They don't posses any advanced knowledge, their knowledge is in fact irrelevant, just their possible actions (it doesn't matter how irrational those actions might be) that would cause significant environmental changes.
- Any kind of climate change. Making Earth less hospitable, making a very earth-like world more like Earth - it doesn't matter.
- A long period of time is allowed (let's say a maximum timeframe is 100 000 years). Natural climate change doesn't count. Reaching industrialization doesn't count, so let's assume we halt their civilization's development at an earlier point if it takes them too long.
- Whole mankind globally is involved.
- If it would still not be possible, what changes to the world would allow it?
How could mankind of ancient times cause significant climate change?