What are the major contributors to the amount of infra-red emitted by the earth?
It depends on the wavelength.



The figure shows the most absorbing species between 6 and 16 µm for a U.S. standard tropical atmosphere (Note: this figure does NOT include the Earth's surface!).  Absorption data is taken from Anderson et. al (1986) and simulations are performed with the open-source Atmospheric Radiative Transfer Simulator (ARTS; Eriksson et al., 2011).  The plotting was carried out with LaTeX and pgfplots.  ARTS is open-source and includes the Anderson et al. data, so you can try different wavelengths for yourself.

Depending on wavelength, we have dominant absorption from H₂O, O₃, CO₂, N₂O, CH₄, and others.

I have also uploaded a PDF version (PDF, 117.68 mm × 228.21 mm, 5.1 MiB) for higher-resolution viewing.