We often get requests to provide an easy-to-understand explanation for why increasing CO2 is a significant problem without relying on climate models and we are generally happy to oblige. The explanation has a number of separate steps which tend to sometimes get confused and so we will try to break it down carefully.
Step 1: There is a natural greenhouse effect.
The fact that there is a natural greenhouse effect (that the atmosphere restricts the passage of long wave (LW) radiation from the Earth’s surface to space) is easily deducible from i) the mean temperature of the surface (around 15ºC) and ii) knowing that the planet is roughly in radiative equilibrium. This means that there is an upward surface flux of LW around (~390 W/m2), while the outward flux at the top of the atmosphere (TOA) is roughly equivalent to the net solar radiation coming in (1-a)S/4 (~240 W/m2). Thus there is a large amount of LW absorbed by the atmosphere (around 150 W/m2) – a number that would be zero in the absence of any greenhouse substances.
Step 2: Trace gases contribute to the natural greenhouse effect.
The fact that different absorbers contribute to the net LW absorption is clear from IR spectra taken from space which show characteristic gaps associated with water vapour, CO2, CH4, O3 etc (Harries et al, 2001; HITRAN). The only question is how much energy is blocked by each. This cannot be calculated by hand (the number of absorption lines and the effects of pressure broadening etc. preclude that), but it can be calculated using line-by-line radiative transfer codes. The earliest calculations (reviewed by Ramanathan and Coakley, 1979) give very similar results to more modern calculations (Clough and Iacono, 1995), and demonstrate that removing the effect of CO2 reduces the net LW absorbed by ~14%, or around 30 W/m2. For some parts of the spectrum, IR can be either absorbed by CO2 or by water vapour, and so simply removing the CO2 gives only a minimum effect. Thus CO2 on its own would cause an even larger absorption. In either case however, the trace gases are a significant part of what gets absorbed.
Step 3: The trace greenhouse gases have increased markedly due to human emissions
CO2 is up more than 30%, CH4 has more than doubled, N2O is up 15%, tropospheric O3 has also increased. New compounds such as halocarbons (CFCs, HFCs) did not exist in the pre-industrial atmosphere. All of these increases contribute to an enhanced greenhouse effect.
Step 4: Radiative forcing is a useful diagnostic and can easily be calculated
Lessons from simple toy models and experience with more sophisticated GCMs suggests that any perturbation to the TOA radiation budget from whatever source is a pretty good predictor of eventual surface temperature change. Thus if the sun were to become stronger by about 2%, the TOA radiation balance would change by 0.02*1366*0.7/4 = 4.8 W/m2 (taking albedo and geometry into account) and this would be the radiative forcing (RF). An increase in greenhouse absorbers or a change in the albedo have analogous impacts on the TOA balance. However, calculation of the radiative forcing is again a job for the line-by-line codes that take into account atmospheric profiles of temperature, water vapour and aerosols. The most up-to-date calculations for the trace gases are by Myhre et al (1998) and those are the ones used in IPCC TAR and AR4.
These calculations can be condensed into simplified fits to the data, such as the oft-used formula for CO2: RF = 5.35 ln(CO2/CO2_orig) (see Table 6.2 in IPCC TAR for the others). The logarithmic form comes from the fact that some particular lines are already saturated and that the increase in forcing depends on the ‘wings’ (see this post for more details). Forcings for lower concentration gases (such as CFCs) are linear in concentration. The calculations in Myhre et al use representative profiles for different latitudes, but different assumptions about clouds, their properties and the spatial heterogeneity mean that the global mean forcing is uncertain by about 10%. Thus the RF for a doubling of CO2 is likely 3.7±0.4 W/m2 – the same order of magnitude as an increase of solar forcing by 2%.
There are a couple of small twists on the radiative forcing concept. One is that CO2 has an important role in the stratospheric radiation balance. The stratosphere reacts very quickly to changes in that balance and that changes the TOA forcing by a small but non-negligible amount. The surface response, which is much slower, therefore reacts more proportionately to the ‘adjusted’ forcing and this is generally what is used in lieu of the instantaneous forcing. The other wrinkle is depending slightly on the spatial distribution of forcing agents, different feedbacks and processes might come into play and thus an equivalent forcing from two different sources might not give the same response. The factor that quantifies this effect is called the ‘efficacy’ of the forcing, which for the most part is reasonably close to one, and so doesn’t change the zeroth-order picture (Hansen et al, 2005). This means that climate forcings can be simply added to approximate the net effect.
The total forcing from the trace greenhouse gases mentioned in Step 3, is currently about 2.5 W/m2, and the net forcing (including cooling impacts of aerosols and natural changes) is 1.6±1.0 W/m2 since the pre-industrial. Most of the uncertainty is related to aerosol effects. Current growth in forcings is dominated by increasing CO2, with potentially a small role for decreases in reflective aerosols (sulphates, particularly in the US and EU) and increases in absorbing aerosols (like soot, particularly from India and China and from biomass burning).
Step 5: Climate sensitivity is around 3ºC for a doubling of CO2
The climate sensitivity classically defined is the response of global mean temperature to a forcing once all the ‘fast feedbacks’ have occurred (atmospheric temperatures, clouds, water vapour, winds, snow, sea ice etc.), but before any of the ‘slow’ feedbacks have kicked in (ice sheets, vegetation, carbon cycle etc.). Given that it doesn’t matter much which forcing is changing, sensitivity can be assessed from any particular period in the past where the changes in forcing are known and the corresponding equilibrium temperature change can be estimated. As we have discussed previously, the last glacial period is a good example of a large forcing (~7 W/m2 from ice sheets, greenhouse gases, dust and vegetation) giving a large temperature response (~5 ºC) and implying a sensitivity of about 3ºC (with substantial error bars). More formally, you can combine this estimate with others taken from the 20th century, the response to volcanoes, the last millennium, remote sensing etc. to get pretty good constraints on what the number should be. This was done by Annan and Hargreaves (2006), and they come up with, you guessed it, 3ºC.
Converting the estimate for doubled CO2 to a more useful factor gives ~0.75 ºC/(W/m2).
