Answer:
human activity
Increased human activity, in agriculture, manufacturing, and services, is the main reason of global warming or climate change. This is why the scientific community has concluded that global warming is anthropogenic: which means that it is caused by humans.
greenhouse gases
Greenhouse gases are one of the main causes of climate change. These gases clog up the atmosphere and retain solar heat, causing a "greenhouse effect". The main greenhouse gases are carbon dioxide, methane, and water vapor.