So last time I ended with the question of how to present complicated data to 9th graders. Global climate change involves extremely large amounts of complex data. The literature on past climate change is vast, but single or simple conclusions are absent. From what I can tell, climate change poses an ill-structured problem, the kind educational researchers tell us are important for students to be exposed to. Real world problems are complex and often lack a single answer or one correct approach.
Climate change in the past seems to have been caused by a combination of orbital variations of Earth (Milankovitch cycles) internal forcing by greenhouse gases (including water vapor, CO2, methane, ozone, etc.), and various feedback mechanisms. Scientists disagree about the relative importance of each. It is clear from ice cores and other records that CO2 and methane vary along with global temperature, but a clear cause and effect relationship is absent, despite what Mr. Gore might tell us. We are also led to believe that CO2 levels are off the chart compared to Earth's past. This is not quite correct. Current levels of CO2 (around 385 parts per million) are certainly the highest in the last 650,000 years, and while this might seem like a long time, it is very brief compared to the 4.6 billion year history of planet Earth. There is evidence that CO2 levels have been up to 15 times higher than current levels around 400 million years ago and at least 5 times the current levels more recently (geologically speaking). Of course, people weren't around back then, and there were no coastal cities, but still, sometimes Mr. Gore makes it sound like planet Earth itself couldn't survive. This is of course nonsense; as recently as 60 million years ago Earth likely had average temperatures 15 degrees Celsius higher than current averages. And Earth survived, as did its plants and animals. There have been times where there haven't been any glaciers anywhere, and as recently as 120,000 years ago sea levels were 4 to 5 meters higher than current levels.
So if you hear that current warming is unprecedented, that is simply false. Point this out to 9th graders, though, and you see some of them glaze over and start to dismiss the possibility of anthropogenic climate change. The problem is that Mr. Gore and others try to use past climate change as a simple analog to explain what is happening now. This doesn't work for several reasons; as mentioned above, past climate change involves dozens of factors that are not fully understood by any climate scientist. The current scenario is actually new in the history of the world.
Next: uncharted territory