A guest post by Spencer Weart, in collaboration with Raymond T. Pierrehumbert
The simple physics explanations for the greenhouse effect that you find on the internet are often quite wrong. These well-meaning errors can promote confusion about whether humanity is truly causing global warming by adding carbon dioxide to the atmosphere. Some people have been arguing that simple physics shows there is already so much CO2 in the air that its effect on infrared radiation is "saturated"— meaning that adding more gas can make scarcely any difference in how much radiation gets through the atmosphere, since all the radiation is already blocked. And besides, isn’t water vapor already blocking all the infrared rays that CO2 ever would?
The arguments do sound good, so good that in fact they helped to suppress research on the greenhouse effect for half a century. In 1900, shortly after Svante Arrhenius published his pathbreaking argument that our use of fossil fuels will eventually warm the planet, another scientist, Knut Ångström, asked an assistant, Herr J. Koch, to do a simple experiment. He sent infrared radiation through a tube filled with carbon dioxide, containing somewhat less gas in total then would be found in a column of air reaching to the top of the atmosphere. That’s not much, since the concentration in air is only a few hundred parts per million. Herr Koch did his experiments in a 30cm long tube, though 250cm would have been closer to the right length to use to represent the amount of CO2 in the atmosphere. Herr Koch reported that when he cut the amount of gas in the tube by one-third, the amount of radiation that got through scarcely changed. The American meteorological community was alerted to Ångström’s result in a commentary appearing in the June, 1901 issue of Monthly Weather Review, which used the result to caution "geologists" against adhering to Arrhenius’ wild ideas.
Still more persuasive to scientists of the day was the fact that water vapor, which is far more abundant in the air than carbon dioxide, also intercepts infrared radiation. In the infrared spectrum, the main bands where each gas blocked radiation overlapped one another. How could adding CO2 affect radiation in bands of the spectrum that H2O (not to mention CO2 itself) already made opaque? As these ideas spread, even scientists who had been enthusiastic about Arrhenius’s work decided it was in error. Work on the question stagnated. If there was ever an “establishment” view about the greenhouse effect, it was confidence that the CO2 emitted by humans could not affect anything so grand as the Earth’s climate.
Nobody was interested in thinking about the matter deeply enough to notice the flaw in the argument. The scientists were looking at warming from ground level, so to speak, asking about the radiation that reaches and leaves the surface of the Earth. Like Ångström, they tended to treat the atmosphere overhead as a unit, as if it were a single sheet of glass. (Thus the “greenhouse” analogy.) But this is not how global warming actually works.
What happens to infrared radiation emitted by the Earth’s surface? As it moves up layer by layer through the atmosphere, some is stopped in each layer. To be specific: a molecule of carbon dioxide, water vapor or some other greenhouse gas absorbs a bit of energy from the radiation. The molecule may radiate the energy back out again in a random direction. Or it may transfer the energy into velocity in collisions with other air molecules, so that the layer of air where it sits gets warmer. The layer of air radiates some of the energy it has absorbed back toward the ground, and some upwards to higher layers. As you go higher, the atmosphere gets thinner and colder. Eventually the energy reaches a layer so thin that radiation can escape into space.
What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from lower down slips through, adding more greenhouse gas molecules means the layer will absorb more of the rays. So the place from which most of the heat energy finally leaves the Earth will shift to higher layers. Those are colder layers, so they do not radiate heat as well. The planet as a whole is now taking in more energy than it radiates (which is in fact our current situation). As the higher levels radiate some of the excess downwards, all the lower levels down to the surface warm up. The imbalance must continue until the high levels get hot enough to radiate as much energy back out as the planet is receiving.
Any saturation at lower levels would not change this, since it is the layers from which radiation does escape that determine the planet’s heat balance. The basic logic was neatly explained by John Tyndall back in 1862: "As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial [infrared] rays, produces a local heightening of the temperature at the Earth’s surface."
Even a simple explanation can be hard to grasp in all its implications, and scientists only worked those out piecewise. First they had to understand that it was worth the trouble to think about carbon dioxide at all. Didn’t the fact that water vapor thoroughly blocks infrared radiation mean that any changes in CO2 are meaningless? Again, the scientists of the day got caught in the trap of thinking of the atmosphere as a single slab. Although they knew that the higher you went, the drier the air got, they only considered the total water vapor in the column.
The breakthroughs that finally set the field back on the right track came from research during the 1940s. Military officers lavishly funded research on the high layers of the air where their bombers operated, layers traversed by the infrared radiation they might use to detect enemies. Theoretical analysis of absorption leaped forward, with results confirmed by laboratory studies using techniques orders of magnitude better than Ångström could deploy. The resulting developments stimulated new and clearer thinking about atmospheric radiation.
Among other things, the new studies showed that in the frigid and rarified upper atmosphere where the crucial infrared absorption takes place, the nature of the absorption is different from what scientists had assumed from the old sea-level measurements. Take a single molecule of CO2 or H2O. It will absorb light only in a set of specific wavelengths, which show up as thin dark lines in a spectrum. In a gas at sea-level temperature and pressure, the countless molecules colliding with one another at different velocities each absorb at slightly different wavelengths, so the lines are broadened and overlap to a considerable extent. Even at sea level pressure, the absorption is concentrated into discrete spikes, but the gaps between the spikes are fairly narrow and the "valleys" between the spikes are not terribly deep. (see Part II) None of this was known a century ago. With the primitive infrared instruments available in the early 20th century, scientists saw the absorption smeared out into wide bands. And they had no theory to suggest anything different.
