Guest Post by Willis Eschenbach
OK, a quick pop quiz. The average temperature of the planet is about 14°C (57°F). If the earth had no atmosphere, and if it were a blackbody at the same distance from the sun, how much cooler would it be than at present?
a) 33°C (59°F) cooler
b) 20°C (36°F) cooler
c) 8° C (15°F) cooler
The answer may come as a surprise. If the earth were a blackbody at its present distance from the sun, it would be only 8°C cooler than it is now. That is to say, the net gain from our entire complete system, including clouds, surface albedo, aerosols, evaporation losses, and all the rest, is only 8°C above blackbody no-atmosphere conditions.
Why is the temperature rise so small? Here’s a diagram of what is happening.
Figure 1. Global energy budget, adapted and expanded from Kiehl/Trenberth . Values are in Watts per square metre (W/m2). Note the top of atmosphere (TOA) emission of 147 W/m2. Tropopause is the altitude where temperature stops decreasing with altitude.
As you can see, the temperature doesn’t rise much because there are a variety of losses in the complete system. Some of the incoming solar radiation is absorbed by the atmosphere. Some is radiated into space through the “atmospheric window”. Some is lost through latent heat (evaporation/transpiration), and some is lost as sensible heat (conduction/convection). Finally, some of this loss is due to the surface albedo.
The surface reflects about 29 W/m2 back into space. This means that the surface albedo is about 0.15 (15% of the solar radiation hitting the ground is reflected by the surface back to space). So let’s take that into account. If the earth had no atmosphere and had an average albedo like the present earth of 0.15, it would be about 20°C cooler than it is at present.
This means that the warming due to the complete atmospheric system (greenhouse gases, clouds, aerosols, latent and sensible heat losses, and all the rest) is about 20°C over no-atmosphere earth albedo conditions.
Why is this important? Because it allows us to determine the overall net climate sensitivity of the entire system. Climate sensitivity is defined by the UN IPCC as “the climate system response to sustained radiative forcing.” It is measured as the change in temperature from a given change in TOA atmospheric forcing.
As is shown in the diagram above, the TOA radiation is about 150W/m2. This 150 W/m2 TOA radiation is responsible for the 20°C warming. So the net climate sensitivity is 20°C/150W-m2, or a temperature rise 0.13°C per W/m2. If we assume the UN IPCC canonical value of 3.7 W/m2 for a doubling of CO2, this would mean that a doubling of CO2 would lead to a temperature rise of about half a degree.
The UN IPCC Fourth Assessment Report gives a much higher value for climate sensitivity. They say it is from 2°C to 4.5°C for a CO2 doubling, or from four to nine times higher than what we see in the real climate system. Why is their number so much higher? Inter alia, the reasons are:
1. The climate models assume that there is a large positive feedback as the earth warms. This feedback has never been demonstrated, only assumed.
2. The climate models underestimate the increase in evaporation with temperature.
3. The climate models do not include the effect of thunderstorms, which act to cool the earth in a host of ways .
4. The climate models overestimate the effect of CO2. This is because they are tuned to a historical temperature record which contains a large UHI (urban heat island) component. Since the historical temperature rise is overestimated, the effect of CO2 is overestimated as well.
5. The sensitivity of the climate models depend on the assumed value of the aerosol forcing. This is not measured, but assumed. As in point 4 above, the assumed size depends on the historical record, which is contaminated by UHI. See Kiehl for a full discussion.
6. Wind increases with differential temperature. Increasing wind increases evaporation, ocean albedo, conductive/convective loss, ocean surface area, total evaporative area, and airborne dust and aerosols, all of which cool the system. But thunderstorm winds are not included in any of the models, and many models ignore one or more of the effects of wind.
