Scientists have known about the greenhouse effect since 1824, when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere.
The rapid rise in greenhouse gases is a problem because it is changing the climate faster than some living things may be able to adapt.
Also, a new and more unpredictable climate poses unique challenges to all life.
As the mercury rises, the climate can change in unexpected ways.
In addition to sea levels rising, weather can become more extreme.