CO2 is a colorless gas that is 60% denser than air. It is composed of one carbon atom double bonded to two oxygen atoms. CO2 occurs naturally in small amounts in the atmosphere and is released from volcanoes. It dissolves easily in water and is found in oceans, lakes, and groundwater. CO2 levels have risen from 280 parts per million before the industrial revolution to 0.04% currently due mostly to fossil fuel combustion.