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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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CO2 is not the only driver of climate

What the science says...

Select a level... Basic Intermediate

While there are many drivers of climate, CO2 is the most dominant radiative forcing and is increasing faster than any other forcing.

Climate Myth...

CO2 is not the only driver of climate
CO2 is not the only driver of climate. There are a myriad of other radiative forcings that affect the planet's energy imbalance. Volcanoes, solar variations, clouds, methane, aerosols - these all change the way energy enters and/or leaves our climate.

Understanding what drives climate does not occur by a process of elimination. It's happens by a process of integration. There are many influences of climate that all need to be considered together to gain the full picture. The following lists the radiative forcing, loosely defined as the change in net energy flow at the top of the atmosphere, from the various factors that affect climate (IPCC AR4 Section 2.1). Positive radiative forcing has a warming effect (so obviously, negative radiative forcing has a cooling effect).

  • Surface Albedo has changed due to activity such as deforestation. This increases the Earth's albedo - the planet's surface is more reflective. Consequently, more sunlight is reflected directly back into space, giving a cooling effect of -0.2 Wm-2.
  • Ozone affects the climate in two ways. The depletion of stratospheric ozone is estimated to have had a cooling effect of -0.05 Wm-2. Increasing tropospheric ozone has had a warming effect of +0.35 Wm-2.
  • Solar variations affect climate in various ways. The change in incoming Total Solar Irradiance (TSI) has a direct radiative forcing. There is an indirect effect from UV light which modifies the stratosphere. The radiative forcing from solar variations since pre-industrial times is estimated at +0.12 Wm-2. Note that the radiative forcing from solar variations may be amplified by a possible link between galactic cosmic rays and clouds. However, considering the sun has shown a slight cooling trend over the last 30 years, an amplified forcing from solar variations would mean a greater cooling effect on global temperatures during the modern warming trend over the last 35 years.
  • Volcanoes send sulfate aerosols into the stratosphere. These reflect sunlight, cooling the earth. A strong volcanic eruption can have a radiative forcing effect of up to -3 Wm-2. However, the effect of volcanic activity is transitory - over several years, the aerosols wash out of the atmosphere and any long term forcing is removed.
  • Aerosols have two effects on climate. They have a direct cooling effect by reflecting sunlight - this is calculated from observations to be -0.5 Wm-2. They also have an indirect effect by affecting the formation of clouds which in turn affect the Earth's albedo. The trend in cloud cover is one of increasing albedo which means a cooling effect of -0.7 Wm-2.
  • Stratospheric Water Vapour has increased due to oxidation of methane and had a slight warming effect of +0.07 Wm-2.
  • Linear Contrails from aviation have a slight warming effect of +0.01 Wm-2.
  • Nitrous Oxide reached a concentration of 319ppb in 2005. As a greenhouse gas, this contributes warming of  +0.16 Wm-2.
  • Halocarbons (eg - CFC's) were used extensively in refrigeration and other industrial processes before they were found to cause stratospheric ozone depletion. As a greenhouse gas, they cause warming of +0.337 Wm-2.
  • Methane is actually a more potent greenhouse gas than CO2. Pre-industrial methane levels, determined from ice core measurements, were around 715 parts per billion (ppb). Currently methane rates are at 1774 ppb (eg - 1.774 parts per million). The radiative forcing from methane is +0.48 Wm-2.
  • CO2 levels have increased from around 280 parts per million (ppm) in pre-industrial times to 384 ppm in 2009. The radiative forcing from CO2 is +1.66 Wm-2. CO2 forcing is also increasing at a rate greater than any decade since 1750.

Here's a visual summary of the various radiative forcings:

Figure 1: Global mean radiative. Anthropogenic RFs and the natural direct solar RF are shown. (IPCC AR4 Section 2.1)

Putting it all together, Figure 2 compares the warming from human caused greenhouse gases to the total radiative forcing from all human sources.

Figure 2: Probability distribution functions (PDFs) from combining anthropogenic radiative forcings. Three cases are shown: the total of all anthropogenic radiative forcings (block filled red curve); Long-lived greenhouse gases and ozone radiative forcings (dashed red curve); and aerosol direct and cloud albedo radiative forcings (dashed blue curve). Surface albedo, contrails and stratospheric water vapour RFs are included in the total curve but not in the others. Natural radiative forcings (solar and volcanic) are not included in these three PDFs. (IPCC AR4 Figure 2.20b)

Greenhouse gases and ozone contribute warming of +2.9 Wm-2. The majority of this is from CO2 (+1.66 Wm-2). This warming is offset by anthropogenic aerosols, reducing the total human caused warming to 1.6 Wm-2. So the warming from CO2 actually exceeds the final total radiative forcing. The other important point to glean from Figure 2 is that we have a relatively high understanding of greenhouse gas radiative forcing. The probability density function (PDF) shows a much higher probability than the aerosols PDF, meaning the uncertainty associated with greenhouse gas forcing is much lower. This is also confirmed by experimental observations from both satellites and surface measurements which confirm the enhanced greenhouse effect from rising greenhouse gases.