Measurements done for the US Air Force drew scientists’ attention to the details of the absorption, and especially at high altitudes. At low pressure the spikes become much more sharply defined, like a picket fence. There are gaps between the H2O lines where radiation can get through unless blocked by CO2 lines. Moreover, researchers had become acutely aware of how very dry the air gets at upper altitudes — indeed the stratosphere has scarcely any water vapor at all. By contrast, CO2 is well mixed all through the atmosphere, so as you look higher it becomes relatively more significant. The main points could have been understood already in the 1930s if scientists had looked at the greenhouse effect closely (in fact one physicist, E.O. Hulbert, did make a pretty good calculation, but the matter was of so little interest that nobody noticed.)
As we have seen, in the higher layers where radiation starts to slip through easily, adding some greenhouse gas must warm the Earth regardless of how the absorption works. The changes in the H2O and CO2 absorption lines with pressure and temperature only shift the layers where the main action takes place. You do need to take it all into account to make an exact calculation of the warming. In the 1950s, after good infrared data and digital computers became available, the physicist Gilbert Plass took time off from what seemed like more important research to work through lengthy calculations of the radiation balance, layer by layer in the atmosphere and point by point in the spectrum. He announced that adding CO2 really could cause a degree or so of global warming. Plass’s calculations were too primitive to account for many important effects. (Heat energy moves up not only by radiation but by convection, some radiation is blocked not by gas but by clouds, etc.) But for the few scientists who paid attention, it was now clear that the question was worth studying. Decades more would pass before scientists began to give the public a clear explanation of what was really going on in these calculations, drawing attention to the high, cold layers of the atmosphere. Even today, many popularizers try to explain the greenhouse effect as if the atmosphere were a single sheet of glass.
In sum, the way radiation is absorbed only matters if you want to calculate the exact degree of warming — adding carbon dioxide will make the greenhouse effect stronger regardless of saturation in the lower atmosphere. But in fact, the Earth’s atmosphere is not even close to being in a state of saturation. With the primitive techniques of his day, Ångström got a bad result, as explained in the Part II . Actually, it’s not clear that he would have appreciated the significance of his result even if he had gotten the correct answer for the way absorption varies with CO2 amount. From his writing, it’s a pretty good guess that he’d think a change of absorption of a percent or so upon doubling CO2 would be insignificant. In reality, that mere percent increase, when combined properly with the "thinning and cooling" argument, adds 4 Watts per square meter to the planets radiation balance for doubled CO2. That’s only about a percent of the solar energy absorbed by the Earth, but it’s a highly important percent to us! After all, a mere one percent change in the 280 Kelvin surface temperature of the Earth is 2.8 Kelvin (which is also 2.8 Celsius). And that’s without even taking into account the radiative forcing from all those amplifying feedbacks, like those due to water vapor and ice-albedo.
In any event, modern measurements show that there is not nearly enough CO2 in the atmosphere to block most of the infrared radiation in the bands of the spectrum where the gas absorbs. That’s even the case for water vapor in places where the air is very dry. (When night falls in a desert, the temperature can quickly drop from warm to freezing. Radiation from the surface escapes directly into space unless there are clouds to block it.)
So, if a skeptical friend hits you with the "saturation argument" against global warming, here’s all you need to say: (a) You’d still get an increase in greenhouse warming even if the atmosphere were saturated, because it’s the absorption in the thin upper atmosphere (which is unsaturated) that counts (b) It’s not even true that the atmosphere is actually saturated with respect to absorption by CO2, (c) Water vapor doesn’t overwhelm the effects of CO2 because there’s little water vapor in the high, cold regions from which infrared escapes, and at the low pressures there water vapor absorption is like a leaky sieve, which would let a lot more radiation through were it not for CO2, and (d) These issues were satisfactorily addressed by physicists 50 years ago, and the necessary physics is included in all climate models.
Then you can heave a sigh, and wonder how much different the world would be today if these arguments were understood in the 1920’s, as they could well have been if anybody had thought it important enough to think through.
For Further Reading
References and a more detailed history can be found here and here.
Some aspects of the "thinning and cooling" argument, and the importance of the radiating level are found in the post A Busy Week for Water Vapor, which also contains a discussion of water vapor radiative effects on the top-of-atmosphere vs. surface radiation budget. A general discussion of the relative roles of water vapor and CO2 is given in Gavin’s post on ths subject.
You can get a good feel for the way CO2 and water vapor affect the spectrum of radiation escaping the Earth by playing around with Dave Archer’s online radiation model here. It would help, of course, to read through the explanation of radiating levels in Archer’s book, Understanding the Forecast. A discussion of radiating levels for real and idealized cases, at a more advance level, can be found in the draft of Pierrehumbert’s ClimateBook; see Chapters 3 and 4.
The Monthly Weather Review article commenting on Ångström’s work is here, and Ångström’s original article is here.
Excellent article! I have been seeing the “saturation” argument a fair amount recently, and this is a nice summary of why it’s not true. Thanks for posting.
Thank you very much realclimate.
Top stuff.
Any idea what levels of CO2 required would cause saturation ?
I used an excerpt from this realclimate article – which I put at the bottom of my recent article at Newsvine called: 15 cartoon finalists on science, policy and climate change, at:
http://npat.newsvine.com/_news/2007/06/25/802021-15-cartoon-finalists-on-science-policy-and-climate-change
Any idea what levels of CO2 required would cause saturation ?
Looking at the graph on the Part II article it looks like there is still extra areas to absorb at 100,000 times pre-industrial CO2 levels, up at the 21-22 micron wavelengths, and some down at 11.5 microns or so that would take 10,000 times as much to be absorbed at the other end of the peak. It seems unlikely there is enough carbon around for us to burn to get it that high, so in practise we aren’t going to reach saturation at any point.
