Final answer:
The concentrations of CO2 in the atmosphere were 45% higher in 2016 than the highest estimated levels in the 65,000 years prior to 1900. This increase is mainly due to human activities like deforestation and fossil fuel combustion.
Step-by-step explanation:
The concentrations of CO2 in the atmosphere were 45% higher in 2016 than the highest estimated levels in the 65,000 years prior to 1900. Atmospheric CO2 concentrations have increased from approximately 280 parts per million (ppm) in the 18th century to 408 ppm in 2018, which is the highest level in at least 800,000 years.
This increase in atmospheric CO2 is mainly the result of two sets of human activities: (1) deforestation and (2) the combustion of fossil fuels. Human activities currently release over 30 billion tons of CO2 into the atmosphere every year, which is more than 135 times as much as what volcanoes emit annually.
This human-caused build-up of CO2 in the atmosphere is responsible for the current high levels, which is an alarming concern for our climate and environment.