So in summary, there are two reasons for the focus on CO2:


  • CO2 is the most dominant radiative forcing
  • CO2 radiative forcing is increasing faster than any other forcing

Intermediate rebuttal written by John Cook

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 15 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 18:

  1. cleanwater,
    you forgot to mention Ångström! It's a pitty. Apart from your random quotes and some standard shouting, you're welcome to discuss science, if you'll ever will.

    much too easy to copy and paste to leave garbage around ;)
  2. A great addition to this page would be a link to the page with the evolution of relative forcings over the past century or so. I just can't find that page at the moment.
    Response: Good idea. I may reshape the content to include the graph you're talking about but in the meantime, here it is:

    Separate global climate forcings relative to their 1880 values (GISS).
  3. Hot off the press from the Journal of Climate. Cloud data supports the higher end of the estimates of climate sensitivity :-/
  4. In the rediative forcing graph it shows "well-mixed greenhouse gases".  Is there a breakout of CO2, H2O, methane etc to show the relative importanct to the effect?

    and a related question, when we burn a fuel we get CO2 and H2O in a 3:4 ratio (typical). Since H2O is a more potent green house gas why all the hupla on CO2 and not the incresed H2O?


    [TD] Water vapor's concentration in the atmosphere is limited by temperature.  Putting more water vapor into the air than the air's temperature will support causes the excess water to drop out in about 10 days.  Therefore water vapor is a feedback to temperature increase, not a forcing.  See the rebuttal to the myth Water Vapor is the Most Powerful Greenhouse Gas.

  5. roosaw...  Here's the RF chart from IPCC AR5 that has them broken out...

  6. Reply to likeithot from here.

    Thank you for responding. Now what climate science actually states is that climate will respond to the net effect of all forcings. A huge amount of climate science also goes into understanding the internal variability that is inevitable when you unevenly heat an ocean-covered planet. Unfortunately, important processes (especially ENSO) for determining surface temperature defy predictive modelling. So, to quote the modellers - "climate models have no skill at decadal level prediction".

    Given these constraints, and the multiple forcings at work in climate, what then do you think the data should like that would convince you that the attribution to CO2 is accurate?

    If you are stumped, then perhaps you should read the IPCC WG1 chapter on attribution to see the approaches that have been done so far.

    One very important consideration to think about is that while surface temperature has a very high degree of variability, you do expect total ocean heat content to vary a great deal less in response to a constant forcing.

    And as an aside, if you dont want to have your comment moderated, then try reading and complying with the comments policy. If you want to bluster with uninformed rhetoric, then there plenty of sites on the internet that will welcome your comments. If you want to discuss the science, then welcome, and please study what the science says so we can have an informed discussion.

  7. likeithot wrote: "...for me to "believe" in AGW there would have to be a clear correlation between the beginning of human CO2 emissions and evidence of warming."

    Um, there is a clear correlation between the beginning of human CO2 emissions and evidence of warming... so long as you are looking at the full picture.

    If you look at the five minutes after the first coal power plant went online, no you won't see any correlation. Nor is it clear for the first decade or two. However, look at CO2 levels and temperature levels for the first hundred years since the industrial revolution and there is a very clear correlation. Both have gone nearly straight up at rates faster than anything seen previously in century level resolution proxy data.

    Thus, this argument amounts to, 'I will cherry pick a time frame too short to see the correlation and then pretend it is not happening'.

  8. Greetings - I'm new to this webpage / forum, which I find fascinating. The brief bio's on the Skeptical Science Team is as impressive as it is varied. I realize this is an old thread, but it looks like a good place to start.

    Let me say at the outset I agree the climate is changing - warming in most areas of the globe - with a net temperature increase. That is obvious in the data, of which I spent a 40 year career collecting, analyzing and processing. Living in the plains, the changes are apparent - latter first snows, earlier spring, increases in non-native insects, plant and human diseases; the list goes on.

    While I have my own understanding and response to Change skeptics, I've learned to consider all the facts. Science is after all based in question,  discovery, hypothesis, observation, proof, testing proof. Therefore I have two questions.

    1) Water Vapor and it's impact on the changing climate. Per the older NASA article from November 2008

    and others since, it seems H2O is somewhat downplayed. What is the Teams sense on H2O impacts on climate change?

    2) Science by concensus (Bill Nye's famous videos). Other disciplines outside of meteorology have differing views on the changing climate. By education I was a Soil Science major. In my 'weather' career I've seen this lack of cross discipline discussion result in all manner of issues. Again, what is the Teams sense on "Science by concensus" versus the time tested steps of the Scientific Method?

    Thank you. I look forward to your response.

  9. While some threads are dated, no threads are dead here.

    Water vapor is best addressed on this SkS thread here:

    Explaining how the water vapor greenhouse effect works

    I have placed a followup to your question there.  Any questions you may have after reading the post and my response should be placed there.

    Many other threads deal with consensus.  The Search function will find many; pick one and re-ask your question there.

  10. ClimateTool - Regarding consensus, I would suggest following Daniels excellent suggestion, and looking at threads using the Search box on the upper left.

