> Human beings have been releasing CO2 for a lot longer than the Otto cycle has been around
When there were a lot fewer human beings and each individual released far less CO2. Industrialisation has brought leverage to our use of carbon, and the rate of change has become unprecedented.
> The Earth had far higher CO2 concentrations in the past
...and has also had sea levels a hundred feet or more higher than today. Which would do what to our major cities?
Do we wait for our cities to start being lost and the outside air to feel stuffy all the time before we take this remotely seriously? Or perhaps just wait until there's immediate and catastrophic danger? (ie when it's too late to do anything about)
> human behavior is not the only possible cause of net changes in atmospheric composition
I don't believe anyone would seriously claim that, but that humanity has added to the numerous existing sources. There are now enough humans using enough carbon through vehicles, heat, light, consumer goods and all the other things of development that the ecosystem can't simply squelch the results of human activity any more. We're seeing it in a whole range of areas outside of CO2, like loss of habitats and species etc.
On a long enough timeline you're right that there is nothing we can do because among other things the Sun is going to explode, but that's a phenomenally stupid reason to suggest we shouldn't manage greenhouse gasses responsibly. Humans live on a very short timeline, but we're producing effects better fit for a geological scale timeline.