Step 6: Radiative forcing x climate sensitivity is a significant number
Current forcings (1.6 W/m2) x 0.75 ºC/(W/m2) imply 1.2 ºC that would occur at equilibrium. Because the oceans take time to warm up, we are not yet there (so far we have experienced 0.7ºC), and so the remaining 0.5 ºC is ‘in the pipeline’. We can estimate this independently using the changes in ocean heat content over the last decade or so (roughly equal to the current radiative imbalance) of ~0.7 W/m2, implying that this ‘unrealised’ forcing will lead to another 0.7×0.75 ºC – i.e. 0.5 ºC.
Additional forcings in business-as-usual scenarios range roughly from 3 to 7 W/m2 and therefore additional warming (at equilibrium) would be 2 to 5 ºC. That is significant.
255 Responses to "The CO2 problem in 6 easy steps"
Ben Kalafut says
I’ve received email–and I’m not even a climatologist–from someone who’s a denier or denialist because he’s trying to think of this as a black-body radiation problem. The guy even did an experiment with painted ping-pong balls to convince himself that AGW must not be happening.
That’s the wrong approach–and I wish people would stop and do their homework before trying to apply high-school physics to this–but it’s instructive. You can’t say “QED” until you’ve put together at least some two-layer radiative balance equations and shown how this actually works.
Michael Farinha says
Thanks for the link David B.
Steve Horstmeyer says
Comments on #5, #7 and #19
First, can anyone be surprised about the state of confusion on AGW among the members of the (under educated) general public after reading the comments here?
As a TV weatherman in Cincinnati, OH, USA I run into the problem of attribution constantly. In fact in January 2007 after a very unusually warm month I made the statement that this very warm month cannot be necessarily attributed to GW. Jan. 2007 was 5.3F (9.5C) warmer than normal. I stated long term records were stronger evidence for the effect and proceeded to present the 1.0F increase in mean temperature for the previous decade as evidence in support of GW, stating that AGW is likely a part.
Following that was an avalanche of emails telling me to read Michael Crichton’s recent book, to stop taking issue with the Bush administration’s position, to stop advocating the destruction of the American automotive industry and on and on ad infinitum even though I made no statements about automobiles or Mr. Bush.
What is the point?
First people in accordance with Abraham Maslow’s “hierarchy of needs” will worry about putting food on the table first and global warming second, and second only if those of us concerned about the issue properly present the supporting arguments.
If we present confusing information or forcefully and without consideration present information that overtly threatens a person’s well being (as that person perceives it) we have lost our audience.
To Aaron Lewis (comment #5)
Well…. are there other factors? Is AGW the only factor involved in the flooding in England? If as you seem to imply that AGW is the only factor in these storms how do you account for the great storm of November 1703. It may not have had as much rain as the event under discussion but it devastated southern England. It had to derive its energy from some source and the 1703 storm occurred long before anthropogenic global warming kicked into high gear.
Please do not accuse me of being absurd here. My point is that it is much more complicated than Aaron states. Many factors always contribute to a particular event and in fact there are nonlinear, chaotic and linear feedbacks and interactions at a continuum of time and space scales from the nanosecond and molecule to the multi-century and global in every weather event.
Steve Reynolds (comment #7)
I agree more with you than with Gavin’s comment in reply to you. You are clearly saying that if we find no Katrina magnitude hurricanes in the past then maybe Katrina is a child of AGW. However historically there have been many hurricanes that equal or exceed the magnitude of recent hurricanes. Hurricanes require multiple minimum thresholds be surpassed to attain a given magnitude so unless the thresholds other than AGW are shown to be unimportant in a particular event then one cannot claim AGW is the sole cause.
Be careful, this does not say that there is no role of AGW in the increase of hurricane strength it merely states that more work must be done to demonstrate how important a role.
As I read Gavin’s comment it looks to me like he is not requiring a rigorous enough standard be met to make the case for the DOMINANT role of AGW. If on the other hand Gavin is just saying that a continuous chain of cause and effect need be followed to state SOME role of AGW in an event then I agree with him.
From Terry Gilliam and his movie “Brazil”,
“…even the complications have complications…”
Bob Bergen (comment #19)
I too teach, in my case introductory meteorology and oceanography. It seems weather and oceans have much more appeal than chemistry and physics so I get the less mathematically inclined students who are surprisingly naive about math being necessary for a detailed study of the atmosphere or ocean.
Yes, as I have argued in earlier comments, even something as simple as sigma*T**4 will send most students into a math panic. I cannot use basic trig functions to explain the effect of sun angle on radiation intensity at the surface. Just wait until you try to describe kinetic theory and life at the scale of a molecule.
I have, with mixed success, tried to avoid math altogether and teach conceptually. I know heresy! But I am faced with two choices: fail most of them or make the best of reality and have them leave with an intuitive knowledge of the physical world.
Think of it this way, what is the mathematical description of a phenomenon without an underlying concept. The concept comes first the formalization after that.
Gavin, for a career scientist to whom the concepts of basic atmospheric physics have become second nature and to whom the mathematics describing the phenomena can be read as clearly as a popular novel, I imagine it is very hard to understand that the level of mathematical competence in the general populace is so removed from yours.
After teaching part-time for nearly 30 years I have seen a remarkable decline in the ability of students to deal with even basic math and have gradually, for introductory and non-major courses only, migrated to a conceptual approach.
That is the first mountain to climb and once I accepted conceptual teaching the second mountain was separating unnecessary details from the core general concept. Of course one person’s unnecessary detail is another’s foundation cornerstone.
It is a continuing challenge.
Mike Alexander says
Post #29 raises a good point. The simple toy model referenced is not easy to follow. The author doesn’t “show the work”. He should, it’s algebra. In my simple greenhouse model (see link below) I work through the issues step by step.
My model is naive, clouds are treated as opaque radiation screens (like carports) and the cloud-free atmosphere is treated as a pane of translucent glass. Nevertheless, with a single adjustable parameter (chosen to make the forcing for a doubling in CO2 equal to 5.35) it produces a predication of 2.8 C for a doubling of CO2, which is very close to the consensus of far more complex models. That is, it does a good job with sensitivity (without any effort on my part ot make it do this).
I ignore oceanic damping, except to estimate the size of the solar forcing from a putative solar cycle-linked temperature effect. And I don’t consider other greenhouse gases. I plan to add improvements as I come to understand the phenomena (this is complicated stuff). Most importantly I currently include the cosmic ray mechanism that skeptics like to cite and show that it doesn’t help their case.