Note that the climate sensitivity figure of half a degree per W/m2 is an average. It is not the equilibrium sensitivity. The equilibrium sensitivity has to be lower, since losses increase faster than TOA radiation. This is because both parasitic losses and albedo are temperature dependent, and rise faster than the increase in temperature:
a) Evaporation increases roughly exponentially with temperature, and linearly with wind speed.
b) Tropical cumulus clouds increase rapidly with increasing temperature, cutting down the incoming radiation.
c) Tropical thunderstorms also increase rapidly with increasing temperature, cooling the earth.
d) Sensible heat losses increase with the surface temperature.
e) Radiation losses increases proportional to the fourth power of temperature. This means that each additional degree of warming requires more and more input energy to achieve. To warm the earth from 13°C to 14°C requires 20% more energy than to warm it from minus 6°C (the current temperature less 20°C) to minus 5°C.
This means that as the temperature rises, each additional W/m2 added to the system will result in a smaller and smaller temperature increase. As a result, the equilibrium value of the climate sensitivity (as defined by the IPCC) is certain to be smaller, and likely to be much smaller, than the half a degree per CO2 doubling as calculated above.
Re: scienceofdoom (Mar 17 00:35),
It is not only in the night that the ground temperature is different than the air.
I live in Greece, in the summer the ground in sunlight is more than 50C, the air temperature may be anywhere from 38 to 42, not higher. One could fry eggs on the rocks.
Searching for ground temperatures, I found one study in Puerto Rico where they measured in the ground in the sun for planting studies, and the temperature 20cm in the ground was the same as the air temperature ( in that particular study 22C). This means that the surface, which is the radiator, would be much higher, as soil is not a good conductor.
That is why I think that average SST temperatures would be more realistic, since they are being measured.
Willis: Do the models take account of the endothermic photosynthesis process? The vegetation on the earth is cooling the surrounding air because conversion of CO2 and water through photosynthesis is an endothermic reaction. When biomass is burned, liberating CO2 and water again, this energy is released. As CO2 concentration increases the rate of photosynthesis of plants using the C3 photosynthetic pathway (trees, most grasses, most plants) increases much more rapidly than the increase in CO2 concentration.
Ronaldo asked:
On my comment about downward longwave radiation in the bands of various “greenhouse” gases and outgoing longwave radiation with an equivalent “notch” in it
It’s both. When a “greenhouse” gas absorbs energy there is the possibility that it will reradiate this same energy in a very short space of time in a random direction.
But especially in the troposphere and generally in the first 100km of the atmosphere, the gas in the excited state is rapidly deactivated by collisions and the energy absorbed from the original photon is distributed thermally.
This layer of the atmosphere therefore warms up and radiates energy both up and down. This process is the dominant one.
Willis is just “testing the class” by changing horses a little bit with the answers to his pop quiz.
Using 342 W/m² and 14°C, here’s the “point spread”:
The 8°C cooler is correct if comparing “now” to a blackbody (albedo = 0), but if comparing “now” to a denuded Earth with its current average surface albedo (≈ 0.16), the 20°C is correct. If we turn the Earth into a “big Moon” (albedo ≈ 0.12), we get 17°C. In any case, it’s nice to have our cosy blanket, and even nicer for it to be so stable. 🙂
/dr.bill
Willis . You are confusing things. Is it deliberate?
1) You do not define which temperature you are talking about. From the 14degC average you’d think it was surface temperature, but then you stumble by looking at TOA fluxes. Try looking at the total non-reflected downwelling fluxes at the surface instead and apply stefan-boltzman to that. In that case you’d get an earth which is ~50 degC warmer than a blackbody. This is reduced to ~33C, if you account for non-radiative heat loss terms accounting for the mixing in the atmosphere (i.e. sensible and latent).
2) Wikipedia says: the surface temperature of Earth as a blackbody would be -19C. That is 33C colder than you 14C with an atmosphere. See T_E calculation here: http://en.wikipedia.org/wiki/Black_body.
Brian W said:
I don’t expect to convince Brian W.
Max Planck was the original promulgator of this “nonsense”. Born 1858, won the Nobel prize for physics in 1918. Many people know his name.
