NASA offers a great collection of resources for teaching real math topics embedded in real science. I was skimming through their archive of materials today when I came across these interesting lessons:
Did you know that the length of an Earth day changes over time, meaning that the ratio between Earth's revolution and its rotation around the sun is not constant? That is so cool!
Also, based on the idea of cell (waste) equilibrium, we can estimate cell sizes.
Here is another nice tie-in to geometric proportionality: Estimating rate of glacial retreat via photos.
They, in fact, offer an entire book of well-organized "Earth math" for free to help educators with teaching sustainability math. A lot of the concepts can be tied easily to middle-school math.