Meehl, G. A., Washington, W. M., Ammann, C. M., Arblaster, J. M., Wigley, T. M. L., & Tebaldi, C. (2004). Combinations of natural and anthropogenic forcings in twentieth-century climate. Journal of Climate, 17(19), 3721-3727.
Stone, D., Allen, M. R., Selten, F., Kliphuis, M., & Stott, P. A. (2007). The detection and attribution of climate change using an ensemble of opportunity. Journal of climate, 20(3), 504-516. Link to paper
Lean, J. L., & Rind, D. H. (2008). How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters, 35(18). Link to PDF
Huber, M., & Knutti, R. (2012). Anthropogenic and natural warming inferred from changes in Earth/'s energy balance. Nature Geoscience, 5(1), 31-36. Link to PDF
In theory, the decrease in atmospheric d13CO2 could come from deforestation instead of fossil fuels, because the the d13C content of biomass (trees) is very similar to fossil fuel (which is very old biomass). However, the amount of fossil fuel burning is much higher than deforestation (now about 10 billion tons C/yr compared to about 1 billion tons C/yr), so we know from the amount that the main reason for the 13C decrease in the atmosphere is fossil fuel burning.
Meehl, G. A., Washington, W. M., Ammann, C. M., Arblaster, J. M., Wigley, T. M. L., & Tebaldi, C. (2004). Combinations of natural and anthropogenic forcings in twentieth-century climate. Journal of Climate, 17(19), 3721-3727.
Stone, D., Allen, M. R., Selten, F., Kliphuis, M., & Stott, P. A. (2007). The detection and attribution of climate change using an ensemble of opportunity. Journal of climate, 20(3), 504-516. Link to paper
Lean, J. L., & Rind, D. H. (2008). How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters, 35(18). Link to PDF
Huber, M., & Knutti, R. (2012). Anthropogenic and natural warming inferred from changes in Earth/'s energy balance. Nature Geoscience, 5(1), 31-36. Link to PDF