He found the equation that described radiation from a body according to its temperature, and the shape of the waveform. You can see the formula here:
http://en.wikipedia.org/wiki/Planck's_law – along with typical radiation vs wavelength curves.
The energy from the sun, as measured by satellite, amazingly follows this curve. So misguided, that sun..
It’s sad when so little is understood of the absolute basics and yet the passion and the belief is so strong.
Of course, Brian W. can win the Nobel prize by showing that Max Planck – and the sun – are wrong.
The http://www.worldscinet.com/cgi-bin/details.cgi?id=pii:S021797920904984X&type=html paper disagrees with the idea of back radiation too.
The net is not equivalent to 50/50. The atmosphere is not made up of a single atom re-emitting half of what it absorbed back to earth. For air particles that are 1km up, the earth would be about 170 degrees of the field of view. For those that are 10km up, the earth would be something like 80 degrees of the field of view. This is just an example. I made up the numbers, but the fact there is a big difference is what is important.
John M Reynolds
My quick 2 cents on this is I’m not sure Trenberth’s energy balance is accurate. It ignores readings we get from satellites and thermometers in favor of equations taken from Hansen’s climate model.
I’m not going to call Trenberth a quack, as someone else did, but I will say it’s not clear to me that we have the technology needed to build an accurate energy balance model at this time.
Which means it’s not clear to me that anything build upon a foundation of our current energy models is accurate.
I seem to remember reading that Earth’s atmosphere causes 33C heat up vs no atmosphere. Sometimes writers say the “greenhouse effect” for earth is 33C.
Was 33C wrong?
Guess this echos a post just above.
Willis, I have a question for you. Want to see if this seems real to you. You say they measure the down dwelling LW and I assume you are speaking of 321 W/m2 but I can’t get the math to jive. Using their numbers I always come up with the surface being too hot.
If you take all radiation absorbed by the ground, 169 for SW Absorbed By Surface and 321 for LW Back Radiation Absorbed By Surface, you get 490 W/m2. That is the radiation from the sun the chart says you would need to warm the earth to 14 C as if there was no atmosphere at all, right?
Take (490/SB)^0.25 = 304.9 K or 89 F for surface temperature according to the chart. Does that seem correct to you? Am I viewing it wrong? To me it seems too hot and the back radiation is way too high.
To correct that would recalculate to SB*(14+273.15)^4 – 169 = 216.5 for the LW back radiation absorbed by the surface. That is where I don’t connect right to K&T math. Seems 216 not 321, or 105 Wm-2 too high.
Don’t think this would affect your sensitivity logic at all, just the internal numbers.
jmrSudbury:
Which one are you going with?
No “back-radiation” according to the entertaining Gerlich and Tscheuschner or back-radiation but modified by the volume integral?
Equally divided up and down is just a useful approximation to explain the basic principles to people who are starting out. Mathematic treatments take this effect into account.
The Planck formula for radiance is in units of W/m^2/sr. “sr” is steradians, or solid angle, i.e. the energy is radiated in all directions.
I’m confused. At the beginning, the answer to the Quiz is said to be (c), 8° C cooler. But latter on, I read
“If the earth had no atmosphere and had an average albedo like the present earth of 0.16, it would be about 20°C cooler than it is at present.”
And Leif chose (b). So which is it? (b) or (c)? While I don’t see eye to eye with Leif on some things, the “smart money” would bet (b).
Has anyone actually measured the incoming wattage per square meter?
[quote anna v (03:24:15) :]
Searching for ground temperatures, I found one study in Puerto Rico where they measured in the ground in the sun for planting studies, and the temperature 20cm in the ground was the same as the air temperature ( in that particular study 22C). This means that the surface, which is the radiator, would be much higher, as soil is not a good conductor.
That is why I think that average SST temperatures would be more realistic, since they are being measured.
