CO2 is naturally found in the earth’s atmosphere in small amounts.
It also has the neat function of acting as an insulator for the earth’s atmosphere, allowing it to keep the heat from the sun at a temperature we’ve become accustomed to – the Greenhouse Effect.
Because of human activity the amount of CO2 and other greenhouse gases being released into the atmosphere has risen dramatically since the beginning of the industrial age.
It’s now reaching artificially high levels because of the burning of fossil fuels needed to power all the stuff we are building and producing.
These artificially high levels of CO2 consequently produce the even more famous effects of Global Warming and Climate Change.
The Intercontinental Panel on Climate Change (IPCC) in early 2007 announced irrefutable evidence exists that human activity is the main driver behind the big G W. 
More recently many agency and government reports have added further evidence. In Australia the Climate Commission has published their report ‘The Critical Decade’
 Climate Change 2007:Synthesis Report,
 Climate Commission – a critical decade report: