70 degrees is not too hot and it's completely normal for a working GPU. Something beyond 85 degrees would start getting concerning. Anyways, you may set the tile size to a low value (16 x 16 for instance) to get what you want. Here you can see a table that shows how different Tile sizes can affect rendering time when using a GPU.
However, from a more environmental perspective ( :) ) I don't recommend you doing this since you will have to leave your computer on for a much longer time to get the final rendering. In the meanwhile, your computer is using way more electricity than it would if you use the GPU efficiently by setting the tile size to a higher number (e.g. 224 x 224). Normally, a mid-range GPU uses around 200 watt-hours or less working with the highest load. When idle, it uses around 30-50 watt-hours. The rest of the hardware for your computer also use at least around 70-100 watt-hours in idle and maybe around 200-300 watt-hours when they are under descent load. Assuming that you don't do anything else overnight (8 hours) and just leave your computer on for rendering your computer will be using something around 8 x 200 = 1600 watt-hours. The number 200 represents 100 watt-hours for your [almost] idle computer and 100 watt-hours for your GPU which is not under heavy load due to small tile size. However, if you set the tile size to a higher number and the rendering takes 2 hours your computer would use something around 2 x 300 = 600 watt-hours. 300=100(everything except GPU) + 200 (the GPU under heavy load).
Note that all of these numbers are just a rough approximation and could widely vary depending on your hardware but one thing that won't change is that leaving your computer on for longer will potentially use more watt-hours than finishing your rendering faster and turning your machine off :)