What is one consequence of increased atmospheric carbon dioxide levels?

Increased atmospheric carbon dioxide levels contribute significantly to the greenhouse effect, which leads to global warming. Carbon dioxide is one of the primary greenhouse gases that trap heat in the Earth's atmosphere. When sunlight reaches the Earth's surface, some of it is reflected back to space, but the carbon dioxide and other greenhouse gases trap some of this outgoing heat, preventing it from escaping. This process leads to an overall increase in the Earth's average temperatures.

As atmospheric carbon dioxide levels rise, more heat is trapped, exacerbating warming trends that have various climatic impacts, such as more frequent and severe weather events, melting ice caps, rising sea levels, and disruptions to ecosystems. This understanding is critical in assessing environmental and climate responses to human activities, particularly those related to fossil fuel combustion and deforestation, which are major sources of carbon dioxide emissions.

In contrast, decreased global temperatures would suggest a cooling effect, and enhanced biodiversity would not be a direct consequence of increased carbon dioxide; rather, it's often negatively impacted by climate change. Reduced energy efficiency is typically linked to technological and systemic factors rather than a direct effect of carbon dioxide levels. Hence, increased global warming is the most accurate answer regarding the consequences of rising atmospheric carbon dioxide levels.