Guest Post by Willis Eschenbach
Of late there has been a lot written about the effect of “black carbon”, a.k.a. “soot”, and also “brown carbon”, a.k.a. wood and dung smoke, on the climate. Me, I think it’s worthwhile controlling black and brown carbon solely because of the health effects. Inhaled soot and wood smoke kill a lot of people every year. So reducing atmospheric black and brown carbon is an example of the “no-regrets” actions I have been advising that we should take. It is of value whether or not black carbon affects global climate.
And city dwellers are familiar with the phenomenon that when soot falls on snow, it absorbs sunlight, warms, and speeds the melting of the snow. In the country, people spread firewood ashes on frozen walkways to melt the ice. So black carbon tends to melt snow and ice, and thus reduce snow and ice albedo, and thus warms the climate. How much? Unknown, but estimates say black carbon is a definite factor in the Arctic warming.
However, there is one major misconception out there about the effect of black carbon on climate. It shows up in a recent editorial by Richard Kerr in Science magazine.
A Quick (Partial) Fix for an Ailing Atmosphere
Science 13 January 2012: , Vol. 335 no. 6065 p. 156 , DOI: 10.1126/science.335.6065.156
The world’s air could use a quick scrubbing. So a group of scientists has come up with 14 practicable approaches to doing just that. The researchers say the selected cleaning methods, described on page 183, would more than pay for themselves in lives saved and crop yields increased while cutting global warming to boot. “Technically, it can be done,” says atmospheric scientist Mark Jacobson of Stanford University in Palo Alto, California, who was not involved in the work. “It’s a question of will power.”
Scientists and policymakers alike have long known how, in principle, to get a quick start on cleaning up the atmosphere: Stop the gush of short-lived pollutants. Carbon dioxide will remain in the atmosphere for centuries, warming the world all the while, but pollutants like soot and methane remain airborne just a few weeks and a decade or so, respectively. Stop their emissions and their concentrations would promptly start dropping, sharply.
And that would be a good thing. Inhaled soot, also called black carbon, kills or debilitates millions of people each year, while soot in the atmosphere tends to warm climate, mainly by absorbing more sunlight.
It is the last statement, “soot in the atmosphere tends to warm climate, mainly by absorbing more sunlight”, that is in error. I can show this by means of a curious thought experiment, by taking black carbon to extremes.
The logic of their claim goes like this. The earth receives a global 24/7 average of 342 W/m2 at the top of the atmosphere. Of this, about 107 W/m2 is reflected back into space. Black carbon is very much like an ideal blackbody, it absorbs just about all of the light that hits it. The claim is that black carbon in the atmosphere absorbs the incoming solar radiation, so it cannot be reflected back to space. In addition, it also absorbs sunlight reflected from the ground and prevents it from escaping to space. So it intercepts and absorbs sunlight in both directions.
As a result, the system has to end up warmer than it is at present.
And to be fair, that all sounds eminently logical. We end up with more energy in the system, the atmosphere ends up warmer, because the black carbon is absorbing both more sunlight and more reflected sunlight. “Simple physics”, as the AGW folks are fond of saying.
So, here is the thought experiment. Suppose we have a planet just like the Earth, that receives a global 24/7 average of 342 W/m2 at the top of the atmosphere and reflects about 107 W/m2 back into space
We start adding black carbon to the atmosphere. We note that as Richard Kerr says, the black carbon absorbs more and more of both incoming (solar) and outgoing (reflected solar) radiation. Just as their logic says, there’s less and less energy reflected back into space.
We add more and more black carbon, slowly absorbing more and more sunlight and reflecting less and less sunlight. Finally we have added so much black carbon that it forms a shell of solid black carbon entirely surrounding the planet, say 20 kilometres above the surface. This shell is not reflecting anything at all, it is absorbing all the sunlight.
What happens to the temperature of the planet? This is the extreme case of black carbon in the atmosphere, and so it will tell us what the net effect is of adding black carbon to the atmosphere.
Well, we know that the shell has to radiate the same amount of energy that it receives, both inwards and outwards. Since the shell is the only thing heating the planet, that means the planet must be at the same temperature as the shell.
And what temperature would that be? Well, it would be the blackbody temperature sufficient to radiate 342 W/m2, which is … wait for it …
5.5°C or 42°F
This is well below the current temperature of the planet, which is usually taken to be about 14-15°C, or 58°F.
And this means that black carbon in the atmosphere cools the planet.
So where did the logic go wrong?
Their logic went wrong by not considering the effect of atmospheric black carbon on the poorly named planetary “greenhouse effect”. The greenhouse effect works because sunlight strikes the surface. When that energy is radiated back out towards space, some of the energy is absorbed by the atmosphere.
About half of that energy is radiated from the atmosphere back to earth, while the rest is radiated back to space. As a result, the earth ends up warmer than it would be without “greenhouse” gases in the atmosphere.
But when atmospheric black carbon absorbs the solar energy, only about half of the absorbed energy is radiated down to the surface, with the rest radiating upwards towards space.
And as a result, the surface only receives half the radiant energy from the sun that it would have gotten if the black carbon were not there.
In other words, atmospheric absorption of solar energy by any aerosols or molecules, including black carbon, reduces the efficiency of the greenhouse effect. Instead of the surface receiving energy from both the sun and the atmosphere, when black carbon intercepts the sunlight, the surface receives energy solely from the atmosphere.
For the greenhouse effect to work, the sun has to strike the surface. Any solar absorption in the atmosphere reduces the greenhouse effect, and in the extreme, total solar absorption in the atmosphere reduces the greenhouse effect to zero.
And as a result, as the thought experiment shows, adding black carbon (or anything that absorbs sunlight) to the atmosphere cools the planetary surface.
I cannot let this go by without expressing my displeasure at the use of bad science in pushing public policy. As Richard Kerr has just amply demonstrated, the understanding of climate even among scientists is still far too poor to serve as a base for any kind of policy decisions.
w.
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Zeke Hausfather says:
February 7, 2012 at 7:31 pm
“Generally, when you have a bright idea that makes smart folks like Ramanathan look stupid…”
____
You will generally lose on that proposition.
Markus Fitzhenry,
This should help explain your dilemma: http://www.globalwarmingart.com/wiki/File:Atmospheric_Transmission_png
Compare the absorption bands under the visible and infrared spectra.
Layman’s question: What goes up must come down. Is that true? If we intentionally put a lot of black carbon (or anything else) into the atmosphere, wouldn’t it eventually come back to earth? Would my white car turn black?
R. Gates says:
February 7, 2012 at 7:22 pm
2) I’m am wondering about the internal heat generated by the Earth if there is an external shell of black carbon 20 Km up in the atmosphere? Over time, this internal heat would warm up the Earth under the black carbon, as no heat would flow from the cooler Earth to the black carbon shell until such time as the Earth under the shell was warmer. Thus, even if the geothermal energy released by the Earth under the black carbon shell was only .1 w/m2, it would not flow to the much warmer black carbon shell, and would just be trapped, and over millions of years, the atmosphere and surface under the shell would get warmer and warmer, until…
—-
The shell and Earth are assumed to be in the steady state already. The small fraction of additional radiation received by the shell from the Earth would be made up in the higher temps of both.
Nice post Willis. Amazing how it has the warmists here confused :-). It is climate science 101 from the alarmist text books – Aerosols cool! It does not matter if they are black carbon, sand or volcanic ash. How Richard Kerr made such a mistake I don’t know! Maybe he got confused after drinking to much kool-aid. After all, kool-aid lets you ignore the sign and use upside down series ;-).
/ikh
Joel Shore says:
February 7, 2012 at 6:33 pm
Joel, if you want to propose that some hugely unlikely thing interferes to make it so a little bit of carbon warms and a lot cools, be my guest. You might even have a chance of someone believing you if you can propose a plausible mechanism that does that.
Until then? I’ll go with Occam and say you’re grasping at straws, as AGW folks are doing all over the world these days.
w.
Alexander Feht says:
February 7, 2012 at 6:36 pm
Sounds like something a proctologist might be able to help you with, and I’d get help quickly if I were you, because it sounds like it’s backed up all the way to your little gray cells …
w.
Zeke Hausfather says:
February 7, 2012 at 7:31 pm
Zeke, you left out the part where when an amount of energy “X” hits the surface and is radiated away, the surface gets back part of the radiated energy from the atmosphere, well call that “Y”. So the surface gets energy X plus Y.
But when the sunlight hits the BC in the atmosphere, the surface doesn’t get energy X, it only gets Y from the atmosphere.
You are right that it does gain the ~ 15% lost to surface albedo. So the final equation is without BC the surface gets 0.85X + Y, and with the black carbon, just Y.
Perhaps you can explain how the surface will be warmer with incident radiation “Y” than with “0.85X + Y”.
Finally, using a throwaway about Ramanathan like that is very bad manners. You get to pretend authority without either a) actually having the nerve to make a clear claim about what Ramanathan said or b) citing your claim. I’d wait until dawn before you start crowing …
Your move.
w.
KR says:
February 7, 2012 at 7:39 pm
KR, you’re talking to the wrong boy. I’m discussing what Richard Kerr said, that “soot in the atmosphere tends to warm climate, mainly by absorbing more sunlight.”
Not soot on the ground. Soot in the atmosphere. Soot on the ground is a separate question.
w.
“Markus Fitzhenry says:”
What is going on is incoming radiation is almost entirely short wavelength radiation that is almost unaffected by CO2 (although water vapor does absorb some incoming radiation). Both water vapor (~85+%) and CO2 (~9%) absorb outgoing long wave radiation except for a big spectrum hole around 10 microns. All the atmospheric radiation incoming and outgoing absorbed by the atmosphere is re-emitted at its characteristic temperature in all directions.
Carbon black absorbs all wavelengths and emits at all wavelengths omnidirectionally. Since this is high in the atmosphere and affects the incoming short wave (high energy) radiation the net effect is cooling. The radiation is coming in, strikes the carbon, and has a less than 50% chance of continuing in (due to geometry and the absorption gradient). Radiation that doesn’t strike the earth doesn’t warm the earth.
“”Zeke Hausfather says:
February 7, 2012 at 7:47 pm
Compare the absorption bands under the visible and infrared spectra.””
No worry, if I was to accept SB BB equation offhandedly, I’d agree with the spectra. As it is, I don’t.
“Poriwoggu says:
February 7, 2012 at 8:25 pm
The radiation is coming in, strikes the carbon, and has a less than 50% chance of continuing in (due to geometry and the absorption gradient). Radiation that doesn’t strike the earth doesn’t warm the earth.”
Given that the atmospheric density increases, doesn’t the convection lapse rate near surface slow and heat, from the radiation that does strike the surface?
Willis,
Here is the Ramanathan citation I was referring to: http://www.nature.com/ngeo/journal/v1/n4/full/ngeo156.html
If you need a paywall-free version, try here: http://www.climate.org/PDF/Ram_Carmichael.pdf
As to your question, you are assuming that heat reradiated from incoming solar radiation is only transported upward. It reaches the surface both through direct radiation (those black carbon particles reradiate in all directions) and through convection, which is rather significant in the troposphere. And even if it were radiated only upward, a good portion of it would be captured and reradiated downward by greenhouse gases higher up in the troposphere (recall that black carbon tends to remain in the lower part of the troposphere due to its weight; it is decidedly not well-mixed).
