Final answer:
The current carbon dioxide concentration in Earth's atmosphere is about 400 ppm, which is significantly higher than historical levels and is primarily due to human activities.
Step-by-step explanation:
The present carbon dioxide concentration in the Earth's atmosphere is about 400 ppm (parts per million). Atmospheric CO2 concentrations have significantly increased by 45% since pre-industrial times, rising from approximately 280 ppm in the 18th century to about 408 ppm in 2018. This is a substantial increase considering that the Earth's atmosphere historically cycled between 180 and 300 ppm. The current CO2 level is the highest it has been in at least 800,000 years, as demonstrated by evidence from ice cores. Human activities contribute vastly to this increase, emitting over 30 billion tons of CO2 into the atmosphere annually, which is more than 135 times as much as volcanoes emit each year.