Identify the largest human-contributed factor to climate change.

• A. Deforestation
• B. CO2 emissions from human activity
• C. Industrial aerosols
• D. Methane from enteric fermentation

Answer: (B)

The dominant human factor is carbon dioxide released from burning fossil fuels and cement production, because it builds up in the atmosphere and stays there for centuries, continually trapping heat. This long-lasting buildup means each ton of CO2 adds to warming for a very long time, and the total amount emitted by human activities is larger than any other single factor.

Other factors matter, but in different ways. Methane from enteric fermentation is very potent per molecule, but its overall contribution to warming is smaller than CO2 because total methane emissions are less and it clears from the atmosphere relatively faster. Industrial aerosols can reflect sunlight and produce a cooling effect, offsetting some warming, but they do not represent a larger warming driver than CO2. Deforestation releases stored carbon and reduces the land’s ability to absorb CO2, adding to emissions, but the scale of fossil-fuel CO2 emissions still dominates as the primary contributor to human-caused warming.