[Response: Quite true. In fact, you need to look in the spectrum beyond the graph in Part II to see when CO2 really gets saturated, because the portions of the spectrum outside the wavelength range shown can be considered transparent to thermal infrared for the purposes of discussing Earth, but start to absorb significantly at extremely high CO2 values like those on Venus. Of particular interest in this regard is the CO2 continuum, which starts just to the shortwave side of the graph in Part II. When you take the CO2 continuum into account you find that for gravity like Earth or Venus, CO2 starts to become saturated for a surface pressure of about 10 bars (10 Earth atmospheres). Venus has a surface pressure of about 90 bars, and has an almost pure CO2 atmosphere. Even for Venus, to infer that CO2 absorption is saturated one needs to go to absorption data beyond what’s in the HITRAN database, since the high surface temperature causes the surface of the planet to radiate into shorter wave parts of the infrared than does Earth. On Earth, both the lack of saturation and the “thinning and cooling” argument come into play in determining the climate. Venus is an example of a planet that can be considered saturated in the sense imagined by Angstrom for Earth, but which nevertheless gets warmer as you add additional CO2 because of the “thinning and cooling” argument. –raypierre]
Well done.
This article makes the physics involved relatively easy to understand, without oversimplifying. It also clearly explains why the current crop of saturation “arguments” out there are irrelevant. It nicely ties together the science with the relevant history, which I find fascinating. Finally, it summarizes the main points of the article in a concise closing paragraph.
5 stars.
If I am reading this correctly (and I may not be), as co2 concentration rise, the greenhouse effect shifts somewhat to the upper atmosphere where water vapor is nearly absent and co2 has a larger relative effect. Two questions. First, why then do model predictions and actual measurements show the troposhere warming and the stratosphere cooling? Second, the infra red absorption rate is proportional to the number of molecules of co2 encountered by the radiation. With co2 well dispersed in the entire column of the atmosphere at 380 ppm, are not there many fewer co2 molecules in the thin air of the stratosphere and therefore limited absorption of radiation?
[Response: You are reading this correctly, though you should keep in mind that the level which controls the greenhouse effect depends on which wavelength you are looking at (see Part II). Near the centers of strong absorption lines, even the stratosphere by itself is strongly absorbing, and hence (by something known as Kirchoff’s Law) strongly emitting. The reason the stratosphere cools upon increase of CO2 is that the balance in the stratosphere is between absorption of solar radiation by ozone and cooling by infrared emission. As you increase the CO2, there is excessive radiative cooling, so the stratosphere has to cool down to come back into balance. As for your second question, it is precisely because there are fewer molecules of CO2 in the thin upper air that CO2 increases continue to increase the greenhouse effect even when the column as a whole is saturated. Because you are adding more molecules throughout the column, the extra molecules you add at high layers where there isn’t initially enough CO2 to absorb everything can make a difference. –raypierre]
Of potential interest to people here, I recently completed making a figure showing atmospheric absorption bands for the principle gases. The primary goal of the figure was to compare downgoing solar and upgoing thermal radiation, so it doesn’t do much to clarify the saturation arguments per se, but does show the relative importance of water vapor and other gases and help understand where the saturation arguments are coming from.
Though not really a complete argument, for unsophisticated audiences, I find that saying that more CO2 allows heat to be held “closer” to the Earth’s surface is often an effective response to the saturation line of thought.
All of this is greatly appreciated – including the links. Undoubtedly it will take a while for me to absorb (absorp?) all I can from the light with which you both (and David Archer) have illuminated this subject, but that means a journey of discovery which I will enjoy for quite some time to come.
Even if rising CO2 levels were not a global warming concern, there is another reason to be concerned about rising CO2: toxicity. Humans evolved in conditions of pre-industrial CO2 levels, so presumably we tolerate 300 ppm just fine, but the physiological effects of higher CO2 levels on a long-term basis have perhaps not been adequately studied. Do you feel a little drowsey sitting in the auditorium or class room? It may well be related to higher CO2 levels – upwards of 600 ppm would not be unusual. Levels as low as 2000 ppm may result in unconsciousness, but lower levels may result in impairment in our mental functions and have other health implications.
Re Raypierre’s response to 4
It would help clarify matters further to point out that because the peak wavelength of a planet’s radiated energy depends on the fourth power of its temperature , absorption at 20 micron wavelengths is much less important on Earth than absorption around 10 microns , for having a temperature of roughly 300K, Earth is essentially a ten micron peak black body.
The 21-22 micron CO2 band corresponds to a blackbody temperature of ~150 Kelvin, too low to be of much Earthly interest because of the feeble radiated power. In the outer solar system many bodies have even lower temperatures, but CO2’s gaseous absorption spectrum is moot because it freezes out around 200K.
