Another round of the Carbon Cycle or can we move on?

My 8th grade students have supposedly learned the water cycle, the carbon cycle, and the nitrogen cycle.  They’ve completed a lab “How does combustion affect carbon dioxide levels in air” and today they submitted a lab “How do plants affect the nitrate and ammonia levels in a bin of water containing two fish”.   I’m ready to have them undertake a global warming lab that will hopefully help them make the link between increasing CO2 levels, greenhouse gases, and global warming.  However, I wasn’t sure if they have really grasped the concept of the cycles.  In a PowerPoint I put together some formative assessment questions, all multiple-choice and all designed to expose misconceptions.  They answered on their whiteboards.  WOW was that ever interesting and incredibly informative!  3/5 of the students clearly had NO deep understanding. 1/5 was 100% ready to move on and the other 1/5 was 99% ready to move on.   So, it very easily told me that I needed to do an additional, very hands-on activity that I had tucked away “in my back pocket”.  I saw several “light bulbs go on” and at the end of class everyone (including those that had been ready to move on) commented, “That was very helpful”.  One more round of formative assessment on Monday and I'm quite sure we’ll be able to start the lab that day as well.