We humans are on track to increase the amount of co2 in the atmosphere so that the concentration in the future more than double the atmospheric co2 concentration that existed for the few millennia before the industrial revolution. if we could cause just a doubling and then hold the co2 level constant at that doubled level for the next thousand years, we very likely would see:

Rapid increase in the temperature of the earth.

Explanation:

CO2 is known as a greenhouse gas. It is able to trap and retain infrared radiation in the earths atmosphere thereby causing a gradual rise in the temperature of the earth. If this doubled concentration of CO2 is sustained over a long period of time, it will eventually lead to melting of polar ice caps and consequent flooding. Gradually, the earth would be destroyed due to the scorching heat and attendant flood.