It would be great if RealClimate could persuade Weart to apply his clear-eyed prose to _An Inconvenient Truth_ with the same vigor he devoted to hyperbolic Cold War icons in his superb book _Nuclear Fear : A History Of Images’_
[Response: You’re quite wrong about much of what you’ve said in this comment. For one thing, you’re confusing the Stefan-Boltzman law with the Wien displacement law; the wavelength of peak emission is inversely proportional to the temperature itself, not to the fourth power of the temperature. Further, because of the Planck law, the emission tails off very sharply at wavelengths shorter than the peak, but only rather gradually at wavelengths longer than the peak. For that reason, the emission of the Earth at wavelengths where CO2 is a good absorber is extremely important. You’d understand this if you had bothered to play around with Dave Archer’s online spectrally resolved model. Really, you are rather ignorant for one who makes pronouncements of this sort with such confidence. I won’t even get into how your prejudice against Al Gore blinds you to the essential correctness of what he presents in “An Inconvenient Truth.” And please, let’s not get into any further discussion of either Al Gore or nuclear weapons or nuclear power in this thread. Any such discussion will be considered off-topic and will be expunged. –raypierre]
Here is an explanation as to why, 17 years after global greenhouse warming was acknowledged as ‘real’, it is necessary to have RC to point out the lies and distortions of ‘skeptics’ and bogus scientists.
http://www.motherjones.com/mojoblog/archives/2007/06/4723_cheney_stovepip.html
That link doesn’t explain why there were only a few scientists in government who took an honest stand on climate change.
Are you assuming that CO2 concentrations are consistent (evenly distributed) throughout the vertical entirety of the atmosphere? It seems to me (based purely on intuition) that CO2 concentrations would be decreasingly significant with altitude, such that the top-most layers of the atmosphere would experience very little change in CO2 concentration. How much of the CO2 in the atmosphere actually exists up there? What percentage of all CO2 exists in the stratosphere?
DaveS (#13) wrote:
I don’t have the figures, and no doubt it differs by latitude, but you are right: the carbon dioxide isn’t evenly distributed.
The concentration is higher in the stratosphere. And we have known this since 1969.
The following is the citation to a recent peer reviewed study on the effects of CO2 enrichment and depletion upon cucumber growth in a greenhouse environment. I hope that your readers and scientists will comment on it. It appears that CO2 is a powerful gaseous fertilizer. Reduced CO2 diminishes plant growth. What will happen to the world’s food production capacity if CO2 levels are reduced below current levels?
[Response: That’s hardly likely… – gavin]
Segura, M.L., Parra, J.F., Lorenzo, P., Sánchez-Guerrero, M.C. and Medrano, E. 2001. THE EFFECTS OF CO2 ENRICHMENT ON CUCUMBER GROWTH UNDER GREENHOUSE CONDITIONS. Acta Hort. (ISHS) 559:217-222
http://www.actahort.org/books/559/559_31.htm
Re # 15 “What will happen to the world’s food production capacity if CO2 levels are reduced below current levels?
Well, cucumbers have apparently been cultivated for some 3000 years (http://www.foodreference.com/html/a-cukes-history.html), so I’m willing to bet they would do just as well as they did at pre-industrial revolution levels of CO2. But, I doubt we will return to those lower levels any time soon.
In the mean time, as CO2 levels continue to rise, poison ivy will also thrive (http://www.sciencenews.org/articles/20060603/fob1.asp). More cucumbers and poison ivy – it doesn’t get much better than that!
Off-topic, but the ‘Review comments and responses’ of IPCC WG1 AR4 are now on line. A fascinating read:
http://ipcc-wg1.ucar.edu/wg1/Comments/wg1-commentFrameset.html
Perhaps readers would care to use this post to critique Steven Milloy’s recent statement on Fox News:
“Based on the physics of the greenhouse effect, a doubling of carbon dioxide levels from the pre-industrial period (supposedly around 280 parts per million) to 560 parts per million (about 48 percent higher than present levels), might lead to an increase in average global temperature on the order of less than 1 degree centigrade – and we’ve already experienced about 60 percent of that increase.”
“A further doubling of atmospheric carbon dioxide to 1,120 parts per million would result in even less of an increase in temperature because of the energy absorption properties of carbon dioxide.”
“Essentially, the Earth only radiates so much energy back into the atmosphere that is available to be absorbed by carbon dioxide. Once all that energy is absorbed, superfluous carbon dioxide will not add to the greenhouse effect.”
Briefly, the equilibrium climate sensitivity is estimated to be around 3C, not ‘less than 1C’, and could be higher (1.7-4.5 are the current 95% confidence limits). If it is 3C, than we’ve only experienced 20% of the estimated warming over the pre-industrial case. The second paragraph is just wrong – doubling CO2 from preindustrial to 2X should have the same temperature effect as going from 2X to 4X. However, you could see accelerating CO2 emissions leading to a faster rate of temperature increase due to carbon cycle-feedback effects. The third paragraph? This article deals with that.
Why can’t ‘science journalists’ get their facts straight? Why does Fox hire ex-tobacco lobby employees to write articles claiming that global warming is a hoax?
[Response: Under no definition is Milloy a ‘science journalist’. On the contrary, science journalists generally do a pretty good job (and I’ve dealt with a lot). – gavin]
Regarding Steve Milloy (#18):
I thought it was more like 1.2 C for CO2 doubling – and that is before the water vapor feedback which would bring it up to 2.9 C.
Perhaps Steve is trying to land a job at Exxon?
In support of real science journalists:
Ten predictions about climate change that have come true, Tim Flannery, TO June 2007
and Coastal zones set agenda on climate, Mike Lee, SDUT, Jun 2007
On the minus side, Gloom and Doom in a a Sunny Day, by Emily Yoffe, WP is worth looking at. This is not really science journalism, but rather opinion. “I, however, refuse to see the apocalypse in every balmy day…”
Here we have no mention of science, just an appeal to be ‘free from fear’ and a condemnation of the use of ‘the politics of fear’ by ‘global warming activists’.
However, few if any of the science journalists who discuss climate change and global warming ever make the connection to renewable energy. Articles on global warming focus on the need to reduce emissions of to sequester carbon from coal, but they rarely attempt to discuss the plausibility of replacing all CO2-emitting fuel sources with renewable energy.
