Carbon dioxide is already absorbing almost all it can – really?

On the list of debunked myths that “skeptics” like to promote in an attempt to downplay the significant role of CO2 as a greenhouse gas is the old “More CO2 has little effect” card; Nova’s no exception with her “Carbon dioxide is already absorbing almost all it can” post. This has been debunked many times over. Interestingly in the early 1900’s it was a valid skeptical argument, but it was rebutted and ever since climate scientists knew better.

That doesn’t stop political bloggers from dragging out old myths. Nova goes on to confuse the issue even further.

Nova suggests that adding more CO2 has little impact on global temperatures because of the logarithmic equation governing the effect of CO2. She’s partly right, the logarithmic part of the equation is to do with CO2’s radiative forcing, not the temperature. To find the impact on the temperature you need to use a climate sensitivity value and that’s where Nova fails.

Nova’s first graph looks like this …

… and whilst the shape of the graph is correct, the values along the Y axis directly depend on which climate sensitivity value you wish to use. In this instance Nova has selected a low climate sensitivity of 0.5°C from the Lindzen & Choi 2009 paper. Unfortunately for Nova, and for Lindzen & Choi, their paper has numerous problems.

More is Less?

Nova titles the graph “As carbon dioxide increases it has less warming effect” which is a little ambiguous (or perhaps deliberately misleading?). Each additional 20 ppm of CO2 will still cause additional warming, just less additional warming than the previous 20 ppm. To work out the total amount of warming you need to combine each of the columns beyond the 280 ppm mark.

How much warming would we really get?

We are warming – the science on that is settled.

Manmade CO2 is contributing to that warming – the science on that is settled. Even Nova has agreed that CO2 produces a direct warming of around 1 degree per doubling.

Where climate science is uncertain is with regards to how much warming we will encounter after feedbacks – this is what “climate sensitivity” is all about. Uncertainty is not cause for celebration however. A number of climate sensitivity studies use various data and methods in order to reduce the amount of uncertainty. Summarised by Knutti and Hegerl 2008, the figure is likely to be between 2 and 4.5°C, with the most likely value being 3°C.

Real scientists examine all of the data to constrain the climate sensitivity figure. Nova wishes to discard all other climate sensitivity studies and instead use the flawed Lindzen & Choi 2009 paper. That gives her a figure of 0.5°C instead of 3°C which better suits her politically-based arguments.

Lower than low.

Not content with that, Nova’s second graph supports the absurd in order to grab the lowest climate sensitivity figure possible. She cites another Exxon-sponsored Idso scientist (recall the Ocean Acidification “science” they performed) for a climate sensitivity figure.

Climate sensitivity includes the planet’s “fast feedbacks” including water vapor, clouds, sea ice, and aerosols. Idso “climate science” from the 1980’s included one method that examines how Mars climate responds to CO2 in order to determine Earth’s climate sensitivity. Mars and Earth, and I probably don’t have to point this out, have very different atmospheres with very different weather and lay at vastly different distances from the sun. Only one has an ocean. This gives you some insight into the “genius” of Idso – more on his studies in a future post. They were rebutted by scientific method and only appear on politically motivated websites since.

As David Archer pointed out,

Archibald’s low-ball estimate of climate change comes not from the modtran model my server ran for him, but from his own low-ball value of the climate sensitivity.

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3 Responses to “Carbon dioxide is already absorbing almost all it can – really?”

  1. richard Says:


    Another important point from the above discussion is that all greenhouse gases are not equally efficient at trapping terrestrial radiation. Consider a greenhouse gas absorbing at 11 mm, in the atmospheric window ( Figure 7-8 ). Injecting such a gas into the atmosphere would decrease the radiation emitted to space at 11 mm (since this radiation would now be emitted by the cold atmosphere rather than by the warm surface). In order to maintain a constant terrestrial blackbody emission integrated over all wavelengths, it would be necessary to increase the emission flux in other regions of the spectrum and thus warm the Earth. Contrast this situation to a greenhouse gas absorbing solely at 15 mm, in the CO2 absorption band ( Figure 7-8 ). At that wavelength the atmospheric column is already opaque ( Figure 7-13 ), and injecting an additional atmospheric absorber has no significant greenhouse effect.

  2. John Smith Says:

    Sorry, but ANY “rebuttal” to Linzen &Choi that comes from “ClimateSkeptic” is laughable on its face, especially given all the pie on John Cooks face after his egregious “Cook at al 2013” 97% concensus paper was utterly destroyed

    Cook, and his site have ZERO credibility.

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