Arguments
How do we know CO2 is causing warming?
Posted on 8 October 2009 by John Cook
We've just perused the empirical evidence that humans are raising atmospheric CO2 levels. In earlier posts, we noted that tallying up the planet's heat content shows that our climate is accumulating heat, proof of global warming. But is there any evidence that links the two? Is there empirical data proving that increased CO2 contributes to the energy imbalance that causes global warming?
The greenhouse gas qualities of CO2 have been known for over a century. In 1861, John Tyndal published laboratory results identifying CO2 as a greenhouse gas that absorbed heat rays (longwave radiation). Since then, the absorptive qualities of CO2 have been more precisely measured and quantified by laboratory results and radiative physics theory (Herzberg 1953, Burch 1962, Burch 1970, etc).
Satellite measurements of the change in outgoing longwave radiation
So according to lab results and radiative physics, we expect that increasing atmospheric CO2 should absorb more longwave radiation as it escapes back out to space. Has this effect been observed? The paper Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997 (Harries 2001) attempts to find out. In 1970, NASA launched the IRIS satellite that measured infrared spectra between 400 cm-1 to 1600 cm-1. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Harries 2001 compared both sets of data to discern any changes in outgoing radiation over the 26 year period. The resultant change in outgoing radiation was as follows:
Figure 1: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001).
What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as CO2 and methane (CH4) absorb energy. The change in outgoing radiation over CO2 bands was consistent with theoretical expectations. Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect".
This result has been confirmed by subsequent papers using the latest satellite data. Griggs 2004 compares the 1970 and 1997 spectra with additional satellite data from the NASA AIRS satellite launched in 2003. Chen 2007 extends this analysis to 2006 using data from the AURA satellite launched in 2004. Both papers found the observed differences in CO2 bands matched the expected changes based on rising CO2 levels. Thus we have empirical evidence that increased CO2 is preventing longwave radiation from escaping out to space.
Measurements of downward longwave radiation
What happens to longwave radiation that gets absorbed by greenhouse gases? The energy heats the atmosphere which in turn re-radiates longwave radiation. This re-radiated energy goes in all directions. Some of it makes its way back to the surface of the earth. Hence we expect to find increasing downward longwave radiation as CO2 levels increase.
Philipona 2004 finds that this is indeed the case - that downward longwave radiation is increasing due to an enhanced greenhouse effect. Evans 2006 takes this analysis further. By analysing high resolution spectral data, the increase in downward radiation can be quantitatively attributed to each of several anthropogenic gases. The results lead the authors to conclude that "this experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming."
So we have multiple lines of empirical evidence for CO2 warming. Lab tests show CO2 absorbing longwave radiation. Satellite measurements confirm that less longwave radiation is escaping to space. Surface measurements detect increased longwave radiation returning back to Earth at wavelengths matching increased CO2 warming. And of course the result of this energy imbalance is the accumulation of heat over the last 40 years.
Acknowledgements: A big thanks must go to AGW Observer. Their lists of papers on changes in outgoing longwave radiation, changes in downward longwave radiation and laboratory measurements of CO2 absorption properties made this post possible.
The number 2 comment by WAG exhibits a classical mistake of logic. The comment points to a correlation between CO2 and temperature. Then the comment asserts that causation is present, unless somebody proves otherwise.
nodeniro - See the link that John Cook gave in his response to that very comment by WAG, pointing to Empirical evidence that humans are causing global warming. We start from the science that indicates CO2 is a powerful greenhouse gas (spectroscopy, atmospheric structure, thermodynamics), and from there correlations are useful for determining exactly how much of an influence CO2 is relative to other forcings, such as solar changes, aerosols, land use, etc. Correlation without causation (common in climate change denier arguments searching for alternatives) is an empty assertion, but correlation with causation is a valid tool - when, as here, it's backed by physics.
nodeniro.
Since WAG is probaly long since moved on - it was 7 years ago, perhaps you would like to open a conversation on a current issue?
Is there any measured evidence that the heat energy retention by CO2 and methane has had any measureable effect on the temperatures shown on the official graphs of yearly global air temperature?
billev - read the main article? If you want a really direct measurement, then this article explains a 2015 method. However, you have to be a disbeliever in Planck's Law if dont think that increasing the radiative flux on a surface will not increase its temperature. Doubling CO2 directly increases surface temperature by about 1.1C - the calculation is pretty straightforward. The difficulty with climate sensitivity determination however is that increasing temperature causes other feedbacks to cut in as well notably decreasing albedo and water vapour. As to teasing out of the various influences on surface temperature, then this is known as attribution studies. The IPCC AR5 summaries the published science on this in Chpter 10.