What’s really lacking is a reasonable global plan to replace current fossil-fueled electricity generation and transportation with renewables.
re: #15
Given the Wall Street Journal’s editorial views, I was amused to find, today, June 26:
1) “Climate Changes are Making Poison Ivy More Potent”, by Tara Parker-Pope.
U of Md research, published in magazine “Weed Science” shows poison ivy (@ 400ppm CO2) compared to 1950’s 300ppm:
– grows at almost twice the rate.
– is hardier plant that recovers faster from grazing animals’ ravages.
Duke U research shows that more CO2 creates more potent irritants as well.
Kudzu is another plant that responds well to increased CO2.
2) “Drought Wreaks Devastation in West, Southeast”, Jim Carlton & Lauren Etter. Drought, bugs, fires, not enough hay for cows (in Florida).
====
Putting these two together, there’s a simple rule, which any modern farmer knows perfectly well (actually farm kids typically learn this stuff by the time they’re 10 or 12):
Up to a plant’s genetic limits, you can increase its growth by giving it more of what it needs {sun, water, soil nutrients, CO2, right temperature range/climate pattern}, but it is always limited by whichever is *least* sufficient. You can plant cucumbers in the middle of the Sahara, and no matter how much CO2 you give them, they’re dead.
Some plants are amazingly specific in their optimum conditions, which is one of the reasons many California towns specialize in the one fruit or vegetable that fits the best right now. This is why people like Borlaug spend their lives tinkering with plants to get variants tuned for specific local conditions.
If warming moves a crop’s comfortable temperature zone nearer the poles, that doesn’t mean:
– that it necessarily gets the same amount of water
– that it has the same soil
– that it gets as much sunlight (well, actually it gets less, for sure)
Of course, such motion normally takes quite a while, I don’t expect the Napa vineyards in Alaska any time soon.
It is useful to increase yield in greenhouse crops via extra CO2, in places with adequate sun and water, but that is a tiny fraction of the world’s agriculture, and doesn’t really give any practical help for staples like wheat, corn, rice, etc.
Spence and Raymond
This came up with respect to another post but seems relevant here. Based on what you have written, is it correct that we should expect greater temperature changes at higher altitudes (in the mountains, not upper atmosphere)? In addition, is there data somewhere that actually shows that the increases in temperature are greater than the average for the planet as a whole.
You said:
“Moreover, researchers had become acutely aware of how very dry the air gets at upper altitudes â�� indeed the stratosphere has scarcely any water vapor at all. By contrast, CO2 is well mixed all through the atmosphere, so as you look higher it becomes relatively more significant.”
On this site, I was told before that the relatively small amount of water vapor at high altitudes should result in a greater impact from co2 increases. This seems to track with the above.
I am not talking here about the stratosphere, but it seems we do not need to go that high to observe the relative differences in changes of high vs low altitudes.
Living at 8500 feet in the Rockies, my observation would be that the average temperature is several degrees warmer than when I was a child in the 50s. Is there any corroboration for this.
A bit off-track here, but something that has nagged at me for a while… When we get statements like “…an increase in average global temperature on the order of less than 1 degree centigrade…”, why are the numbers not also given in degrees F as well? Most Americans (and probably many people in other countries with a British heritage) simply don’t think in centigrade. (Much less the now PC “Celsius”.) I’ve used that scale in science since high school, and I still have to stop and mentally translate to really feel what it means.
This might be a big part of the reason many Americans see climate change as less important than the rest of the world, because the potential temperature increases are psychologically discounted by almost half. We think that say a 3 degree increase isn’t all that much, when it’s actually, in our familiar terms, a much larger increase of almost 6 degrees F.
Spencer Weart> …That’s only about a percent of the solar energy absorbed by the Earth, but it’s a highly important percent to us! After all, a mere one percent change in the 280 Kelvin surface temperature of the Earth is 2.8 Kelvin…
That statement seems either incorrect or misleading to me. With the T^4 energy radiation dependence on temperature, doesn’t the one percent forcing cause only ~1/4 percent change (0.7K) in temperature?
Of course that is without positive feedback effects.
[Response: Actually, I’m to blame for that particular bit of verbiage. It wasn’t meant to be a quantitative estimate of the amount of warming you’d expect from a 1% change, but just to make the point that the order of magnitude of a percent of the Earth’s temperature in Kelvin is a significantly large number. It’s a way of saying that a 1% change in the radiation budget is not small enough to discard out of hand, and so it is necessary to do the work to find the (order unity) numerical factors needed to translate that 1% change into the corresponding percentage change in the Earth’s temperature. It’s Kelvins that are important rather than Celsius in this regard, because both the radiation and the thermodynamics work in Kelvins.
By the way, you can get part of the way to the actual climate sensitivity using the T^4 law if you put in the correct (cold) radiating temperature of the planet, but water vapor feedback makes the curve of emission vs. temperature more linear than T^4, and therefore enhances the sensitivity. –raypierre]
The text above state that doubling carbon dioxide “adds 4 Watts per square meter to the planets radiation balance for doubled CO2. That’s only about a percent of the solar energy absorbed by the Earth, but it’s a highly important percent to us! After all, a mere one percent change in the 280 Kelvin surface temperature of the Earth is 2.8 Kelvin.”
If I understand the Stefan-Boltzmann law correctly, a body radiates at the fourth power of its temperature. Therefore the first four W/m2 will cause more warming than the last four W/m2. This is why a forcing of 4 W/m2 (I thought it was 3.7 for doubled CO2) give a direct warming (before feedbacks) of 1.2 K rather than 2.8 K.
Uhh, isn’t “Herr” simply a German way to say “Mister”?
