos-chem-phys.org/6/4559/2006/acp-6-4559-2006.pdf
“Recently, numerous kinetic studies on iodine compounds have been carried out and the understanding of iodine chemistry has improved considerably (e.g., Turnipseed et al., 1997; Gilles et al., 2000). As a result, in this study, we have attempted to reevaluate the role
of iodine chemicals on the ozone depletion using the updated evaluations of iodine chemistry.”
and a bit earlier:
http://linkinghub.elsevier.com/retrieve/pii/S1352231004005849
“The role, if any, that iodine chemistry plays in the polar ozone depletion episodes is not known. These events are rationalized today largely in terms of Br2- and BrCl-initiated reactions. The potential for enhancement of ozone depletions through the presence of iodine-containing molecules (I2, IBr, ICl, CH2I2, CH2IBr, CH2ICl, and CH3I) is investigated in this study….”
Hank,
Be careful. The second paper you refer to there is dealing with tropospheric emissions of iodine-containing compounds. The ozone depletion they are referring to in that paper is occuring at the surface (in the polar spring), not in the stratospheric ozone layer.
As far as I know, algae represent the largest (tropospheric) source of iodine to the atmosphere — probably much larger than any anthropogenic emissions. Presumably these compounds are scrubbed by the atmosphere, and don’t make it to the stratosphere where they could play a role in ozone destruction.
The first paper you cite is interested in looking at potential direct emissions of Iodine into the upper troposphere from aircraft, and how that might affect the ozone layer.
Sean, agreed; several of the papers I read said iodine moves up from troposphere to stratosphere over time, that was just meant as a pointer to searching for people, hoping others will find more info.
Bromine’s far more persistent than chlorine.
Anyone know if iodine is more persistent than bromine?
— see Crutzen’s Nobel Prize speech; I posted excerpts here about how lucky we happened to be, in that regard:
http://scienceblogs.com/deltoid/2006/01/global_warming_alarmism.php#comment-1891
Re 25
The point is that we are not finding a clear and convincing science that says the large-scale discharge of methyl iodide/ iodomethane to the atmosphere is benign or beneficial.
Consider the EPA’s risk management analysis at : http://www.epa.gov/oppsrrd1/reregistration/soil_fumigants/
and note that it does not address atmospheric chemistry.
The EPA should be encouraged to include atmospheric chemistry in its risk analysis. Thinking before we release might be much easier than thinking after we release.
As I recall the absorption spectrum of methyl iodide is shifted to higher wavelengths than methyl bromide and methyl chloride. Since the upper state is dissociative, that should translate into a shorter photochemical lifetime in the troposphere and less getting into the stratosphere. A little googling shows that the Ozone Depletion Potential of CH3I is very low and the GWP is also low, probably because of the short lifetime. From biological problems I know nothing.
Here’s how the ice melting tipping point plays out in the press:
http://www.planetark.com/dailynewsstory.cfm/newsid/44590/story.htm
Bromine’s far more persistent than chlorine.
I don’t think bromine persists any longer in the atmosphere than chlorine does. What bromine has going for it, IIUC, is a lack of relatively stable low-activity species in the stratosphere that reduce its overall catalytic activity.
Paul, do you have a source on that?
Crutzen in his Nobel speech saidbromine was 100x as active as chlorine, and more of a problem, for several chemical reasons.
Did you read Crutzen’s speech, or the excerpts from it I posted:
http://scienceblogs.com/deltoid/2006/01/global_warming_alarmism.php#comment-1891
Aside — I find that methyl bromide was being argued about 12 years ago and Robert Parson cited Paul Dietz there:
http://yarchive.net/env/ozone.html
Layman tipping-point Q: How far does the runnaway effect go? How has the planet recovered from warming events in the past? Can we hope for the same recovery triggers this time around?
[Response: We are a long way from any ‘runaway‘ effects, and yes, the planet (as a whole) will recover, though the timescales to fully remove all the excess carbon we are injecting are longer than you or I are likely to care about. -gavin]
Michael (33) — My understanding is that a global warming of 6 degrees Celcius is about the same as during the PETM event about 55 million years ago. (Wikipedia has more.) This ought to be of considerable interest because all large (greater than 2 kilogram) mammalian species went extinct then.
Eventually the so-called greenhouse gases leave the atmosphere. This takes, for some of these, are very long time. The recovery is not a trigger-like process.
I encourage you to note the ‘Start Here’ link at the top of the page to discover some useful readings for beginners.
[Response: I don’t think the PETM extinctions went beyond benthic ocean fauna, do you have a citation? – gavin]
The question was meant as a discussion piece. I hesitate to use RC as an educational resource, because it’s purpose is not education, but an attempt to combat misinformation – and therefore not very balanced, or exhaustive.
If earlier GW events were due to excess carbon, did the excess carbon sources have to exhuast themselves before the climate recovered? Or are there feedbacks that overcome the excess carbon?
Re 34 David B. Benson “This ought to be of considerable interest because all large (greater than 2 kilogram) mammalian species went extinct then.”