One of the reasons I have developed my model is because I found that most web resources dealing with AGW are either extremely watered down or way over my head. I have a Ph.D. in chemical engineering so my head isn’t *that* low. So I have sought to produce a model that gives pretty good results that an intelligent citizen with some math and science background can actually work through (with some effort).
I still have some problems with figures on Internet explorer (it works with Firefox) that I am trying to fix. Anyways I would appreciate comments at firstname.lastname@example.org
Simple climate model: http://my.net-link.net/~malexan/Climate-Model.htm
“You can’t take any value that has the same units as the radiative forcing and multiply it by the sensitivity and expect to get anything sensible. For one thing, the 150 W/m2 net LW absorption includes all feedbacks already, secondly expecting climate sensitivity to be linear from no greenhouse gases to today’s level is rather optimistic. Bottom line, you can’t estimate sensitivity from the mean conditions today – you need to look at a climate change. – gavin”
Ah, feedbacks, yes. Pretty models not tested by experimental data.
But you do estimate sensitivity using a rule of thumb:
“As we have discussed previously, the last glacial period is a good example of a large forcing (~7 W/m2 from ice sheets, greenhouse gases, dust and vegetation) giving a large temperature response (~5 ºC) and implying a sensitivity of about 3ºC (with substantial error bars).”
The fact that CO2 was not the trigger for the change in the Earths temperature, is ignored.
[Response: Not ignored, irrelevant. And if you weren’t paying attention, none of the values come from models, all of it is from observational constraints. – gavin]
You based the 3ºC on:-
1) An analysis of the last glacial period which you now appear to believe was CO2 driven (if not then it adds nothing to effects of CO2 on temperature).
2) a paper which used Bayes’ Theorem to analyse all the various gusses in the literature. Models Gavin.
[Response: Your reading of this is bizarre to say the least. The LGM example does not depend on what leads or lags – it is an equilibrium calculation – read Lorius et al 1991, or Hansen et al 2006 for more details – part of the forcing is GHGs, part is not. Second point: Bayes Theorem is not a GCM, and none of the estimates used in AH06 were model based either. – gavin]
B Buckner says
Re:44 Timothy Chase
Now you have gone and misspelled “spelling B” (its Bee), which is almost as funny as dust bowel :)
Thank you for this article. In point 4 you said “Most of the uncertainty is related to aerosol effects”. This of course is one of the points that Lindzen appears quite troubled by. In his 2005 article Is there a basis for global warming alarm?, Lindzen says “Unfortunately, the properties of aerosols are largely unknown. In the present instance, therefore, aerosols constitute simply another adjustable parameter (indeed, both its magnitude and its time history are adjustable).” In the same article he also says “aerosols and their impact are unknown to a factor of ten or more; indeed, even the sign is in doubt.”
Are you able to point me to resources that set out how the net cooling affect of aerosols has been calculated? Also, are you able to tell me what the accepted estimate of the degree of uncertainty associated with net cooling affect of aerosols? Is it the factor of “ten or more” that Lindzen suggests?
B Buckner says
Gavin: In step 4 you state: “The stratosphere reacts very quickly to changes in that (radiation) balance and that changes the TOA forcing by a small but non-negligible amount. The surface response, which is much slower, therefore reacts more proportionately to the ‘adjusted’ forcing and this is generally what is used in lieu of the instantaneous forcing.”
Temperatures in the lower stratosphere stopped going down in 1993 according to published UAH, RSS and HadAT2 data series. Since the stratosphere reacts very quickly to changes in the radiation balance, shouldn’t the temperatures there continue to decline as CO2 builds up in the atmosphere?
[Response: Lower stratospheric temperatures (as measured by MSU4) are dominated by ozone trends, with a large contribution from volcanoes, but only a minor effect from CO2. Further up in the stratosphere, cooling is much larger and continues apace. – gavin]
Chirs Moran CPA says
Even though the United States has not adopted a mandated reduction in carbon dioxide emmissions, at least a lot of people are still speaking about the issue.
Steve Reynolds says
Steve Horstmeyer> Be careful, this does not say that there is no role of AGW in the increase of hurricane strength it merely states that more work must be done to demonstrate how important a role.
I certainly agree. AGW will have many effects, some bad, some good. Having a scientific understanding of these effects (and the associated economics) seems very important to me. Unfortunately, too many here appear unwilling to consider that some short term ‘solutions’ could cause more harm than the original problem.
Timothy Chase says
Chuck Booth (#40) wrote:
European Space Agency?!
This is whats coming out of NASA:
Atmospheric InfraRed Sounder – some of the vids I mentioned…
AIRS – Multimedia: Videos: Animations
Total Column Ozone Time Series – 8/1/2005 to 9/30/2005
Carbon Monoxide Time Series – 8/1/2005 to 9/30/2005
Water Vapor Time Series – 8/1/2005 to 9/30/2005
Atmospheric Temperature Time Series – 8/1/2005 to 9/30/2005
Outgoing Longwave Radiation Time Series – 8/1/2005 to 9/30/2005
Cloud Fraction Time Series – 8/1/2005 to 9/30/2005
A 3D Look At Atmospheric Water Wapor
Supertyphoon Pongsona Isotherms
Hurricane Isabel Isotherms
Transport of Dust from China Dust Storm of April 2006
The Alaska Fire Season of 2004
Upper Tropospheric Water Vapor
Atmospheric Temperature at 500 millibars
… and vids on how its done:
From Data Collection Swath to Atmospheric Temperature Profile
Light Travels Through AIRS Optics
False Color Thermal Images From AIRS channels In Three Spectral Regions
Anyway, this sort of thing might help – as it demonstrates that absorption and reemission takes place throughout the atmosphere – and that it is fairly well understood.
Phil. Felton says
Re #16 and Gerhard Gerlich, Ralf D. Tscheuschner. Falsification of the atmospheric CO2 greenhouse effects within the frame of physics.
This runs to ~90 pages, the first 40 of which are devoted to proving that real greenhouses rely on cutting off convection rather than differential radiation effects! The authors seem very proud of themselves and slip in several very non scientific sneers as well. They consider the IR portion of the solar spectrum to be the same as the IR of the thermal radiation from the earth, they don’t seem to consider the TOA at all (I may have missed it in all the verbiage).