You keep saying “Herr Koch” as if “Herr” is his first name. (Or is it his first name?) I think we usually don’t say “Mr. X” when talking of someone’s research.
Or perhaps the custom is different for Germans…?
[Response: He is referred to as Herr J. Koch in Angstrom’s paper, so we’ve followed Angstrom’s lead in referring to him. –raypierre]
Re: #22 I’ve tried to relocate that info here before but wasn’t able to recover it w/ the search engine.
A point to make before elaborating might be –
The temperature of Venus (~ all CO2 atmosphere)
…..surface = 467C, (boiling sufur)
…..but the expected value without a greenhouse effect = -42 C
That ‘strongly suggests’ the Earth’s CO2 level isn’t at a saturation value.
put another way, there is a lot of buried C02 and carbon and methane and so on you could eventually release. long before we got to “saturation” we’d be extinct.
this is good for the saturation argument.
As a piecemeal it’s perfect. It leaves out the strong point that temperature and water vapor and C02 are interrelated and there is positive feedback? It kind of includes it – you visualize the C02 piling up higher and higher as it lingers (unlike water vapor at a given temperature).
Just saying, if people still aren’t convinced, then they should be told that even if the column model doesn’t give them a convincing “feeling” contribution (in the amount of time it’s acting) they need to remember that it’s the lingering C02 raising the radiating levels increasing the temperature increasing the water vapor increasing the temperature increasing the emission of C02 which lingers …
This article was enlightening. I hope we will see more guest articles from Dr. Spencer.
A quick question: I occasionally run into the argument that the 0.7 degrees C warming observed as CO2 levels have increased from 273 ppm to 383 ppm suggests a lower climate sensitivity, because we are already 40% of the way to a doubling of pre-industrial CO2 levels and we would expect greater radiative forcing changes to occur from the initial addition of CO2 given the relationship between CO2 and wavelength absorption described in this post.
The obvious answer is that we are not currently at the “equilibrium” temperature that would accompany a 383 ppm CO2 concentration due to the thermal inertia of the ocean, but am I missing anything else? Also, is there a good chart available of the best guess equilibrium temperatures associated with different levels of CO2?
Re 15, 16, 21 The main factor that limits food production is not CO2 or water or crop physiology its money. Farmers with money can apply technology to overcome other constraints to agriculture. Those without can not. How productive would California agriculture be if the only technology available was hand tools and human labour?
[[Are you assuming that CO2 concentrations are consistent (evenly distributed) throughout the vertical entirety of the atmosphere? It seems to me (based purely on intuition) that CO2 concentrations would be decreasingly significant with altitude, such that the top-most layers of the atmosphere would experience very little change in CO2 concentration. How much of the CO2 in the atmosphere actually exists up there? What percentage of all CO2 exists in the stratosphere? ]]
Intuition is misleading in this case. You might expect gases to be stratified by molecular weight, in which case all the carbon dioxide would be low down, since CO2 has a molecular weight of 44 and the mean MW of air is only 29. But it doesn’t work out that way. Turbulence due to convection (a form of heat transfer due to warm parcels of air rising and cool ones falling) keeps the troposphere well mixed. (The troposphere is the lowest layer of the atmosphere, from the ground to about 11 kilometers high on average.)
[[In the mean time, as CO2 levels continue to rise, poison ivy will also thrive (http://www.sciencenews.org/articles/20060603/fob1.asp). More cucumbers and poison ivy – it doesn’t get much better than that!]]
The last time I tried to eat poison ivy my tongue became inflamed and I had to soak it in calamine lotion for 15 days. I had to cancel all my speaking engagements.
For what it is worth, Lubos Motl has a few things to say about this post.
http://motls.blogspot.com
[Response: Indeed. And as usual with Motl’s stuff, it’s not worth much. In a rather confused and roundabout way, he seems to have rediscovered that the radiative forcing due to CO2 is logarithmic in CO2 concentration, and to think that’s news. It’s also a howler that he took Spencer to task for trying to explain it all without graphs or equations; the graphs of course are in Part II, the equations are in the references, and what RC is all about is trying to make climate science comprehensible to people who can’t take off a few years do do a graduate degree in the subject. Naturally, one achieves a deeper understanding on the basis of mathematics, but if something can be said to be understood at all, it should be possible to convey some of the essential truth in plain language. I think Spencer did a fine job of that. Motl is a good example though,of how being capable of doing the mathematics is no guarantee of actually being able to derive understanding from it. –raypierre]
Re 34 poison ivy
Deer seem to love it!
Global warming –> more poison ivy –> more deer
Great! Just what we need.
Zeke Hausfather (#31) wrote:
The most important point we are still not in balance in terms of radiation leaving the planet being equal to the amount of radiation entering the system – it takes a while for the temperature to rise to the point that the radiation leaving the system becomes equal to that which is entering the system – but I would assume that in part this is the inertia associated with the ocean. Then various feedbacks aren’t instantaneous. For example, the effects of carbon dioxide being applified by water vapor.
re: #20
“However, few if any of the science journalists who discuss climate change and global warming ever make the connection to renewable energy. Articles on global warming focus on the need to reduce emissions of to sequester carbon from coal, but they rarely attempt to discuss the plausibility of replacing all CO2-emitting fuel sources with renewable energy.”
This may not respond specifically to your remark, but does seem to addresses the overall problem being faced. It’s from Sigma XI website, the folks who publish “American Scientist” magazine, where I first saw the executive summary of the follwing report:
“Confronting Climate Change: Avoiding the Unmanageable and Managing the Unavoidable”
http://www.sigmaxi.org/about/news/UNSEGReport.shtml
I haven’t had a chance to read it in depth yet, only skim it, but from the perspective of sustainability, it seems pretty solid.
