[This paper](https://www.osapublishing.org/ao/fulltext.cfm?uri=ao-10-6-1376&id=73132) shows that there is a dip in the CO2 absorption spectrum of Earth. In essence the trough of the absorption of CO2 for Earth is cut into two separate troughs instead of 1 large trough. Why is that?

[![Thermal emission spectra recorded by IRIS-D on Nimbus 4. The apodized spectra have a spectral resolution between 2.8 cm−1 and 3 cm−1. A hot desert case, an intermediate case over water, and an extremely cold spectrum recorded over the Antarctic are shown. Radiances of blackbodies at several temperatures are superimposed.](https://i.stack.imgur.com/DUByG.png)](https://i.stack.imgur.com/DUByG.png)

[original without arrow](https://i.stack.imgur.com/VDC9g.jpg)

As there have been some comments on whether this is planet related or molecule related, here is an additional image which shows the absence of the double trough in the CO2 absorption pattern for Venus whilst showing it for Earth:
[![The mid-IR spectrum of the Earth, Venus and Mars at a low resolution (spectra are derived from a variety of published models including Meadows and Crisp, 1996; Tinetti et al., 2005; Tinetti et al., 2006; Kaltenegger et al., 2007; Selsis et al., 2007b).](https://i.stack.imgur.com/pUvWd.jpg)](https://i.stack.imgur.com/pUvWd.jpg)