“The point discussed here was to answer the question, whether the supposed atmospheric
effect has a physical basis. This is not the case. In summary, there is no atmospheric
greenhouse effect, in particular CO2 -greenhouse effect, in theoretical physics and engineering
thermodynamics. Thus it is illegitimate to deduce predictions which provide a consulting
solution for economics and intergovernmental policy.
The authors express their hope that in the schools around the world the fundamentals of
physics will be taught correctly and not by using award-winning “Al Gore” movies shocking
every straight physicist by confusing absorption/emission with reflection, by confusing the
tropopause with the ionosphere, and by confusing microwaves with shortwave.”
It seems like quite a number of posters here are looking for an easy-to-understand explanation for why increasing CO2 is a significant problem. Perhaps someone would be happy to oblige.
Dick Veldkamp says
Re #53 Attribution (Steve Horstmeyer)
You might want to use the loaded dice analogy in the attribution debate (discussed earlier on this site). If we throw a few sixes in a row, that would be consistent with a fair dice. However, 10 sixes in a row would start to raise some eyebrows.
Also you may point at setting of records. For numbers drawn from a non-changing distribution, the chance of setting a new record (say an extremely hot summer) quickly goes down to zero after the first few records. If we keep on seeing new records, clearly something is the matter, and a new record hot summer may reasonably be attributed to GW.
Barton Paul Levenson says
[[As the environmental effects of global warming increase, the disruption to the economy will be far worse than that caused by banning the use of unnecessary transport.]]
Alastair, I can go along with banning UNNECESSARY transport. But if you look at what I was responding to, the original poster was suggesting banning ALL transport, which is idiotic and would kill almost everybody on Earth.
Barton Paul Levenson says
[[According to the Wikipedia source, the value for earth’s albedo is 0,3 only because clouds are taking into account. The albedo is about a third lower without the clouds (earth’s surface = 70% water).
Is it allowed to use part of the atmosphere in this kind of calculations?
Yes. Clouds reflect away much of the sunlight reflected by the Earth, and they do cut down on how much sunlight is absorbed by the Earth system.
You only have to deal with the albedo of the surface if you’re doing a model involving the surface plus layers of air. One value often used for the surface albedo is 0.10, which is an average including land, sea, and ice. The Moon, which has no sea or ice, has an albedo of 0.11, and Mercury, ditto (though it may have small amounts of ice near its poles) is about 0.119.
Nick Gotts says
RE #48 [Supposing that my above statistics are correct It would be more prudent for society to rally around reforming our coal power plants more so than around car’s and airplanes.]
We have to think about what sources are growing fastest as well. For example, air travel is responsible for a relatively small proportion of current global CO2 emissions (though a larger proportion of warming because of other effects), but its projected growth, which would far outpace any likely efficiency savings, and which governments are encouraging, would make it one of the main CO2 sources in a few decades.
Nick Gotts says
RE #66 (Barton Paul Levenson) [[[[As the environmental effects of global warming increase, the disruption to the economy will be far worse than that caused by banning the use of unnecessary transport.]]
Alastair, I can go along with banning UNNECESSARY transport. But if you look at what I was responding to, the original poster was suggesting banning ALL transport, which is idiotic and would kill almost everybody on Earth.]]
No, the original post #10 specified “all car and plane transport”. Not trucks, buses, trains or ships. This doesn’t mean it’s a practicable suggestion, but it’s not obviously crazy, as your misinterpretation here and in #23 would suggest. Given a few months to prepare, I’d guess it would be possible without insupportable disruption. The impossibility is political rather than economic.
Lawrence McLean says
Re #16 Check out: http://en.wikipedia.org/wiki/European_Science_and_Environment_Forum
As far as Tscheuschner is concerned he must be wanting to torpedo his academic reputation for being associated with such rubbish.
Nick Gotts says
Re #69 Sorry, rereading #10 it’s internally inconsistent, at another point saying “all plane and vehicular transport”. And it does say NOW.
Andre Arbour says
I hope our hosts will forgive me for continuing the somewhat off-topic policy discussion.
Michael Farinha #48: Your impulse to focus on ROI is a good one, but be cautious in prioritizing mitigation efforts according to sectoral contribution. Generally, reductions should be prioritized according to what can be done at the lowest marginal cost. For example, say reductions can be realized by changing certain agricultural practices at cost of $5/ton of co2 equivalent or in the electrical utility sector at $10/ton. Even though the agricultural sector contributes a relatively small portion of GHGs it’s more efficient to makes these investments first.
The best way of determining lowest cost reductions is through market instruments such as a tax or cap and tradable emissions permits regime.
The overall sectoral emissions contribution can be a rough guide for inclusion in the mitigation regime but the most important element of the public policy (IMHO) is to apply a nontrivial price on emissions wherever it’s administratively cost-effective and allow for market mechanisms to “find” low cost reductions. Emissions offsets/tax credits allow for reductions to be realized outside of the cap or tax regime.
Ferdinand Engelbeen says
Some remarks on several points in this step program:
– No problems with steps 1-3.
– Step 4:
The efficacies calculated in the Hansen report are within the constraints of the model used. Accoding to Hansen, the efficacy of e.g. solar is 0.9 that of CO2. Hardly to believe, as a test of the HadCM3 model (see here shows that solar may be underestimated with a factor 2.
In the case of HadCM3, the model included a fixed value for aerosols effect, without that, the difference might have been larger.
Indeed aerosol effects are quite important, one can halve the efficacy of CO2 in a simple model (Oxford EBM model), if the real forcing (or efficacy or both) of aerosols is lower than expected, without changing the temperature profile of the past century (see here).
The global cooling effect of aerosols indeed is questionable: Ramanathan found (published in Nature) that the current aerosols above India (and probably China, as they use a lot of dirty coal) are increasing the warming with about 50%. India and China are also the largest contributors to SO2 at this moment. This makes the “cooling” effect of aerosols in general rather dubious…
[Response: This is both wrong and irrelevant. All aerosols cause local heating (due mainly to absorbed LW), and the Asian Brown Cloud additionally has strong absorption of solar due to the presence of black carbon. Sulphates however are a cooling. The stott et al study does not show any differences in efficacy in any case. – gavin]
– Step 5:
As said in the previous point, there are problems with the attribution of efficacies. The main difference between solar and volcanic at one side and GHGs and (man-made) aerosols at the other side is their distribution effect and the kind of radiation.
Incoming solar radiation has two important effects: heating of the stratosphere (mainly in the tropics) and deep penetration of the oceans. Downwelling radiation from GHGs is more distributed all over the latitudes and absorbed in the upper fraction of a mm of the ocean’s surface. The effect of the different radiation spectra on clouds is another item of interest…
From solar it is known that the stratospheric warming shifts the jet stream position, cloud cover (regional and global) and rain patterns (USA, Italy, Portugal, recently South Africa) to the poles and back together with the solar cycle. From GHGs, the influence on these items is not known to any accuracy (especially cloud cover…). Stratospheric dust from volcanoes has the opposite effect of solar. But man-made aerosols should have their maximum cooling effect in the NH (90% of the emissions), where most of the increase in temperature is found…
Most of the ocean warming is found in the sub-tropics (see fig.2 in Levitus), which is mainly caused by changes in cloud cover, inducing 2 W/m2 more insolation in the (sub)tropics over the past 15 years. This is more probably attributable to natural causes (solar or internal terrestrial) than GHGs.
About the attribution of the different efficacies in the last ice age, again this is questionable: the start of the last ice age, at the end of the Eemian, was without any help of CO2 (maybe with some help of methane). Temperature was down near minimum, before CO2 levels started to decrease (see here, without much change in temperature when CO2 started to decline (with 40 ppmv).
Moreover, the Epica C ice core doesn’t show any sign of positive feedback from CO2 for the whole LGM-Holocene transition. I know from my former work as process engineer how a positive (and a runaway!) feedback looks like… See here. Anyway, this points to a much smaller effect of 2xCO2, compared to the other forcings/feedbacks like insolation, ice sheet albedo, vegetation growth,…
[Response: The reason why we look at the LGM is that it is at equilibrium. All of these other issues involve transients where we do not have complete information for what was changing when. Your last line perhaps reveals your confusion. Sensitivity comes from examining all forcings together and for the LGM it includes ice sheet albedo, vegetation and GHGs. – gavin]
– Step 6.
Based on the former remarks, the formula for future warming may not be a simple (sum of forcings) x (general sensitivity) but a sum of (each forcing x its own sensitivity)…
Which makes it a little more difficult (but more exiting) for climate modelers…
[Response: To show that this would make a significant difference you have to demonstrate that efficacies are indeed substantially different from unity (for which there is no evidence) – gavin]
Hank Roberts says
Oh, ick. I’d forgotten. Shudder.
Eric (skeptic) says
I don’t see step 5 as “easy”. For one thing it doesn’t point to a paper or explanation of sensitivity, rather a section. From the section I gather that correlations in climate history are used to derive sensitivity. The biggest problem with doing that is the smoothing and other time distortions in the proxies for CO2 and temperature make them only useful for longer time periods.
So using them to predict short term catastrophe (100 years) without detailed weather models is not going to be particularly useful.
I think that you really need to step back at the beginning and give a broad overview of *why* quantum mechanics and radiative transfer work the way they do. I’m an astronomer, and radiative transfer is a tool that we employ all of the time. There are subtle things that are, in my experience with students, significant conceptual blocks. You’re assuming a lot of prior knowledge here, and I’d begin with the following:
1. For a first approximation the Earth can be treated as being in radiative equilibrium; this is a state where the incoming energy from the Sun is balanced by the outgoing energy radiated by the Earth.
2. Both the Sun and Earth emit light at all wavelengths (or colors, or energies). The temperature influences both how much total light is emitted (the Stefan-Boltzmann law) and what the typical energy of the light is (Wien’s law). The Sun has a surface temperature of 5770 Kelvin, so most of it’s energy is emitted in the form of visible light; the Earth has a surface temperature of about 300 K, so most of it’s energy is emitted in infrared light.
3. Molecules and atoms absorb only certain wavelengths, or colors, of light. This is a fundamental feature of quantum mechanics that can be precisely measured in the laboratory. The atmosphere is very transparent to visible light, which allows sunlight to reach the surface and warm the Earth (with the exception of clouds). However, the molecules in the atmosphere absorb infrared light, and each distinct molecule (water, carbon dioxide, methane, etc.) will absorb different wavelengths of infrared light.
4. When light emitted by the Earth is absorbed by a gas its energy either goes into heating the gas (by collisions between the molecules) or is re-emitted in a random direction (which can include reflecting it back to the surface.) In either case absorption reduces how much heat the Earth can put out, which would drive it out of radiative equilbrium. To compensate the Earth gets hotter, causing it to emit more light. This is the “greenhouse effect.”
5. When light is absorbed by the atmosphere there are two distinct stages. As you add more of something (like CO2), the initial effect is to efficiently absorb certain wavelengths of light. However, once you have enough of an absorber then it is already blocking pretty much everything that it can, and the net effect of adding more becomes progressively smaller. This is why species like methane or carbon dioxide (which are much less abundant than water) matter; they fill in the transparent windows between windows in the spectrum that are blocked by water.
Lynn Vincentnathan says
I agree with #5 that weathermen do a disservice by suggesting certain severe weather events expected in a GW world cannot be attributed to GW. While this may be technically true, since GW & its effects are at a higher order level of statistics & not single events, it grossly misleads the viewing public, who tend to interpret it as “GW is not happening.”
So weathermen could (if they wanted to be honest in spirit, not just technically) say, “Hurricane XXX (or this flood, or brush fire) adds one more piece of evidence that GW is happening.” A piece of evidence (an observation in a statistical data set) out of context of the rest of the evidence does not a case or statistic make, so the statement (I think) would be techically accurate (i.e., no claim is being made that the hurricane by itself proves GW, it just adds one more observation to the data set that is already on the whole linked to GW), especially since GW at the macro-statistical level has already been established. The statement would be accurate both technically and in the spirit of not misleading people into rejecting the idea of global warming.
RE #19, I don’t think one needs any math to understand GW. The greenhouse is a good heuristic device (or a closed car sitting in the sun). Another is a prism, which shows light in many colors (that ought to dazzle them); and it can be explained that certain bands (the ones that generate heat — and we all know from daily experience that the sun generates not only light, but also heat) have longer wavelengths and tend to bounce back, rather than go through, and that the GHGs up in the sky tend to bounce these heat bands back to earth …. Water acts as a prism, and so does air (with the molecules in it). And then there is the black cloth/white cloth experiment re which absorbs more heat.