[/quote]
I’m not sure if this is what you’re looking for, but the ISCCP does produce surface skin temperatures (as opposed to surface air temperatures).
Just follow the link below, select “Mean Surface Skin Temperatures” from the “Select a variable:” dropdown, select a time period, and click the “View” button to see a picture.
http://isccp.giss.nasa.gov/products/browsesurf1.html
magicjava:
Which page on Kiehl and Trenberth’s 1997 paper are you referring to?
[quote Allen63 (04:21:29) :]
I seem to remember reading that Earth’s atmosphere causes 33C heat up vs no atmosphere. Sometimes writers say the “greenhouse effect” for earth is 33C.
Was 33C wrong?
[/quote]
The “greenhouse effect” isn’t going to be the same as “effect of greenhouse gasses”. The greenhouse effect is always a warming effect. The effect of greenhouse gasses can be warming or cooling.
An example of a cooling effect created by greenhouse gasses is water vapor becoming clouds. Overall, clouds cool the Earth by about 20 degrees, even though they’re made from what started out as a greenhouse gas.
[quote: scienceofdoom (04:48:54) :]
magicjava:
I’m not sure Trenberth’s energy balance is accurate. It ignores readings we get from satellites and thermometers in favor of equations taken from Hansen’s climate model.
Which page on Kiehl and Trenberth’s 1997 paper are you referring to?
[/quote]
It’s not the 1997 paper. It’s the updated paper (2005 I believe).
The exact quote is:
[quote: Trenberth :]
The TOA energy imbalance can probably be most accurately determined from climate models and is estimated to be 8.5 +/-0.15 Wm-2 by Hansen, et. al.
[/quote]
I’d consider any earlier paper to be even less accurate than this one.
Recipy (03:39:01) :
Allen63 (04:21:29) :
Regarding the 33°C issue:
That value comes from using an albedo of ≈ 0.31, which is roughly what you get with the atmosphere, clouds, etc, left intact. In that case, the 342 W/m² available for absorption by the surface becomes 236 W/m² (= 0.69×342), Willis’ values (and mine a little further up the page) are found by elimininating the atmosphere altogether. If you don’t have an atmosphere, you can’t have clouds, and you wouldn’t have such a high albedo, so the comparison is between “everything in place as now” versus “bare planet like the Moon”. You can’t logically use the albedo that one gets with the clouds and such if you don’t have an atmosphere to produce them.
/dr.bill
What about internal warming due to radioactivity and tidal forces (although, I guess those are mitigated by the fact that the oceans can move in response to the forces)? My understanding is that Lord Kelvin’s thermodynamic calculation of the age of the earth, which was based on a black-body irradiated by the Sun, failed to produce the correct age because he was unaware of the concept of radioactivity.
[quote 1DandyTroll (04:39:37) :]
Has anyone actually measured the incoming wattage per square meter?
[/quote]
Yes. The CERES satellite measures longwave and shortwave radiation. Their web site is here:
http://science.larc.nasa.gov/ceres/index.html
And an overview of the meaning of it all is here:
http://asd-www.larc.nasa.gov/ceres/brochure/intro.html
scienceofdoom:
Here’s the link to Trenberth’s paper. See page 3 for the quote:
http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/BAMSmarTrenberth.pdf
1DandyTroll:
On page 3 of that paper you’ll see Trenberth saying he ignores data from the CERES satellite when building his energy balance model.
@ur momisugly Bill Parsons (21:03:27) :
Wiki Answer to: “What is the temperature on the moon?”
The average daytime temperature on the Moon is around 107°C (225°F), but can be as high as 123°C (253°F).
When an area rotates out of the sun, the “nighttime” temperature falls to an average of -153°C (-243°F).
253 F
-243 F
_______
10 F”
=============================
For a range of 496 degrees between the light and dark sides. That’s pretty extreme. What is the range on Earth? Thanks to the atmosphere, its nowhere near close to that.
