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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.

What the science says...

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence. Satellite measurements of infrared spectra over the past 40 years observe less energy escaping to space at the wavelengths associated with CO2. Surface

The greenhouse gas qualities of carbon dioxide have been known for over a century. In 1861, John Tyndal published laboratory results identifying carbon dioxide as a greenhouse gas that absorbed heat rays (longwave radiation). Since then, the absorptive qualities of carbon dioxide have been more precisely quantified by decades of laboratory measurements (Herzberg 1953, Burch 1962, Burch 1970, etc).

The greenhouse effect occurs because greenhouse gases let sunlight (shortwave radiation) pass through the atmosphere. The earth absorbs sunlight, warms then reradiates heat (infrared or longwave radiation). The outgoing longwave radiation is absorbed by greenhouse gases in the atmosphere. This heats the atmosphere which in turn re-radiates longwave radiation in all directions. Some of it makes its way back to the surface of the earth. So with more carbon dioxide in the atmosphere, we expect to see less longwave radiation escaping to space at the wavelengths that carbon dioxide absorb. We also expect to see more infrared radiation returning back to Earth at these same wavelengths.

Satellite measurements of outgoing longwave radiation

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. Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period (Harries 2001). 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 carbon dioxide (CO₂) and methane (CH₄) absorb energy. The change in outgoing radiation is 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 more recent satellite data. The 1970 and 1997 spectra were compared with additional satellite data from the NASA AIRS satellite launched in 2003 (Griggs 2004). This analysis was extended to 2006 using data from the AURA satellite launched in 2004 (Chen 2007). Both papers found the observed differences in CO2 bands matching the expected changes from rising carbon dioxide levels. Thus we have empirical evidence that increased CO2 is causing an enhanced greenhouse effect.

Surface measurements of downward longwave radiation

A compilation of surface measurements of downward longwave radiation from 1973 to 2008 find an increasing trend of more longwave radiation returning to earth, attributed to increases in air temperature, humidity and atmospheric carbon dioxide (Wang 2009). More regional studies such as an examination of downward longwave radiation over the central Alps find that downward longwave radiation is increasing due to an enhanced greenhouse effect (Philipona 2004).

Taking this a step further, an analysis of high resolution spectral data allows scientists to quantitatively attribute the increase in downward radiation to each of several greenhouse gases (Evans 2006). 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."

Figure 2: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

There are multiple lines of empirical evidence that increasing carbon dioxide causes an enhanced greenhouse effect. Laboratory tests show carbon dioxide absorbs longwave radiation. Satellite measurements confirm less longwave radiation is escaping to space at carbon dioxide absorptive wavelengths. Surface measurements find more longwave radiation returning back to Earth at these same wavelengths. The result of this energy imbalance is the accumulation of heat over the last 40 years.

Updated on 2010-07-20 by John Cook.

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What the science says...

Satellite measurements of outgoing longwave radiation

Surface measurements of downward longwave radiation