Which of the following is the main cause of global warming?

Last week, a United Nations report said humans are "very likely" the cause of most of the temperature increases in the last fifty years. It said global warming is undeniable, and that the world can expect to feel the effects for centuries to come.
    The Intergovernmental Panel on Climate Change released the report in Paris at a conference of climate experts. Representatives of more than one hundred governments agreed on the findings. This is the most detailed scientific report to date on global warming and the influence of fossil-fuel burning and other human activities.
    The scientists say there is greater than a ninety percent chance that greenhouse gases are the main cause of rising temperatures and sea levels. The report also links global warming to other changes, including increased dryness in some areas and violent storm patterns.
    The U.N. panel released its last climate change report six years ago. The scientists say the new report is based on studies done since then, and stronger agreement on global warming.
    The new report makes no policy proposals. But the aim is to press governments and industries to cut the release of carbon dioxide and other gases blamed for trapping heat.