Science proves that humans are largely to blame for the phenomenon, according to Fletcher:
Earth’s climate has always changed. Modern climate change does not, however, fit geologic history. In the past half century, the rate and extent of climate change has been extraordinary. Despite extensive searching, no known natural processes can account for the present climate trend of extremely rapid warming of the temperature of the lower atmosphere. Furthermore, industrial exhaust, deforestation, and large- scale agribusiness are known producers of heat-trapping gas in the atmosphere. It is only logical to hypothesize that there is a strong likelihood that these human activities are causing the extraordinary warming. Vigorous testing of this hypothesis demonstrates that modern climate change is a consequence of human-caused global warming; in fact, among scientists, this has been known for decades.
Read more about the book here.