Numbers are for the professional number crunchers. It can be said that it may get hot enough by the time your children (or grandchildren) are old to melt enough glaciers and ice sheets and to raise the sea above New York’s (or Houston’s) ground level….Or other non-numerical scenarios….
N Leaton says
[Response: Feedbacks work for everything. That’s why the ‘radiative forcing’ concept works – it doesn’t matter if the initial push is from greenhouse gases or the sun. The change in temperature you’d need to balance a forcing of 4 W/m2 with no feedbacks is around 1.2 ºC and the difference between that and the real sensitivity (around 3 ºC) is a measure of how strong the net feedbacks are. – gavin]
Ok. So what percentage of the 3C change is just down to the CO2, and what is down to the feedback effects?
In the IPCC report, from what I can tell, the small increase for solar radiation includes no feedback effect, but the figure for CO2 does include feedback effects.
Is this correct?
[Response: No. All the forcings in the AR4 figure are without any feedbacks. Feedbacks only go into the calculation of the sensitivity. If you have a forcing from solar or CO2 of 4 W/m2, then you would need to change the temperatures by 1.2 deg C (keeping everything else constant) to restore balance. However, everything else is never constant and the feedbacks multiply this ‘no-feedback’ temperature by about 2.5. The no-feedback response is therefore 40% of the total. But remember this factor is the pretty much the same for a solar forcing as it is for CO2. – gavin]
Aaron Lewis says
Thank you for your concern about the teaching of math and science. Perhaps if we did a better job of teaching math and science we would not be facing the current situation.
My concern about AGW is about the survival of the infrastructure required for civilization. A nomadic herding tribe can survive violent changes in climate. However, civilizations engineer structures based on their experience with the local climate. If the climate changes, then that infrastructure fails. If too much of a civilization’s infrastructure fails, then that civilization fails.
The British know how to build infrastructures that will withstand huge monsoon rains. They learned that in India. However, their experience with the climate in Britain did not lead them to apply such engineering to the structures in Britain. They had a good reason, in the past, it would have been a waste of capital.
The problem with AGW is that it invalidates our long experience with local climate. This disrupts our asset allocation and engineering processes. AGW increases the frequency of the most intense storms. Such storms cause damage to infrastructure engineered to modern standards where capital cost is a major concern. (Low bid mentality)
If we deny AGW, then storm damaged infrastructure is rebuilt to the old standard on the assumption such storms occur rarely, when in fact, as a result of AGW, such storms are now occurring more frequently, and can be expected much more frequently in the future, as the full effects of AGW unfold.
Such building and rebuilding will result in a huge waste of capital. We do not have capital to waste. We have to build to withstand the storms expected to be produced by the climate of the future rather than the climate of the past. That can be a very expensive proposition. For example, it is hugely more expensive to build a house to withstand a cat. 3 hurricane than to build a house to withstand a cat. 1 hurricane. On the other hand, it is much cheaper to build one house that can withstand a cat 3 hurricane then to build a cheaper house, and have it blow down, killing the family inside.
Now, as an experienced weather forecaster, what is your advice to the stakeholders planning public infrastructure?
J.S. McIntyre says
“Given a few months to prepare, I’d guess it would be possible without insupportable disruption. The impossibility is political rather than economic.”
One thing for sure: muscle-powered locomotion would definitely come into vogue. Get a good lock for your bicycles, kiddies – they’ll be worth their weight in gold.
On a related note, I think the economic factors are equally difficult to surmount. Consider – my local supermarket is selling oranges grown in Australia. Now, the last time I checked, Australia is in the midst of a severe drought with no end in sight. Yet they are shipping their water to the U.S. in the form of oranges and who knows what else. Which begs the question: what is really wrong with this picture? I enjoy winter – and summer – veggies as much as the next guy, but how much do we want to really pay for this enjoyment, long-term?
“Planet is in DIRE DIRE straights….”
Minor quibble. The planet is NOT in dire straits; it is the state and content biosphere we depend upon for our existence. The planet and life in general will continue on quite nicely with or without us, thank you very much, though as a member of an interested species, I would prefer the former to the latter.
luminous beauty says
As a ‘physics for poets’ style explanation of the greenhouse effect and ancillary feedbacks, I like to use an empirical comparison with the Moon, using simple arithmetic.
This gives a rough difference of about -35C for an earth stripped of atmosphere, icecaps, oceans, vegetation and clouds, assuming an average earth temperature of ~12C over the last 400,000yrs, which fairly approximates the derivation from Boltzman’s Law, and additionally demonstrates the buffering affect against temperature extremes relative to a near vacuum environment.
A gross over-simplification of a complex system, but it does provide an intuitive handle for those without a scientific/mathematical education.
Aaron Lewis says
Weather models used by forecasters reflect surface sea temperatures that are above historical norms.
Why are the SST warmer now, than in the past? AGW is an (unspoken) assumption of every modern weatherman.
Lynn Vincentnathan says
To add to my previous comment, the Cheshire cat smile the weathermen when they say the temp is “below average” also is a disservice. It reconfirms the skeptics that GW is not happening, since the temp today is below average. (Average of what is another side issue — the last 2 years?)
They could now and then point out that although the temp today is below average, the daily temps go up and down (above and below and just about average), but the overall trend over the years and around the world is increasing temperatures due to GW.
They could now and then take a brief moment to mention aspects of GW. Enquiring minds want to know. Perhaps the local news could get other sponsors, besides oil and car companies. I never ever ever hear weathermen where I live now in S. Texas mention the words “global warming” or “climate change” — but that’s an improvement since a Chicago weatherman (brother of an Enron guy) used to say global warming is NOT happening, at least up until 2002 when we left the area.
Hank Roberts says
I hope Marc will keep working on this explanation from the astronomer’s point of view. Note that the definition of “light”– to an astronomer — includes infrared!
Re #60 —
“Even though the United States has not adopted a mandated reduction in carbon dioxide emmissions, at least a lot of people are still speaking about the issue.”
