For a nice review of orbital forcing you can also watch this series of videos from the Khan Academy.
NASA has a great page explaining the orbital parameters. See NASA Milankovitch.
These suggested articles from the IPCC report of 2013 (WG1 AR5) will give you a more in-depth background on the topics covered in the video in step 1.4:
(Chapter 8 also gives an introduction of anthropogenic as well as natural radiative forcing in general, parts of which will be covered in video in step 1.9 of this week.)