The right answer is global warming.
During the 1990s, virtually all greenhouse gases reached levels of concentration never seen!
Greenhouse gases have a global warming potential that differs according to their lifetime in the atmosphere and their ability to retain heat. Thus, methane has a global warming potential 23 times higher than that of carbon dioxide.
Currently, the natural warming of the earth due to the greenhouse effect is changing with a magnitude and speed never reached, because of human activity.