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What percentage of global warming is due to human causes vs. natural causes?

By Joe Romm  

"What percentage of global warming is due to human causes vs. natural causes?"


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Why increasing CO2 is a significant problem — in six easy steps

NASA scientist Gavin Schmidt continues to do terrific blogging at RealClimate explaining climate science and the immateriality of the illegally hacked e-mails to our broad understanding of human-caused global warming.  That, of course, is why the anti-scientific ideologues are going after him so hard (see “Competitive Enterprise Institute to sue RealClimate blogger over moderation policy“).

He was asked recently on RC, “what percentage of global warming is due to human causes vs. natural causes?”  I’m posting his reply here because it’s a good answer and frankly much clearer than the one science advisor John Holdren gave at today’s House hearing to an almost identical question — though I thought Holdren and Lubchenco were both terrific, especially in their opening statements, and I hope to get those videos up as soon as they are available.  Schmidt explained:

Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been … is caused by a combination of human drivers and some degree of internal variability. I would judge the maximum amplitude of the internal variability to be roughly 0.1 deg C over that time period, and so given the warming of ~0.5 deg C, I’d say somewhere between 80 to 120% of the warming. Slightly larger range if you want a large range for the internal stuff.

Schmidt has a new post today on the emails, in which he urges people to read a 2007 post that provides  “an easy-to-understand explanation for why increasing CO2 is a significant problem without relying on climate models,” which I am reposting below:

The CO2 problem in 6 easy steps

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 [tex]\sigma T^4[/tex] (~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.


And let me add Step 7On our current emissions path, we’re going to blow past 550 ppm, a doubling of CO2 (See U.S. media largely ignores latest warning from climate scientists: “Recent observations confirm “¦ the worst-case IPCC scenario trajectories (or even worse) are being realised” “” 1000 ppm and M.I.T. doubles its 2095 warming projection to 10°F “” with 866 ppm and Arctic warming of 20°F).

I will come back to Step 5 in a later post — for now, see “Another “Must Read” from Hansen: ‘Long-term’ climate sensitivity of 6°C for doubled CO2.”

UPDATE:  If you want to learn the basics of global warming science, click here for a bunch of lectures “for non-science majors” by a real expert.

UPDATE:  If you want a very good and uber-credible written primer on the science, I would suggest, “Understanding and Responding to Climate Change:  Highlights of National Academies Reports 2008.”

Related Posts:


‹ Nature editorial: “Nothing in the e-mails undermines the scientific case that global warming is real ” or that human activities are almost certainly the cause.”

Science advisor John Holdren on the hacked emails and the state of climate science ›

16 Responses to What percentage of global warming is due to human causes vs. natural causes?

  1. Thomas says:

    I think the posts are being munched…

  2. Thomas says:

    well, that got through, try again -

    Thanks for the post Joe, a good read

    Another FYI ’cause I didn’t see a link here (might have just missed it)

    – Professor Archer over at realclimate has posted his class lectures for his “Global Warming” class (a class for the non – science major types) that looks rather informative – those of you who, like me, are trying to get a better understanding of it all might want to take a look. they are downloadable videos.

    Still downloading & just watched part of the intro very quick (real life beckons…), pretty cool though & I’m looking forward to watching them all.

    [JR: Added it.]

  3. This is an extremely useful summation to forward to people with at least a little scientific background. And it’s always good to add to #6, we know it’s a “significant” number because significant things are now happening as a result. The real world confirms the theory all too nicely–that’s why it looks markedly different viewed from a satellite than it did just a few years ago.

    [JR: Thanks. I added a link to another primer.]

  4. Jeff Green says:


    I have been taking notes like I’m back at college again. It is a general ed class and takes a little less work for the math.

  5. Fred says:

    Climate has been changing for billions of years, and will continue to do so for billions more, at least until the sun goes supernova and consumes it. Nothing humans has done or will do, has or will have any notable effect on this

    Some species will adapt(evolve) to survive, some will not. Get over it.

    We don’t need to waste billions of dollars trying to change it, and taxing the hell out of currently viable energy sources and ruining the economy is a bad strategy.

  6. richard pauli says:

    Joe, Thanks so much for keeping an eye out for the John Holdren video…. I want to see it, Holdren is both very scientifically direct and politically careful.

    I really do not want to suspect that there is a political reason that the content is not available.

  7. Thomas says:


    I understand your frustration over the expense – I have some issues with such myself.

    However, I think if you are honestly thinking anything is going to go back to “the good ‘ol days” you need to do some research on the other underlying realities of our energy reserves. “Drill baby drill” isn’t going to work – there are much larger issues at play then that “plan” the slogan is based on would solve. Study, learn, you may find that regardless of if you agree 100% or not at all with AGW the need to change our energy usage is there – regardless of what we all wish.

    Perhaps my more in-depth understanding of some of the other critical issues makes me less resistant to understanding this aspect of it. I hope I wouldn’t have to explain why pollution is bad regardless of outlook, but I would advise spending some time over at theoildrum web site (they are neither “pro” nor “con” oil based resources – they are studying supply limits and some other “inconvenient truths” that are being buried in reports – mostly in regards to natural oil\gas but also in regards to other mined resources) to start coming to terms with why the above slogan is a farce, and why one way or another change is coming in the very near and foreseeable future (unless you missed the how-comes of that dramatic price spike prior to the collapse, and don’t understand why even in the middle of a world-wide recession the price is still rather steep…). We can accept that we need to adjust and be proactive, or we can hang out in blissful ignorance and scream bloody murder when reality finally hits us.

