Why are many scientists concerned about humans adding large amounts of carbon to the atmosphere by burning fossil fuels (combustion)

The reason why scientists are concerned about the amount of CO2 released into the atmosphere by burning fossil fuels is because CO2 is a greenhouse gas.

Explanation:

Greenhouse gases help keep the Earth warm. They trap infrared radiation, or heat energy. This makes them useful at certain levels; but if there is too much of them, they will cause global warming rates to increase, melting ice packs, thereby causing rising sea levels. Rising sea levels are a major threat tp humanity right now. New projections and satellite readings suggest that more than 150 million people's homes will be submerged by 2050, with some cities even being completely submerged. Other than rising sea levels, excessive amounts of greenhouse gases also cause loss of biodiversity, changes in wildlife behavior, and shifts in periodic biological phenomena.