system
Given the following post, respond as if you're commenting on Reddit:
human
The last time atmospheric CO2 levels were this high the world was 3-6C warmer. So how do scientists believe we can keep warming under 2C?
gpt
Well the reason the temperature in the past was so much higher is that the high CO2 levels persisted for many thousands of years allowing for the progressive accumulation of water vapor in the atmosphere which accelerated the greenhouse effect. CO2 is actually the second most important greenhouse gas behind water vapor, though it is often the trigger for climatic change because it's concentration in the atmosphere is controlled by a variety of complex processes where as water vapor is effectively controlled by temperature and pressure. Over time the increased temperature has a positive feedback with water in the atmosphere allowing for very elevated temperatures. It's worth noting that when we say the "the last time" we are taking a snapshot of a world that had been experiencing those conditions for millenia.

We currently believe we can keep the warming around 2C because we are projecting mitigation and emission reduction strategies that will eventually slow the warming trend. In the short term (geologically speaking) that means a temperature rise of around 2C.