Willis – I’ve read the editorial, and there’s certainly no claim of a “20 km shell” there. That’s entirely yours. Much of the effect of the soot is, indeed, on surface absorption, such as darkening polar icecaps.
And it really doesn’t matter whether the soot is at 0 meters or a few kilometers. IR emissivity of the ground and water is ~0.98, not going to be affected much by soot. But visible light albedo is around 0.3 – and that can be dropped considerably with soot. That means more energy retained by the Earth climate, more that has to be dumped back to space by warming.
Again – cleaning up soot benefits both from reducing lung disease and by reducing greenhouse effects. I really don’t see the point of your article.
“”KR says;
Again – cleaning up soot benefits both from reducing lung disease and by reducing greenhouse effects. I really don’t see the point of your article.””
You are going to have to throw out all those incorrect textbooks KR. That is the point of the article. Explain the greenhouse principle as it relates to whole of atmosphere, not just the troposphere, tropopause and lower stratosphere.
Willis, I am sort of with you on this one but I think this is a time when the average incoming power is going to give misleading answers. The reason is t^4 during the daytime and all that that entails.
During the daytime the BC (black carbon) particles are heated very well from all frequencies and re-radiate very well in the IR (T^4…) BC also passes a lot of heat to the adjacent air. At night it continues to radiate heat very efficiently, picking it up from the adjacent air. Thus BC uses air as a heat battery for dumping heat day and night, at different efficiencies of course.
Orbanic carbon (OC) is just a parasol.
So it is true what they say about BC heating things like crazy – about 640 times the effectiveness of CO2 (for PM2.5), but it is also a great radiator in the daytime when it is hottest (remember T^4) and remains one at night too at a lower temperature. No need to go to extremes here, just calcualte a little ∑ this and ∑ that.
It is often said that BC has a short lifetime in the air but Prof Philip Hopke from Clarkson U showed us his pocket aethalometer readings of BC nanoparticles recorded inside an aircraft way above 30,000 ft. The reason you don’t hear much about BC nanoparticles (BCn) is they are really hard to measure below PM0.10. They remain aloft for ages. There are gazillions of them. I suspect the total mass of BCn at PM0.01 is not known at all. They are to the atmosphere what aluminum is to engines – an efficient heat transport system. All that IR coming from the ground is very efficiently absorbed at all frequencies -way better than CO2, and 50% is passed upwards very handily, especially in the daytime.
BC is another leak in the CO2 gasbag.
Grímsvötn put more soot into the atmosphere than Eyjafjallajökull in a much shorter time. That probably explains my mysterious hacking in the mornings this fall and early winter. I don’t smoke, nor get colds to mention. Very healthy, haven’t felt at all that I might have been coming down with one. Glad that’s over with.
Maybe Grímsvötn kept us milder this winter for a while.
Neil says:
February 7, 2012 at 5:53 pm
“This is completely incorrect. The important flux is the one at the tropopause and any extra absorption by BC below the tropopause is basically equivalent to lowering the surface albedo. So, yes, black carbon does warm the system.”
You are redefining the word “surface”. So, your reasoning is entirely incorrect when we take surface to mean what the dictionary says.
KR says:
February 7, 2012 at 8:46 pm
“And it really doesn’t matter whether the soot is at 0 meters or a few kilometers.”
The lowest few kilometers of the atmosphere contain most of the GHGs CO2 and H2O so what you’re saying is that it really doesn’t matter whether we have 40% more or less of these GHGs.
Bad move!
This is a totally logical application of the “precautionary principle”. Sometime around the 1970’s, I think environmentalists “lost the point” of their own protest movement as their concerns veered from very real poisons and harmful elements in the environment to climate. Dare we hope they are hedging their bets, since they only propose to fix the climate “to boot”.
Aerosols, soot and particulates cause very real problems for every human who breathes, and according to Willis’ proof above, few problems for the climate. If somebody can get past the paywall, it might be good to know how the authors intend to address the first problem, since their climate panacea is only secondary. How do they plan to “scrub the air” without hurting business and energy producers?
