A question at the question place regarded this history of climate change science. See that link for the full question. Here's my response, which over-ran the blog comment length limit there.
On the historical link side, the best single source is Spencer Weart's _The Discovery of Global Warming_ http://www.aip.org/history/climate/index.htm I think he under-rates the significance of G. S. Callendar's work in the 1930s-50s, but that's my take and I haven't yet written it up formally. It's an idea I've had, to do so. Unfortunately, a lot of Weart's work will probably pass your students' level. But you should be fine with it yourself and translate suitably to your students.
A different matter for your middle schoolers is the time scale of the history. Perhaps combine this with a project to collect family history?
I'll take your students as being about 11. Let's go with 30 years per generation, which is, at least, fairly accurate for my family. That means your kids born in about 2000, parents about 1970, grandparents in 1940, great-grandparents in 1910, great-great grandparents in 1880, great-great-great grandparents (g3 grandparents) in 1850, and g4 grandparents in 1820.
The history is:
When your students' g4 grandparents were pre-school age, Fourier realized and documented that the greenhouse effect (the name for it now, the name didn't exist then) existed. That the earth was warmer than it would be if the atmosphere didn't do anything to retain heat to the earth's surface. 1824-1827.
When your students' g3 grandparents were middle school age, Tyndall showed that water vapor, carbon dioxide, and some other gases, were greenhouse gases. 1859-1861.
When their great-great grandparents were about to graduate from high school, Arrhenius showed that doubling the atmospheric CO2 levels could lead to about a 4 C warming, and that the warming would be stronger towards the poles than the equator. Both are still considered reasonable predictions, and the latter is now well-observed. 1896.
Around the time their great-grandparents were being born, Angstrom made his error regarding saturation. The error was to think that because the band centers saturated, that increased CO2 and water vapor could not increase the earth's temperature. This was wrong because he ignored the fact that there is more than band centers involved. The off-center parts can give continued warming, even while the centers are saturated.
During their grandparents' early lives (and somewhat before), Callendar was at work, documenting that CO2 levels had risen significantly (he was right, but the objection to his work was also correct -- this is one of the things I want to document properly), that the rise was from human activity, and that meaningful-to-humans climate change can be expected to result. (1938 and following).
As your students' grandparents were graduating from high school and starting to work, Ralph Keeling started observing atmospheric CO2 levels on a regular and reliable basis. Plass conducted his acceptable-to-modern-scientists computation of CO2-induced climate changes. The international geophysical year was held and first earth-observing satellites were launched. (1956-1958)
When your children's parents were younger than your children, the earth experienced its last month that was colder than the 20th century average (in 1976). Your children have never experienced a year when the global mean temperature was colder than the 20th century average. The younger parents never have either.
1988 was something of a watershed year in climate policy, if not the science. 1987-8 was a particularly hot year in the US, and a major drought across the US corn belt (for that time, these days it would be unremarkable, or even a rather cool year). In 1988, Jim Hansen testified to Congress about climate change. One part of that even was that the Whitehouse edited his testimony about the science. The second being that Hansen was predicting that the signature of human-induced climate change would soon come out of the noise. I thought he was ahead of the evidence at the time. He turned out to be correct. But the precedent of politicians trying to (and often being successful) in editing what science was presented was set.
Since 1988, there has (I would say) been little change in the public knowledge of the science in the US on climate change. Your middle schoolers don't really hear much more than their parents did. I have some late 80's, early 90's general press articles on climate, and they could all still be published as-is today.
Another thing I'll point out from the history: The existence of a greenhouse effect was shown long before we knew what gases were greenhouse gases. We knew what gases were greenhouse gases long before we knew that humans could significantly change climate by changing greenhouse gas levels. And (not mentioned above) at the time it was discovered, it was considered (Arrhenius) a beneficial thing. He lived in a very cold climate.
16 minutes ago