Here we present observationally based evidence of clear-sky CO2 surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric CO2. The time series of this forcing at the two locationsthe Southern Great Plains and the North Slope of Alaskaare derived from Atmospheric Emitted Radiance Interferometer spectra together with ancillary measurements and thoroughly corroborated radiative transfer calculations. The time series both show statistically significant trends of 0.2 W m−2 per decade (with respective uncertainties of ħ0.06 W m−2 per decade and ħ0.07 W m−2 per decade) and have seasonal ranges of 0.10.2 W m−2. This is approximately ten per cent of the trend in downwelling longwave radiation. These results confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions, and provide empirical evidence of how rising CO2 levels, mediated by temporal variations due to photosynthesis and respiration, are affecting the surface energy balance.
The measured spectrum in Fig. 1a shows Planck function behaviour near the centre of the fundamental (ν2) CO2 band and exhibits a departure from a Planck curve in the P- and R-branches of this feature, indicating that the emission in these branches is sub-saturated and could increase with increasing CO2. Water-vapour features, continuum emission, and O3 emission are seen in the infrared window between 800 cm−1 and 1,200 cm−1, and lesser features from CH4 are seen around 1,300 cm−1. Calculated transmission and the change in transmission with a 22 ppm CO2 increase are also shown, indicating that weak vibration-rotation features in the far wings of the fundamental and in the infrared window dominate surface radiative forcing from rising CO2.
We can exclude alternative explanations for the change in these measurements, such as instrument calibration or the temperature, water vapour, or condensate structure of the atmosphere because they would produce significant (P < 0.003) trends in other spectral regions outside the CO2 absorption bandssee Fig. 2b and e. Moreover, the spectral forcing from CO2 is a strong function of changes in the CO2 column concentration, and nonlinear interactions between temperature and water vapour were weak, as indicated by the lack of statistically significant differences in the seasonal and annual spectral trends in the CO2 P- and R-branches. Therefore, the atmospheric structure of temperature and water vapour does not strongly affect CO2 surface forcing, which is consistent with the findings of others.