Climate change refers to the fluctuation of the climate over a period of time. A period of time may also refer to decades or millions of years. The range of fluctuations can be regional or global, and the average meteorological index changes. At present, the most discussion on climate change is about the impact of environmental policies on contemporary climate, that is to say, the impact of human factors on the climate, especially the issue of global warming. Regarding the current global warming, most climate scientists believe that "human activities are very likely to be the main cause of global warming for nearly half a century."