I think Gavin is correct, the PETM extinctions were largely limited to deep-sea benthic foraminifera as the deep ocean warmed and became anoxic. What wiki actually says abut mammals is:
“At the start of the Eocene, the Earth remained warm for about 80,000 to 200,000 years. On land, there was a massive turnover of mammals, in which most of the primitive mammals that had developed since the end of the Cretaceous Period were suddenly replaced by the ancestors of most of the surviving modern mammal groups, all of them in small versions which were adapted to Eocene heat.”
Re 33 Michael: “Layman tipping-point Q: How far does the runnaway effect go?”
That depends on how much CO2, methane and other greenhouse gasses are ultimately released from both human activity and natural carbon sinks, and how far temperature rises to reach radiative equilibrium. The warmer it gets the more carbon released from natural sinks, but it’s self limiting so it can not go on indefinitely. But the warmer it gets the less dissolved carbon the ocean can hold, so the slower the draw-down will be.
Michael: “How has the planet recovered from warming events in the past?”
There have been different causes for past warming events (changing Milankovetch cycles increasing solar insolation and the initial warming then leading to higher greenhouse gas concentrations; massive geological disturbances directly injecting huge quantities of methane and CO2), but for all of them recovery was very, very slow.
Michael: “Can we hope for the same recovery triggers this time around?”
Be careful what you wish for: any “triggers” in the opposite direction will be just as disruptive, or more so. An ice age would tend to ruin your whole day.
Michael, you ask a question starting with an assumption:
“If earlier GW events were due to excess carbon”
What particular events are you thinking of, that fit that description? Where are you getting the assumptions?
I looked a bit and this was as close as I could get, and it’s not close — but it’s quite old, you’d want to follow cites forward of course.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11538480&dopt=Citation
Most of the climate changes don’t seem to involve “excess” carbon, just rearrangement subject to solar/orbital variables involved, except perhaps the PETM, near as I can tell.
This may help: http://droyer.web.wesleyan.edu/climate_sensitivity.pdf
This may be the kind of excess you’re talking about, just guessing:
http://www.ncbi.nlm.nih.gov/sites/entrez?Db=PubMed&Cmd=ShowDetailView&TermToSearch=11001051&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus
I’m not claiming any of what I found is current knowledge, check for cites and recent info. Just puzzling ober your assumption there.
Anyhow I’d suggest you first state what you’re assuming to be true and what you base that on, and check on it.
Gavin — That’s not what Wikipedia says (but we know to take that source cum grando salis).
P.D. Ginerich
Environment and evolution through the Paleocene-Eocene thermal maximum
Trends in Ecology & Evolution v. 21 #5, May 2006, 246–253.
doi:10.1016/j.tree.2006.03.006
points out that some species did go extinct and at least one dwarfed during PETM. THe dwarfing was down to around 8 kg (60% reduction), not a mere 2 kg.
The following abstract simply states “a turnover of North American mammal assemblages” (whatever that means).
http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_126223.htm
The following paper, by G.J. Harrington in BioObne, states “fundamental changes to mammalian communities both in North America and Europe”, citing five references.
http://www.bioone.org/perlserv/?request=get-document&doi=10.1669%2F0883-1351(2001)016%3C0266%3AIOPEGW%3E2.0.CO%3B2&ct=1
But any modern paleontology text ought to list Paleocene mammals which went extinct about PETM time, since no fossils are found in the subsequent Eocene. It is tempting to blaim PETM directly for this, as Wikipedia seems to have done, but it could simply be that those older species were simply out-competed by the newer forms which arose, at first quite small, by middle-to-end-PETM times.
Here’s a time sequence that may help, note the positions of the continents changed quite a bit over time:
http://ethomas.web.wesleyan.edu/TOS.pdf
Realizing we’re way off topic here (is it Friday again yet?), I found a nice summary from a blogger I didn’t know, here:
http://thedragonstales.blogspot.com/2007/08/some-bits-on-eocene-ocean-petm.html
_________excerpt below, see link for full text and much more_______
“… the PETM oceans. They were warm and not stratified, at least vertically. The mixing – from top to bottom – still happened, but in a way that seems wrong to those of us that grew up with our oceans and their circulation.
It seems that there were cold provinces – if I may use that word here – in the Arctic and Antarctic. However, they were more like cool provinces since they were ice free as far as we can tell (-1.5 C is the average temperature recorded for the Arctic waters at this point). Whereas the tropical surface temperatures were approximately 8 C. To a lay person, this would seem to say that the oceans would have a normal circulation albeit a sluggish one due to the smaller differential. However, the reality is that it had a completely opposite direction than our own.
Our oceans have the warm waters sweep to the north and then get cold and sink. The water then flows along the ocean basins and upwells in the tropics where it is once again heated and sent back to the north and south. This keeps the waters oxygenated and the bottoms of the oceans from becoming anoxic. Mostly. However, during the PETM it seems that the oceans had the opposite pattern. The upwelling was in the higher latitudes and the sinking was in the tropics. The reason being is that the water evaporation of the very warm tropical waters would produce very salty water which would sink and displace the cooler waters of the south and north along the top to the tropics by upwelling ….”