  11. ClimateTool, in addition to the posts pointing you to other sections of the site, short answers to your questions are;

    1: The amount of water vapor the atmosphere can hold is directly tied to atmospheric temperature (e.g. "relative humidity" is the amount of water vapor in the air relative to the maximum amount possible for the current temperature). Thus, temperature increases from CO2 will lead to more water vapor... and corresponding additional warming. Thus, water vapor is a significant positive feedback in global warming, but cannot 'initiate' the warming itself.

    2: Your question itself contains a fallacy. There is no conflict between, "'Science by consensus' versus the time tested steps of the Scientific Method". There is concensus on AGW because application of the scientific method over the course of the past 100+ years (since Arrhenius proposed the AGW theory in 1896) has overwhelmingly shown it to be accurate. Nor is there any "lack of cross discipline discussion". Oceanographers, botanists, zoologists, astronomers (studying atmospheres on other planets), and scientists in various other fields have all independently found evidence matching what climatologists have shown.

  12. ClimateTool, I'm going to expand a little on CBDunkerson's second point.  The lack of cross-discipline discussion is a complaint I often hear from people in genuine doubt and from fake skeptics.  The complaint is a sure sign that the writer or speaker hasn't engaged the science in any meaningful way.  After all, if one wants to know what the relevant science says, but doesn't want to wade through tens of thousands of publications, all one has to do is go to a summary of the existing relevant science.  

    That's what the IPCC AR5 is.  It's a summary of the relevant science.  It was written by ~850 scientists, regular publishers in their disciplines, who were not paid for their work.  It directly references 20,000+ publications, and tens of thousands more by proxy.  It went through multiple rounds of peer review that generated 150,000+ comments.

    You complain that little cross-discipline discussion takes place, but the second section of AR5 (Working Group 2: Impacts, Adaptation and Vulnerability) is nothing but cross-discipline collaboration to summarize thousands of interdisciplinary studies.  I don't mean to suggest that the other WGs are not interdisciplinary, for they are, but WG2 is obviously so.

    And (speaking to your other complaint about consensus) the IPCC assessment reports represent the consensus of evidence.  If you are serious about understanding the problem, the well-organized AR5 is the place to start.  SkS welcomes both questions and informed discussion.

  13. ClimateTool - One side note: your use of the capitalization in "...what is the Teams sense..." is often a shibboleth of pseudo-skeptics; tied to conspiracy theories that have a 'Team' of scientists somehow organized to deceive everyone else, usually for some vague sort of world domination. 

    Which is silly - organizing scientists across multiple fields, countries, and decades to argue against the facts would be like herding ADHD cats. Just not possible.

    If you are actually interested in the topic of climate change, and (particularly for this site) various myths about it, I would suggest avoiding terminology with implied insults.   

  14. "..organizing scientists across multiple fields, countries, and decades to argue against the facts would be like herding ADHD cats."

    I would add "...away from a field of open tuna cans."  Such an effort would be asking those scientifickyADHD cats to sacrifice their own professional self interest.

    Why would they not chose to make a name for them selves by picking low hanging fruit and bucking an obviously false paradigm?  Makes no sense.

  15. KR

    My apologies - I meant no such thing. My goal was to ask for the concensus of the team members as listed in the "About" section. I shall avoid such faux pas in the future.

    I do argue however that organizing various disciplines does occur on a regular basis. Having been involved in both operational and field level research I have been fortunate enough to work with geologists, physicists and agronomists on Climate Change issues.

    Perhaps my naivete is showing through. But I stand by my ascertion.

    Thank you for your responses.


    [DB] "concensus of the team members"

    While consensus is important, what really matters is what the evidence itself shows. Consensus of opinion does not replace evidence and physics.

    If you have questions on the evidence and the physics of the science, please use the Search tool to find the most appropriate thread on which to place them (after reading it and the comment thread underneath the article).  Thanks!

  16. ClimateTool - Ah, that makes sense; perhaps asking what the "SkS team" is thinking would be a less ambiguous terminology.

    However, I'm a bit confused as to what assertion you are standing by? You've just asked a couple of questions...

  17. Climate Tool:

    As was pointed out upthread, the IPCC reports, and various others, are compendia of cross discipline results.  Your suggestion that cross discipline analysis is not done in AGW is simply false.  Please cite a major summary report that does not contain results from multiple disciplines.

    The SkS team has a variety of opinions, often different.  You should be more specific in your question.  About water vapor specificly, which appears to be your question, without asking I dare say everyone agrees H2O is an important greenhouse gas.  It contributes a large fraction, or a majority of the greenhouse effect.  Everyone knows this.  

    Water vapor concentration is dependent on the CO2 concentration.  CO2 is not dependent on water vapor.  This means CO2 is the temperature control knob.  Future temperature changes (and water vapor changes) will follow the CO2 changes.

  18. Just to simplify a little here...

    CO2 is a long-lived, non-condensing greenhouse gas that is well mixed from pole-to-pole and through the full column of the atmosphere.

    H2O is a short-lived, condensing greenhouse gas that freezes out at higher latitudes and altitudes.

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