Answer :
Probably the CO2 concentration. If the earth gets warmer by rising greenhouse gases (one of which is CO2), the amount of rainfall and precipitation increases. I am not an expert, but that's my first guess.
Carbon Dioxide would be your answer. Plants love Carbon Dioxide and as a gift they produce oxygen, its also because of greenhouse effects because of the sun and gases around. However, the amount of Carbon Dioxide on our planet Earth is running low so soon Earth's landscapes will soon decay.