People are doing more than “speaking”. I’ve been looking at cost savings from energy efficiencies a lot over the past few months (ROI strongly favors just ditching energy wasting products — so strongly I’ve learned to not worry so much about cost, and now save enough that the $400 rechargeable mower I just bought will be paid off in 9 months or less from what I save now on lighting. It’s just insane.), and what I see is a severe shortage of products (a sign of demand outstripping supply), or a major increase in variety (a sign of healthy demand being met by supply). That’s a good sign that people are DOING a lot.
I forget the conversion rate (someone here can supply, I’m sure), but in the last several months I think I’ve avoided something on the order of 2 megawatt-hours of electric consumption. There are still a lot of people who’ve not jumped on the CFL bandwagon yet, but based on what I see in stores, a lot of people have already. Multiply those people by 4 or 5 (or more) megawatt-hours saved per year and you’re talking more than a small reduction in CO2 emitted. If you check the news in regards to declining US car manufacturer market (we make more gas guzzlers than the Japanese), and the increase in Japanese market share, I’d wager there is a shift in national fleet fuel economy as well — and if we could just kill all the SUV owners, we’d be golden.
John Mashey says
re: #68, #69, Nick
I recommend David Strahan’s, “The Last Oil Shock: A Survival Guide to the Imminent Extinction of Petroleum Man” (2007), which can be ordered from Amazon in UK or Canada, but not in US. You can get some of it from http://www.davidstrahan.com/. It misses some issues, but overall is pretty readable.
Given that Hubbert’s Peak for world oil = 2015 (+/- 5 years, depending on reality level of OPEC numbers), it seems unlikely that the amount of air travel is going to keep going up for very long. If it does, it means somebody will be converting a lot of coal to kerosene, not A Good Thing for The Climate. Note that petroleum (down) may not be a good thing if it means (unsequestered) coal (up).
Re: #16, “According to the second law of thermodynamics …”
Oh, NOW I get it. The same people who believe the earth is only a few thousand years old, deny evolution, and don’t believe people cause global warming, also don’t believe the Sun is a giant fusion reactor 93 million miles away. The sun is really a god who mounts his chariot at the gates of dawn every morning, rides across the sky, and then descends into the underworld at night.
This makes the Sun an atmospheric phenomenon, which means it is part of the “planetary system”, which means it is a closed system, which means the 2nd law of TD applies, which mean AGW isn’t possible!
Steve Horstmeyer says
Comments to Lynn Vincentnathan (comments #77 and #83)
I get the feeling your approach would be applauded by the nay-sayers concerning AGW – generalize and lump all TV weathermen into one category – talk about confusing the issue. The nay-sayers do the same with “climate scientists”. Stereotypes will do nothing but come back to haunt anyone who uses them to argue a concept that should be based on fact and specifically cited instances. I have spent too much time in my career educating the public to let you get away with that. At the same time I will not defend American television’s abysmal record when it comes to informing the public.
I know nothing about you Lynn but it sure seems to me that you have done very little in the way of trying to explain complex issues in a clear and general way in a time-constrained format. Basically we must use the K-I-S-S approach (i.e. keep it simple stupid. With the widespread use of the web we can drive the audience to sources of good information so then can learn more.
Am I the only one who is asking why you are surprised that you hear nothing about global warming from TV weathercasts now? You live in south Texas what do you expect in an environment where oil is so important? Call your local TV weathercasters (the gender neutral term I prefer that also does not imply each and every one is educated as a meteorologist) and ask them if they are permitted by management to address the issue of AGW. You may be surprised! I am lucky I have no such constraints.
FYI when a TV weathercaster (meteorologist or other) says the temperature is below average they are using the internationally recognized definition based on a 30 year period of record. That period of record changes every 10 years.
Currently “average” is based on the 30 year period 1971 – 2000 and next it will be based on the 1981 – 2010 thirty year period.
To add a bit of confusion to the mix “average” is different from “climatic normal”. Because 30 values of high temperature for May 4th, for example, are too prone to influence by a single extreme value the data are fit to a mathematical function called a spline which stabilizes the data. A simple example of why this is necessary: If May 4th 1988 had a high temperature 30 degrees below average it would change the 30 year average by 1 degree. It is then possible that May 4th could have a lower average high temperature than May 3rd or even April 30th for the 30 year period. At that time of year the long-term average high temp is rising each day and over a sufficiently long period of record the “average” will not decline but rise steadily. The influence of that single observation would not be so great in a 150 year data base. That single May day with a high temperature 30 degrees below average would have an influence of 30/150 or 0.2 degree. (All temperature references in °F).
It is likely that most TV weathecasters do not know which (average or climatic normal) they are speaking about and a good bet they do not know the difference.
Dick Veldkamp (comment #65)- good suggestions and also nicely pointed out in “An Inconvenient Truth”. Thanks for the suggestion.
Steve Reynolds (comment #61) – great point, my favorite of the short term “solutions” with potential for damage from unintended consequences is seeding the ocean with iron filings to sequester CO2.
Dermod O'Reilly says
The IPCC use the diagram on p4 of this doc. to illustrate their position http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch01.pdf perhaps someone can point out the important points of comparison with the “Six Easy Steps”
Chuck Booth says
Re # 76 “The Sun has a surface temperature of 5770 Kelvin, so most of it’s energy is emitted in the form of visible light”
I’m afraid that is not true – visible light makes up less than 50% of the emitted solar radiation. http://earthobservatory.nasa.gov/Library/SORCE/sorce_02.html
The peak of the solar spectrum is in the visible, and just under half is pretty close to most. (Technical point: how you do your accounting also depends on whether you count photons or energy). I do agree that you should communicate that the Sun emits all sorts of light, and that UV, visible, and IR differ only in energy and wavelength.
In a guide like the original proposed one, you have an overall goal. One very common question that I get from students is about why the greenhouse effect is one way – why does light from the Sun get in and light from the Earth get out? It does help to know that there are predictable rules that explain why sunlight is different from the light emitted by the Earth.
Lawrence Brown says
Bob Bergen’s comment in,#19, that he has college freshmen reading at a tenth grade level and that have little or no math background, and Hank Roberts comment,in #27 That John and Jane Q Public are too distracted or too apathetic to take time to get on speaking terms with climate change, worries me almost as much as AGW. Perhaps we’re not challenging our kids enough in high school. PBS ran a docu drama some years ago about a math teacher in California teaching calculus to high school kids from a poor neighborhoods with remarkable success. The kids even enjoyed it! I hope we don’t have to have a 9/11 equivalent or a Pearl Harbor, to wake people up.
