What is one effect of excess carbon dioxide in the atmosphere?

Excess carbon dioxide in the atmosphere significantly contributes to global warming. This occurs because carbon dioxide is a greenhouse gas, which means that it traps heat in the Earth's atmosphere. As the concentration of carbon dioxide increases, more heat is retained, leading to a rise in global temperatures. This warming can have numerous effects on climate systems, including altered weather patterns, more severe storms, rising sea levels, and impacts on ecosystems and biodiversity.

The other choices do not directly represent the primary effects of excess carbon dioxide. Decreased plant growth is generally not associated with higher levels of carbon dioxide, as CO2 is a critical component of photosynthesis, which plants use to grow. Enhanced photosynthesis may occur up to a point with increased CO2, but it's not the main effect discussed in relation to excess atmospheric levels. Increased soil fertility is also not a direct consequence of elevated CO2 levels; rather, soil fertility is influenced by factors like nutrients, organic matter, and moisture, which can be affected by climate change but are not directly caused by carbon dioxide in the atmosphere.