Also, given as you brought up alternatives, I was wondering if you could address something that came up in conversation recently regarding water vapor as a GHG. Perhaps a stupid question, but what the heck.
It was suggested mentioned that hydrogen powered vehicles would be less polluting than the standard internal-combustion engine largely because they expel water vapor. (I’m sure I’m being imprecise here).
So, he wondered, is anyone aware of any calculations regarding what might happen if over a billion hydrogen powered cars were all operating and expelling water vapor? Would this cause an increase in GHGs that would rival the problems we see with CO2? What about local humidity rates, that sort of thing?
Thanks in advance.
Lastly, a little more off-topic but still in the arena, the current American Scientist has an interesting piece on coal:
http://www.americanscientist.org/template/AssetDetail/assetid/55574;jsessionid=baacFyMVD1uQq8
Regards,
[Response: It’s off-topic, but at the risk of inconsistency with my plea further down, I have to say I’m glad you posted a pointer to this article. It’s really thought-provoking. I have been looking into coal reserves myself a bit, and I think that eventually I ought to do a post on the question of how much coal there really is, and how we know; that would provide a good forum for discussing these issues. –raypierre]
Is rising CO2 level really good for farmers? Maybe not…
check out this:
http://globalecology.stanford.edu/DGE/Dukes/JRGCE/home.html
PS Response to Zeke Hausfather (#31) from #37
One really good feedback to include which takes time to fully come into effect is the melting of ice, including polar sea ice. It is nice and visual. Melting will take years – as each summer will eat away at the ice a little more – and the increase in absorbed light will result in more melting until the long-term equilibrium is reached. This is included in the sensitivity to carbon dioxide doubling.
However, by definition, we are not including feedbacks from the carbon cycle itself. For example, the release of methane from thaw lakes in Siberia or the reduced ability of the ocean and plants to absorb carbon dioxide. These will result in a higher long-term level of carbon dioxide than what we ourselves emit – and this is particularly important to keep in mind when comparing our artificial climate change which was is do to our driving the carbon/temperature feedback to the natural climate change of the past where it was an increase in temperature which drove the carbon/temperature feedback loop.
This difference will increasingly become a factor the longer it takes for us to reduce our emissions.
re 38:
So, he wondered, is anyone aware of any calculations regarding what might happen if over a billion hydrogen powered cars were all operating and expelling water vapor? Would this cause an increase in GHGs that would rival the problems we see with CO2? What about local humidity rates, that sort of thing?
The atmosphere is very good at regulating water vapor. Once you saturate water vapor in the air, adding more just gives you fog and rain. So the GW calculations would mostly fall in an area that is still under study, the effect of clouds. I think they still don’t have solid answers on clouds, even to the point of being able to say that they would be a net positive or negative feedback. Lots of particulars come in to play with clouds (cloud hight, time of day, etc)
As to the effect of a billion hydrogen powered cars, if each emits the vapor equivalent of 10 liquid gallons per day(which seems high, but is useful for ease of calculation) then you have an extra 10 billion gallons of rain per day. By comparison, average US stream flow is 1,200 billion gallons per day (a USGS number). So there would be more rain, but not an overwhelming amount, globally. What that might do the local ecologies in places like Phoenix is another question, however.
Andrew Varga (#39) wrote:
By itself, increasing the level of carbon dioxide up to a point will be good for many plants. However, raising the temperature will result in heat stress, and raising the temperature will tend to result in water evaporating more quickly from the soil. This will become increasingly important in the US south west, then in the US south east. Additionally, it will change the precipitation patterns – with more rain falling close to where the evaporation takes place, namely the ocean.
Then as glaciers disappear, it will result in less glacier runoff. In this case a good example would be the six major rivers of China which will dry up as the glaciers in the Himalayas disappear. These glaciers will be gone by 2100, resulting in a drastic reduction of agricultural output in the region – and will affect food prices worldwide.
[Response: We’re getting rather off-topic here with this discussion of agriculture. Could we get back to saturation, water vapor, and perhaps feedbacks instead? –raypierre]
raypierre (inline #42) wrote:
Agreed. My apologies.
[Response: No problem. People should feel free to bring up what’s of interest to them, and if things stray too far, one of us will just gently nudge the discussion back in the right direction. –raypierre]
#35 The clearest statement in Motl’s essay that differs from Ray Pierrehumbert’s is his conclusion about C02 saturation: he argues that saturation would have already occurred by the time C02 doubled and the warming effect would be only 0.3C. Whereas R.Ph is saying that saturation will never be reached. Of course, waiting to find out who’s right isn’t an option, though I suspect that’s what Motl & Co want.
One of his in-house regulars also dismisses the question of stratospheric C02 having any significance. From what I’ve read though, this has been studied from the 60’s onwards by aircraft and radiosonde studies and they’ve found increasing concentrations in the stratosphere. So I wonder what the real story is on this?
A couple of obvious problems strike me with Motl’s arguments right away:-
1) He seems to totally ignore the question of whether observed global warming has been less than predicted due to the oceanic absorption of C02 – which has been observed to be breaking down in the southern ocean.
2) His observational evidence is virtually nil. He constantly excludes evidence of warming, while any tiny localised event that demonstrates cooling is seized upon as significant.
At least he’s now admitting the “greenhouse effect” actually exists. If so, why doesn’t he discuss the evidence seriously?