If the North Atlantic Conveyor Belt turns off, or arctic permafrost melts and suddenly releases huge amounts of methane, I don’t think you’ll have to dumb down the conversation to anybody!
Here’s what climate scientists are up against: (I’m glad someone is saying it.)
Gore: Polluters manipulate climate info
By Gillian Wong, Associated Press
SINGAPORE — Research aimed at disputing the scientific consensus on global warming is part of a huge public misinformation campaign funded by some of the world’s largest carbon polluters, former Vice President Al Gore said Tuesday.
“There has been an organized campaign, financed to the tune of about $10 million a year from some of the largest carbon polluters, to create the impression that there is disagreement in the scientific community,” Gore said at a forum in Singapore. “In actuality, there is very little disagreement.”
Gore likened the campaign to the millions of dollars spent by U.S. tobacco companies years ago on creating the appearance of scientific debate on smoking’s harmful effects.
Ferdinand Engelbeen says
Re #73 (comment)
Is there any period in history which can prove that different forcings have the same sensitivity of 0.75 K/W/m2? All I can see is that in most periods, including ice ages/interglacials, there is a huge overlap of forcings, where CO2 in the past was a feedback of temperature changes. And the above sensitivity is based mainly on solar/insolation changes (at least initially). Which isn’t necessarely similar for CO2 or other greenhouse gases.
The overlap of forcings makes the attribution of real sensitivities quite difficult, as we have one dependent variable (temperature) and four more or less independent variables (solar, GHGs, tropospheric and stratospheric aerosols). I know, there are some attribution attempts, but these depend very much on the pre-industrial reconstruction chosen as comparison…
[Response: Effectively yes. The response to volcanic eruptions has been used to determine sensitivity – that is relatively clean, volcanic+solar was used by Crowley (2000) and Hegerl et al (2006), the transient effects of GHGs+ ice sheets was looked at in Hansen’s latest. The PETM CO2+CH4 estimates and response were looked at by in a paper of mine (though you get a reasonable match, that isn’t as good a constraint). All of these are consistent with efficacies close to unity. – gavin]
Don Condliffe says
Re comment number 2 by Mike Alexander one possible explanation for why the early 20th century temperature rise was so big compared to the post-fifties warming is the rebound from cooling imposed by three large volcanic eruptions with a long gap thereafter until 1991:
KRAKATAU Indonesia 1883 Aug 27 VEI 6
SANTA MARIA Guatemala 1902 Oct 24 VEI (probable 6)
NOVARUPTA Alaska Peninsula 1912 Jun 6 VEI 6
PINATUBO Luzon (Philippines) 1991 Jun 15 VEI 6
from Large Holocene Eruptions table by Smithsonian Global volcanism program, http://www.volcano.si.edu/world/largeeruptions.cfm
David B. Benson says
Powering the Planet, by Nathan S. Lewis, is certainly a forceful, sobering read. At least read the last two paragraphs.
and download the first featured pdf file.
J.S. McIntyre says
Seeing as you brought up the environment, I’d like to mention the absolutely marvelous time I am having reading Alan Weisman’s new book “‘The World Without Us” and recommend it accordingly.
I originally read a short excerpt a year or so ago in Discover magazine wherein Weisman described how, as I recall, New York would eventually disappear, beginning to disintegrate almost immediately upon the disappearance of the humans needed to maintain its infrastructure. I picked the book up upon its release expecting more of the same – you know, the crumbling majesty of man’s works, the exposure of the weakness of our technology, the inability of our monuments to actually endure without us, and to a satisfying degree I got what I paid for.
But that is only the icing on the cake. This is a truly clever book, both in structure and execution. It would appear the author has engaged a particularly subversive technique in selling his book (discussion of mankind’s demise and its effect on the planet, something that has a perverse appeal, IMHO, to human psychology) in order to discuss something much more immediate: the fragile environmental state of affairs we find ourselves in at the beginning of the 21st century.
The set-up is delightful: in order to first discuss what the world would be like without us, one first must discuss what the world was like before we arrived and built our civilization, and then examine how the world has changed due to our civilization’s “footprint”. Only then can you have a contextual reference from which to understand the effect of our passing.
Thus, in very clear and accessible language, Weisman details how we have really made a mess of things – and global warming is part of the discussion, on more than one level – without ever appearing to lecture his reader. I would suggest this (seriously) as a gift book for people who are on the fence or just a little over the other side on these issues. I think it one of the best ways to get people to see the damage we are doing to ourselves without engaging in arguments or to pleading with them to “read this and you’ll understand!” knowing all the while they won’t.
Instead, appeal to that perversely morbid curiosity I mentioned, that odd conceit that makes humans wonder what survives past their deaths. The only downside: it can be a disquieting read in the sense of how thoroughly he lays out a case for how badly we have been mucking up our world.
Jerry Steffens says
There really aren’t many points of comparison, as the figure you cited is an estimate of the earth’s CURRENT energy budget and the post concerns CHANGES in that budget.
Re: #41 Where does the CO2 come from?
This chart (World Resources Institute: GHG Emissions Flow Chart) gives a very nice overview of what contributes what to what (as it were).
[Response: Yes! that was the graph I was thinking of. Thanks! – gavin]
Aaron Lewis says
I feel like I am arguing over how many angels can stand on the head of a pin, but I strongly suspect that chart under reports nitrogen oxides.
Most fuel combustion produces some NOx. Then, more are produced by organic free radicals in hot exhaust flows. Some of these compounds have significant greenhouse gas properties.
Thus, while the percentage of NOx produced is small, the size of the underlying flows and their importance as greenhouse gases suggest that NOx from transportation, residential, and commercial buildings should be visible flows on the chart.
Note that auto emissions form a concentrated layer just above a (black asphalt) road surface making a perfect small scale heat trap. The effect of such a heat trap will not be captured in climate models. the emmissons form buildings will be concentrted in urban areas. Does this explain some small part of the urban heat island effect?
[Response: NOx compounds are most important climatically as ozone precursors, rather than greenhouse gases in their own right. See the previous posting… – gavin]