that is actually a number derived from scientific studies that calculated those probabilities based on a meta-analysis of the available data. I quoted the studies in this old post of mine.
To be precise, it's >99% for human causing and 99.999% for greenhouse gases causing climate change.

https://link.springer.com/article/10.1007%2Fs00382-014-2128-2
and
http://www.sciencedirect.com/science/article/pii/S2212096314000163?via%3Dihub# - in the full text.