Its because the amount of CO2 humans have pumped into the atmosphere correlates to the increase in average temperatures. If we keep pumping the gasses the temperature rises and all we depend on changes.
We continue to change the natural stuff as well - deforestation, changes to fire regimes.
CO2 is not the only driver of climate change, but it contributes. CO2 does not disappear each year - it hangs around for a long tie. AS we continue to depress the natural ability of the planet to absorb and sequester CO2 and add more CO2 to the atmosphere we will change the balance of natural and manmade. Have a look at all the mass extinction events in the past - they correlate with changes in atmospheric conditions and sea levels. Look at the change in Atmosphere when the cyano bacteria flourished in the CO2 rich atmosphere- taking CO2 and releasing oxygen- they breathed themselves to the point of oblivion. Look at the KT boundary - massive and abrupt changes in atmospheric conditions, look at the effect of the Siberian trap formation or the Decan traps - both associated with changes in atmospheric conditions. Now look at what we release each year. And you think it will have no impact.