[Response: You don’t need to wait to find out who’s right. The lab spectroscopy measurements already show that the standard view (represented by my piece with Spencer and my technical addendum) is correct. The spectroscopy is simply not in question — this is absolutely standard stuff. That shows that the atmosphere isn’t even saturated in the sense thought of by Angstrom. As for the “thinning and cooling” argument, that is explained with crystal clarity using the analytic solution for a grey or semi-grey atmosphere, available many places (my book included). It only involves the solution to the simplest kind of first order constant-coefficient ordinary differential equation, which a string theorist like Motl should be quite capable of handling. Moreover,the “thinning and cooling” argument — that the brightness temperature depends only on the layer from which radiation escapes to the observer — is absolutely standard stuff in physics. The core of the Sun is some tens of millions of degrees. You don’t see that when you look at the sky, do you? The core of the Earth is several thousand degrees. You don’t see that when you look at the ground, do you? It would be incandescent if you did. You don’t see it because you see radiation at the temperature of the level from which the radiation can escape. It’s that simple. Atmospheric IR is no different. If you are used to the “photosphere” of the sun, just think of what we’re talking about as the “IRsphere” of the Earth. Same stuff. You can draw your own conclusions about why he doesn’t seem to be able to understand this stuff. –raypierre]
Re Schmidt response to #15: Mr Schmidt should read the paper before shooting from the hip. Perhaps you can enlighten your readership as to why increased and diminished plant growth based upon CO2 concentration is unlikely. Are there other peer reviewed papers that show contradictory results? Real Climate is supposed to be a professional scientific weblog. Why not function like one?
[Response: A modicum of politeness please. My comment was related to your last line about the prospects for CO2 levels lower than those of today. I do not need to know anything about plant growth to know that neither you nor I will see CO2 levels lower than today’s in our lifetime. Thus discussions about the fate of plants under those circumstances are, to say the least, moot. – gavin]
W F Lenihan (#45) Re carbon dioxide and plant growth…
See #42 but if you wish to consider continuing this topic, per #43 consider a different thread as a matter of courtesy to the authors of the current essays.
Ray, your explanation to the first question raised in comment 6 (as to why the stratosphere cools while the troposphere warms when atmospheric CO2 concentration increases) is a bit off-target and incomplete.
The main cause for the stratospheric cooling & tropospheric warming effect is really due to the “split spectrumâ�� nature of the terrestrial atmosphere, i.e., there is a “window” region in the 10 micron vicinity with little opacity, while there is very strong opacity (due to CO2) in the 15 micron region.
In a simple grey-opacity greenhouse model, as atmospheric opacity is increased, the greenhouse effect causes to surface temperature to increase. If the atmospheric opacity is very small, the ratio of the local temperature at the top of the atmosphere (TOA) to the local temperature at the bottom of the atmosphere (BOA) will be near unity. As the atmospheric opacity increases, the atmospheric temperature gradient will increase, and TOA/BOA temperature ratio will decrease, going to zero as atmospheric opacity (and surface temperature) approach infinity.
With the crude grey-opacity model, as the atmosphere becomes more and more opaque, the top of the atmosphere has to maintain a local temperature that is equal to the effective radiating temperature in order to maintain energy balance with the absorbed solar radiation. Thus there can be no real stratospheric cooling with the grey-opacity model while the troposphere continues to warm as CO2 is increased.
But with a more realistic radiative model (one that properly accounts for the atmospheric window region), the ground surface can radiate directly to space within the 10 micron window region. In this case, when CO2 is increased, the greenhouse effect will warm the surface temperature (and increase the radiative flux that is emitted directly from the ground to space), and the stratosphere will cool because it is being shielded more strongly from upwelling radiation from below by the increased opacity in the 15 micron region. Thus, energy balance with absorbed solar radiation will be maintained by an increase in 10 micron spectral flux (from the ground) and a corresponding decrease in 15 micron spectral flux (from the stratosphere).
[Response: There are no end to wrinkles on this problem of stratospheric cooling, and of course there are a whole lot of things going on in the stratosphere. The mechanism I outlined does work in simple models, but I do appreciate the additional insights. Clearly, the stratospheric emission has to be in a limited wavenumber band if you’re going to get the cooling while still respecting the planetary radiation balance. In my book I describe the split-spectrum issue as well as the absorption/emission issue, but I couldn’t figure an easy way to explain that in a comment. Hopefully your explanation will be of some use to our readers. I’m not sure I entirely agree with you regarding your comment on the grey-opacity case, if you include the effects of upper level solar absorption. That’s apt to take us into technical issues that may not be of interest to the readers, so we can pursue that elsewhere. –raypierre]
#44 “You can draw your own conclusions about why he doesn’t seem to be able to understand this stuff”
Don’t worry, I drew those conclusions some time ago.
Some telling points in your last comment.
Just to clarify, there don’t seem to be separate estimates for the doubling CO2 climate sensitivity for the case without water vapor feedback. The uncertainties in the climate sensitivity estimates (1.7-4.5 C) are related primarily to the strength of the water vapor feedback and the role that clouds play. If the global sensitivity of 3C, then it seems certain that all of Greenland will melt, though it make take a thousand years to do so, or a hundred. That will raise sea levels 7 meters, right? The fastest recorded rate of sea level rise in the past is 3-4 mm/year, or 3-4 meters/century. Global warming is accompanied by polar amplification processes, as well.
Here’s an interesting question: suppose we halted all CO2 emissions today. What would be the equilibrium sea level rise for that case? How long would it take to occur?
Ike,
You made a small math error. 3 – 4 mm/yr equates to 0.3 – 0.4 meters/century. I’m concerned that we may see 30 – 40 mm/year in our lifetime, but we haven’t seen it yet.