Global warming is caused by the man-made greenhouse effect from burning fossil fuels which increases carbon dioxide levels in the atmosphere. As carbon dioxide levels rise, it makes the air more dense and warms the planet in a phenomenon known as global warming. Some consequences of global warming could include mass extinction of marine life, major droughts, spread of diseases, more hurricanes, and melting polar ice caps raising sea levels. While some argue global warming may be part of a natural cycle, most scientists believe the current rapid warming is too fast to be natural and is primarily caused by human activity like burning fossil fuels.