Atmospheric carbon dioxide is the primary source of carbon in life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the Precambrian was regulated by photosynthetic organisms and geological phenomena.
At about 55 km, the mesosphere begins. In the mesosphere, the temperature decreases with height again, because there is very little ozone to warm up the air. Finally, the mesopause divides the mesosphere from the thermosphere, which is the section of the atmosphere higher than 90 km. In this region, the temperature increases again. This time, it is molecular oxygen (O2) that causes the temperature increase.
The fear is that adding carbon dioxide to the atmosphere is warming the planet and increasing evaporation from the oceans. The extra water vapour, itself a greenhouse gas, causes more warming and more evaporation in a vicious cycle of temperature increases that eventually result in the ocean boiling away.