Markus Fitzhenry – “Explain the greenhouse principle as it relates to whole of atmosphere, not just the troposphere, tropopause and lower stratosphere”
Well, Markus, as the mesosphere and thermosphere are not terribly relevant to greenhouse gas effects (as the effective altitude, 4-5km for IR radiation [http://scienceofdoom.com/2010/08/15/height-of-emission-of-olr-and-dlr/], is at most in the upper troposphere), I’m not certain what in the world you are talking about. Do you have an an actual point to make?
DirkH
If the soot absorption is below 4km (poles) to 5km (tropics) – it’s in the climate. And hence is energy that must be radiated to space.
You refer me to science of doom? And expect me to think you know enough about atmosphere that you can say;
“”Well, Markus, as the mesosphere and thermosphere are not terribly relevant to greenhouse gas effects”
Except, the atmospheric space above your 4-5klms actually regulates DWLR to the surface, in turn heating atmospheric gases (no not GHG’s) relative to force of pressure on atmospheric mass.
The fact that you, and the consensus of politically granted climate scientists, don’t know what your are talking about, is the point being made, by many.
Community Science is bringing home more bacon, then consensus never could. And no, I don’t reference my knowledge, find your own.
Following Zeke’s veiled hint, I find Ramanathan agrees with me. He says that black carbon warms the atmosphere, but it cools the surface. His exact words are
Ramanathan’s estimates are hardly definitive, however. He estimates surface cooling of -1.7 W/m2, and warming aloft of 2.6 W/m2. However, he adds that
This is where triangular numbers are again useful. Ramanathan’s triangular number for atmospheric warming is [1.3, 2.6, 3.9] W/m2. His triangular number for surface cooling is [0.85, 1.7, 2.55] W/m2. The triangular number for the net forcing is therefore [-1.25, 0.9, 3.05].
So I would hardly call that a definitive answer. Other than the comment about the 50% error, Ramanathan doesn’t mention that the net forcing is not significantly different from zero … in fact, he doesn’t put an error estimate on that number at all …
But in any case, Zeke, Ramanathan agrees with me, that the surface forcing will decrease from BC.
w.
I think that whether black particulates in the atmosphere warm or cool the atmosphere depends on the altitude the particulates are at. During the dust bowl years many record high temperatures were set. Black particulates at low altitudes warm the atmosphere.
Whem volcanoes spewed black particulates high in the atmosphere the climate cooled. Black particulates at high altitudes cool the atmosphere.
High altitude black particulates capture the heat and radiate it into space before conduction and convection can tranfer the heat to the atmosphere. Low altitude black particulates capture the heat and transfer the heat to the atmosphere by conduction and convection better than the heat captured by the earths surface. This causes warming.
“markus says: …Given that the atmospheric density… ”
There has to be a paper on this, it has to have peaked someones interest.
There are two effects that happen when you get out above 10km. One is that the majority of the radiation isn’t absorbed. CO2 has decreased with density and HO2 is reduced by pressure and temperature (at 0 C the water vapor saturation concentration/partial pressure is less than 12% of the saturation level at 30 C – this is what drives thunderstorms – isothermal expansion when the dew point is reached – as opposed to the normal adiabatic expansion). Radiation absorption is a function of the density (concentration) of CO2 and water vapor – no density – no absorption. Granted, the effect of altitude on CO2 absorption is much less pronounced since only about 30 ppm is needed to achieve half the sea level absorption.
A negative gradient causes more radiation to leave the atmosphere since the radiation toward the planet is more likely to be absorbed by the atmosphere and at a shorter distance (and reradiated outward) than the outgoing radiation which is less likely to be absorbed or absorbed after traveling further.
The other component is the part of the radiation sphere from a point in the atmosphere that faces toward the planet (inward) is a cone and the percentage of the residual (outward) part of the sphere increases with altitude.