  8. andrew adams says:


    Climate has been changing for billions of years, and will continue to do so for billions more, at least until the sun goes supernova and consumes it. Nothing humans has done or will do, has or will have any notable effect on this

    But the steps 1-6 above explain in detail the mechanism whereby humans are effecting the climate, so I wonder what specific objection you have to this explanation.

  9. PurpleOzone says:

    Fred’s second point, “ruining the economy”, is also invalid. Much of our reduction in emissions can be accomplished by efficiency, which helps the economy. Green jobs here at home to reduce importing energy from abroad is a win-win. Energy gets more expensive as world demand increases and the world oil supply shrinks.

  10. Jack Kroogman says:

    You quote Gavin Schmidt. He is employed by NASA GISS and not Real Climate. Is there confusion?

    [JR: I don't think so. I've spelled that out in a post that this one links to. I used a slightly ambiguous word that I have changed.]

  11. Craig says:

    Regarding this statement:

    ‘Schmidt has a new post … that provides “an easy-to-understand explanation for why increasing CO2 is a significant problem without relying on climate models.’

    But point #4 begin with this statement:

    “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.”

    So he seems to be saying that a key premise for his simple explanation (e.g., any perturbation to the TOA radiation budget is a pretty good predictor of surface temperature change) is something that is derived from climate models. So how is this simple explanation then independent of climate models?

    I’ll also note that he makes reference to a lot of results from radiative transfer calculations. Isn’t that also modeling? It may not be a full blown GCM, but it is an approximate mathematical representation of the interaction of radiation with the atmosphere. Most scientists and engineers would consider that a model, one that related to climate.

    [JR: Gavin was asked this same question -- "Could you rewrite 4 to leave out models? That point jumps right into models, and the conclusion relies on it." He responded:

    "The conclusion relies on the concept of 'radiative forcing', it doesn't rely on any GCM modelling. The linked post is the simplest explanation of the greenhouse effect that you can write down mathematically, and that is definitely not a GCM. The fundamental point is that if you put more energy into a system, then it will warm up. -gavin."]

  12. Sable says:

    Fred says: “Some species will adapt(evolve) to survive, some will not. Get over it.”

    If you are young enough, are you okay with maybe not being one of the survivors? How about your family? What makes you think that the civilization which nurtures and protects you is invulnerable? Sure, “life goes on”, but with BAU it’s looking ever more probable that it won’t be anything most people would call good.

    As individuals we all have to do things we may dislike, but they must be done, because the alternative is not acceptable. I’m sure you can think of examples in your own life. It doesn’t have to be dealing with an existential threat, it could be brushing your teeth!

    We are called upon to make a hard choice, so our children have a reasonable chance at a secure future, and our civilization can continue in some more or less stable form. Types of work and ways of life can change, how’s that for adaption? We may have to choose to do with less for a time, like less energy usage for one thing. If it came to it, who wouldn’t choose clean water and food over owning a car, or having a jet vacation? All this stuff we have came on credit – an environmental credit. We failed to read or understand the fine print, and now the bill collector is at the door.

  13. David B. Benson says:

    Here is ashorter take, due to Jim Galasyn:
    Fundamentally, climate science is based on well-understood principles of thermodynamics. Before humans burned the sequestered carbon (fossil fuels) and released CO2, Earth was in radiative near-equilibrium with space. Humans introduced a sudden, 500-gigaton excursion in the global carbon budget. Because CO2 is a “heat-trapping gas”, Earth is now in disequilibrium with space. To return to equilibrium, the atmosphere must warm.

    The rest is details. Interesting details, to be sure, but the basic thermodynamics have been understood since Svante Arrhenius published in 1896.

  14. Thomas says:


    I’ve been watching Prof. Archer’s video’s of his lectures – the multiple chalk boards and section on light waves brought back some memories of my college astronomy class and working out the equations for red and blue shifts (26 years ago now??!!)…

    Very much like being “back in college” – only this time I don’t have to stressed out about tests :)

  15. Personally, I have never understood the battle over the human impact on climate change. The way I see it. Given that we are quite obviously moving into a new climatic balance that can be highly destructive, I am delighted that scientists have determined that we are the cause. That at least gives us a chance to try to avoid the worst. If the cause were something outside of our control I would feel much more helpless!

  16. Jean says:

    I have 2 friends in the small antinuclear community in Oklahoma..They know a lot about NUKES..Now we got around to global warming.The 2 mechanisms of global warming:
    > direct production of heat by human industry (nukes, hydrocarbons)
    > production and release of greenhouse gases
    > In fact these 2 are also compatible with solar cycles causing
    > heating/cooling. They insist that water vapor is the greenhouse gas causing global warming..What do I tell them? Should I read up on the water cycle?? Does anyone else think that power plants warm things up?? I guess I do not know what they are saying..These guys have their lives practically dedicated to ending nuclear energy and I cannot figure out what is wrong with them??? Also I am one of the few people in the entire state who says publicly live as we know it is heading for trouble.I am known as an”Alarmist” apparentlya very bad thing..Thank……………..