—————–end excerpt —————-
Hank,
from http://ethomas.web.wesleyan.edu/TOS.pdf
———start————-
“Global cooling started at the end of
the early Eocene to the early middle
Eocene (Figure 1). What triggered that
cooling remains an unsolved question.”
———end—————
I guess I’m asking an unsolved question.
…seems like an important detail.
More than a detail, but it’s a very long time ago. As that Wesleyan article says and also as Gavin points out in his China thread, the ocean cores from drilling are where the answer probably will be found, but it’s very early work yet. The Wesleyan chart shows continental drift changing rate about then with the last big collision, I think.
There’s some discussion and interesting quotes along that line here:
http://www.bautforum.com/general-science/64180-inference-ocean-ph-11-b-10-b-ratios-i-middle-eocene.html
And, um, sorry for prolonging the digression, hosts. Point us elsewhere if you’d rather.
Gavin, PETM extinctions:
http://www.ucar.edu/news/releases/2005/permian.shtml
“… end of the Permian Era, when an estimated 90 to 95% of all marine species, as well as about 70% of all terrestrial species, became extinct…. The warming reached a depth of about 10,000 feet (4,000 meters), interfering with the normal circulation ….
‘The implication of our study is that elevated CO2 is sufficient to lead to inhospitable conditions for marine life and excessively high temperatures over land would contribute to the demise of terrestrial life,’ the authors concluded ….”
Peter Ward quoted: http://news.mongabay.com/2006/0329-extinction.html
[Response: You’re confusing two things here. PETM=55 million ago, Permian/Triassic was 250 million years ago. No question about extinctions then! – gavin]
Paleocene mammals and extinctions:
A summary with links:
http://www.palaeos.com/Cenozoic/Paleocene/Paleocene.htm
A derivative Wikipedia page linking to 32 different pages briefly describing extinct genera and species, most, but not all, in the entirely extinct order Multituberculate.
http://en.wikipedia.org/wiki/Category:Paleocene_extinctions
From the summary it seems that the largest (known) Paleocene mammal weighed about 100 kilos, about like a smaller black bear. Also, “the predominent mammals of the period were members of groups which are now extinct.”
Somewhat late. A lot of the Cl in the strat is tied up in ClONO2. BrONO2 is a lot less, mostly because (my guess) the tail of the absorption spectrum extends further to the red, so photodissociation is a lot more probable.
And a good summary on the PETM event:
http://www.es.ucsc.edu/~jzachos/pubs/Kelly_etal_2005.pdf
Thanks, Gavin, wake me up any time I need it …
Re: Response in 33
I would define “runaway” as anything significantly greater than the best estimate provided to planners, policy makers, and other stake-holders responsible for infrastructure. Now, that is IPCC AR4 numbers. So today, I would consider any global warming/ ice melt/ sea level rise beyond what is stated in the AR4 ES to be “runaway”.
How do you define “runaway”?
To me “runaway,” means runaway from our human control, with “permanent runaway” (as on Venus) as special case. I’m thinking horses — they run away, and the rider cannot stop them, but eventually they get tired and stop. Or a runaway train.
In the case of global warming today, that would mean a situation in which the initial warming caused by anthropogenic GHG emissions would cause nature to contribute (net) to greater warming in positive feedback fashion, such as by causing nature to emit a net increase in GHGs (methane clathrates and permafrost melting) and the warming leading to less albedo from less snow & ice, and many other such factors. This would be a “limited runaway warming,” since even if humans were to reduce their GHG emissions to zero, the warming would continue, not only due to the lag time, but also because nature is emitting GHGs and reducing albedo.
Then eventually the warming would stop and the climate would return to present conditions….maybe in thousands, tens of thousands, or 200,000 years. A large chunk of life may go extinct, and many people would dies from the warming and it many many effects.
I think my idea of “runaway” is somewhat common, though scientists are very adament about limiting the concept only to permanent runaway as on Venus. And at least one scientist suggested that we call the scenario I described “hysteresis.” That does have the implication of something going out of some bounds, but then eventually rebounded to the same conditions.
I suppose if people really reduce their GHGs pronto and drastically we can be more assured that we may only experience “regular” or more linear global warming for a certain period (the lag time for what we’ve already emitted and will emit in the near future, plus a bit of positive feedback effects), then the climate would respond to our reductions and cool down to present conditions. I have no idea about a time scheme, but (knowing nothing) I’d guess maybe within 100 years or so. That would be my idea of a non-runaway or non-hysteresis scenario, in which the climate and nature responds to our decreased emissions — nature helping absorbing much of it (negative feedbacks), and not going off on its own positive feedback expedition into much greater warming.
This latter is how I actually conceived of global warming until about 4 years ago, when I became aware of the greater dangers of “runaway” positive feedbacks kicking in under the hysteresis scenario. So I do think the concept of “runaway” is extremely important. Since people tend not to think much past next quarter’s profits or that Saturday night date, I really wouldn’t worry about them thinking of “runaway” as a permanent Venus-type thing. That a no-problem or only a problem for the “geological time” mind-set.