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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:

  1. CO2 is the most dominant radiative forcing
  2. CO2 radiative forcing is increasing faster than any other forcing

Last updated on 26 June 2010 by John Cook.

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Comments 1 to 7:

  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'.

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