As shone here, the Carbon Dioxide humans produce causes many negative changes to the Earth and its inhabitants.
As explained by the Atmospheric CO2 group, CO2 is produced in many ways - such as the burning of fossil fuels, deforestation, etc. The CO2 then gets released into the atmosphere.
The Ocean Acidification group told us about the bodies of water collecting CO2, which in turns makes the water acidic and the fish unsafe for human consumption.
They also told us about the clams that feed off atoms in the water that the CO2 is destroying, which ends up off-setting the marine food chain. This part alone would make a little over a billion people hungry.
The greenhouse group taught us about the greenhouse effect. Through the greenhouse effect, the CO2 in the atmosphere will warm the earth, which will kill plants and animals that are unable to adapt to the warmer climate.
As proof of this, temperatures and CO2 levels are much higher than they have ever been in the past 400,000 years. This is shown in Ice Cores, where the age of the ice can be told from the depth, the CO2 levels are apparent from the bubbles in the ice, and the temperature directly effects the amount of ice that is formed.