Final answer:
High levels of carbon dioxide in the atmosphere are primarily caused by human activities, especially burning of fossil fuels, which has drastically increased CO₂ concentrations from preindustrial levels and contributes to climate change.
Step-by-step explanation:
The primary charge leading to high carbon dioxide levels in the atmosphere is human activity, specifically the burning of fossil fuels such as coal, oil, and natural gas. This activity releases significant amounts of CO₂, contributing to the greenhouse effect and climate change. Since the industrial era, around 1850, there has been a marked increase in atmospheric CO₂ from approximately 280 parts per million (ppm) to over 400 ppm today. Such levels of carbon dioxide have not been seen in the last 800,000 years, and possibly not in the last 20 million years according to ice core data.
Additional human activities such as deforestation also contribute to rising CO₂ levels by reducing the number of trees that can absorb CO₂ for photosynthesis. Natural processes like volcanic eruptions release CO₂ as well but are overshadowed by the impact of human-induced emissions. The consequences of increased CO₂ are profound, leading to global warming, ocean acidification, sea level rise, and biodiversity loss, posing significant threats to life on Earth.