Trenberth is far less certain than he used to be about the energy budget. Remember he said it was a ‘travesty’ that he couldn’t account for the lack of warming in the last decade. One of his recent papers talks about ‘missing energy’ that he simply can’t account for: ‘Tracking Earth’s energy: From El Niño to global warming’ (February 8, 2010, submitted to ‘Science’)
http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/Tracking%20Energyv5.pdf
“Increasing concentrations of carbon dioxide and other greenhouse gases have led to a post-2000 imbalance at the top-of–atmosphere (TOA) of 0.9±0.5 W m-2 that produces “global warming”. Tracking how much extra energy has gone back to space and where this energy has accumulated is possible, with reasonable agreement between model and observational results for 1993 to 2003…Because carbon dioxide concentrations have further increased since 2003 the amount of heat subsequently being accumulated should be even greater.”
“…although some heat has gone into the record breaking loss of Arctic sea ice, and some has undoubtedly contributed to unprecedented melting of Greenland and Antarctica, it does not add up to be anywhere near enough to account for the measured TOA energy. Closure of the energy budget over the past 5 years is elusive. Thus state-of-the-art observations are unable to fully account for recent energy variability implying error bars too large to make the products useful.”
[“not…anywhere near enough”, “elusive”, “unable to fully account” “too large to make the products useful” etc: read: we’re at a loss to know what’s going on – it’s a travesty.]
“After 2000, observations from TOA…referenced to the 2000 values, show an increasing discrepancy…relative to the total warming observed…The quiet sun changes in total solar irradiance reduce the net heating slightly …but a large energy component is missing…”
“A large energy component is missing” – you can say that again! The discrepancy is huge and increasing by the year – this is the ‘travesty’ because it is a mysterious component that Trenberth knows nothing about, so for five years he’s been unable to ‘close’ the energy budget. Big problem! The contributions from melting glaciers, ice caps, Greenland, Antarctica and Arctic sea ice plus contributions from land and atmosphere warming and changes in solar irradiance all taken together are completely swamped by the “missing energy” due to an energy flux that is not in any way understood.
Of course, it’s only ‘missing’ on the assumption that the greenhouse effect is working as it is alleged to, with positive feedbacks. Lindzen and Choi (though rubbished by Trenberth) showed that, far from positive feedback, there may even be negative feedback – the surface temperature increased AND the escaping IR was increasing just as fast (this is probably Trenberth’s TOA discrepancy, and where the ‘missing energy flux’ is going). This seems to suggest that surface warming through the 1990s was NOT largely due to greenhouse warming, and with increasing CO2 concentration year by year the effect is considerably less than assumed by IPCC. Wind out the positive feedbacks and this ‘missing energy’ component disappears and things start to look sensible again.
The label “back- radiation “Is causing needless confusion.
I take it to mean that all of the 321w/m2 has come “back” from somewhere.
The somewhere must be the atmosphere.
This label seems to separate it from all other IR and long wave from scattering and direct to surface by whatever means EM radiation.
“No “back-radiation” according to the entertaining Gerlich and Tscheuschner”
In fact what G&T say is no HEAT from back radiation which they further state is so small that it can be ignored.
Wayne
….If you take all radiation absorbed by the ground, 169 for SW Absorbed By Surface and 321 for LW Back Radiation Absorbed By Surface,…….
I agree with Wayne does all the 321 come back from somewhere?
This diagram was meant to educate but instead it seems full of obvious ambiguities
[quote Kay (05:30:20) :]
For a range of 496 degrees between the light and dark sides. That’s pretty extreme. What is the range on Earth? Thanks to the atmosphere, its nowhere near close to that.
[/quote]
Looking at the raw data for the aqua satellite from the last 13 months, it gives a high of 274.18 K and a low of 197.06 K, for a difference of 77.12 K.
Obviously, the Earth’s experienced greater extremes than what it’s seen in the last 13 month, but 77.12 K is a decent enough rough estimate.