Step 1 - Human causes of global warming
Amongst the most common causes of human influence on global warming are:
I) Burning of fossil fuels, liberating a lot of CO2, a greenhouse gas, to the atmosphere
II) Agricultural activity. This is still highly debated, but it seems the methane gas (CH4), liberated by cows during their digestion, may impact the global warming as well, given that methane is also a greenhouse gas
III) Industrial activity. To produce some products industrially, a lot of energy is consumed, and eventually greenhouse gases, such as CO2, are liberated to the atmosphere without any concerns
IV) Deflorestation. Plants use photosynthesis to obtain energy. Photosynthesis is a CO2 consumer reaction. By deflorestating, CO2 is not eliminated from the atmosphere in an ideal rate. Therefore, its effect as greenhouse gas is increased
Step 2 - Filling in the blank
You can choose any of the preceding causes for globam warming due to human activities. Since agricultural activity is still uncertain, we could fill the blank with deflorestation, for example.
Answer: deflorestation