Global warming is caused by the increases in the amount of carbon dioxide and other gases being released into the atmosphere by the burning of fossil fuels. These gases add to the natural greenhouse effect. Causing global temperatures to rise.
Deforestation also leads to a greater concentration of carbon dioxide in the atmosphere. Trees act as a carbon store as they absorb carbon dioxide. Removing this store leads to increased levels of carbon dioxide in the atmosphere.
World population growth has led to a greater demand for food. As the result of this farming has intensified and more animals are reared on farms. This naturally leads to an increase in greenhouse gases such as methane.