Final answer:
Human activities have significantly increased atmospheric CO2 levels over pre-industrial levels. If current fossil fuel consumption continues, CO2 concentrations could rise up to 2.75 times the current excess, leading to substantial temperature increases. Atmospheric CO2 levels are currently at their highest in the last 800,000 years.
Step-by-step explanation:
The question pertains to the potential increase in atmospheric CO2 concentrations due to increased volcanism and superplumes. Historically, human activities, particularly the burning of fossil fuels, have increased atmospheric CO2 levels from pre-industrial levels of approximately 280 ppm to over 400 ppm today, marking a 45% increase. While significant volcanic eruptions have contributed to CO2 levels in the past, human emissions now dwarf those from volcanoes, exceeding them by more than 135 times annually.
Estimations suggest that CO2 concentrations could potentially rise to 2.75 times the current excess if fossil fuel consumption continues at the current rate until the end of the century. This scenario could lead to a radiative forcing of 4.25 W/m² and a temperature increase of approximately 3.4°C. Ice core data provide strong evidence that current CO2 levels are the highest in the last 800,000 years, and possibly in the last 20 million years.