Climate change
The Earth's climate has changed many times during the planet's history - from ice ages to long periods of warmth. Most of the variation is caused by natural reasons. Fortunately, the climate has remained on a relatively stable stage since the last Ice Age. However, due to human influence, the Earth's average temperature has been rising and changing the climate since the industrial revolution in the 18th century. Unless the world takes action, climate change will have severe impacts on the well-being of humans and the nature.
Climate.org
Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Programme
United States Environmental Protection Agency
WWF
"You Control Climate Change" by the European Commission