There are a variety of disadvantages of large amounts of carbon dioxide, and I've outlined some of the more common issues below...
Carbon dioxide is a greenhouse gas. What this means is that when carbon dioxide enters the atmosphere, it allows heat from the sun to enter, but doesn't allow it to escape.
Over time, the amount of carbon dioxide in the atmosphere increases due to the burning of fossil fuels. As a result, less and less heat can leave the atmosphere, and therefore the temperature of the earth continues to rise.
This is a huge negative as the increased temperature allows ice caps to melt, which in turn, leads to an increase of water volume in the oceans. As this occurs, the new volume of water covers new area of land.
Plants
Recent studies suggest that excessive amounts of carbon dioxide are reducing the production rate and quality of plants and crops. When this occurs, crops have a much lower nutritional value, and as such, both animals and humans find it more difficult to feed themselves.
Overall, large volumes of carbon dioxide have many disadvantages, and it's important that we rely more on other energy sources, such as solar and win, rather than continue to burn fossil fuels. This will help to reduce the amount of carbon dioxide in the atmosphere over time.
Here's a video documenting how excessive carbon dioxide levels in the atmosphere have become in recent times: