Final answer:
The steep increase in atmospheric CO2 levels to around 410 ppm is primarily due to human activities, notably the combustion of fossil fuels and deforestation. It represents a significant deviation from the natural historical cycle of CO2 concentration, which has led to the highest levels in at least 800,000 years.
Step-by-step explanation:
The most likely cause of the recent upward trend in atmospheric CO2 from historical levels of 250 parts per million (ppm) to the current level of around 410 ppm is human activity. Specifically, two main activities are responsible for the increase: (1) deforestation, which reduces the number of trees that can absorb CO2, and (2) the combustion of fossil fuels, such as coal, oil, and natural gas, as a source of energy.
Human activities release over 30 billion tons of CO2 into the atmosphere annually, which is more than 135 times as much as volcanoes emit each year. The data from ice cores shows that these human-caused emissions have led to atmospheric CO2 concentrations being at their highest in at least 800,000 years and possibly up to 20 million years.
The rapid increase in CO2 levels over the past few centuries, especially the last 50 years, is unprecedented compared to the natural cyclical pattern of CO2 levels historically ranging between 180 ppm and 300 ppm over periods of 50,000 years. This drastic rise correlates with the industrial activities of modern human society.