Final answer:
The major contributor to the rise in atmospheric CO2 levels from 300 ppm to over 400 ppm is industrial emissions, particularly the combustion of fossil fuels.
Step-by-step explanation:
One of the biggest human-caused contributors to the increase in atmospheric carbon dioxide concentrations from 300 ppm to over 400 ppm is industrial emissions. The burning of fossil fuels - such as coal, oil, and natural gas - has been the primary source of the increased CO2 levels. This human activity has resulted in a CO2 concentration that is at its highest in over 800,000 years, and possibly the highest in 20 million years. While activities like deforestation and cement manufacture also contribute to atmospheric CO2, they are not the primary cause of the massive increase that has been recorded.
Carbon dioxide's rise from preindustrial levels of around 280 ppm to now over 400 ppm has been linked directly to the combustion of fossil fuels as a source of energy, leading to a significant increase in greenhouse gases and thus contributing to global climate change. Human activities emit more than 135 times the amount of CO2 than volcanic activities, significantly impacting the balance of greenhouse gases in the atmosphere.