And since CO2 has increased by approximately 1/100th of 1% the tree line may have gone up a proportionate amount. Maybe a foot or so. If it were temperature Siberia wouldn’t have forests. It does. BTW, which is heavier, a molecule made of 2 atoms of Oxygen or a molecule made of 2 atoms of Oxygen and an atom of Carbon? Climb to the top of Pikes Peak and think about it.
Actually, in the last 50 years the CO2 level in the atmosphere has increased about 30%, from ~310 ppm to a little over 400 ppm.