Haven't Global Temperatures Risen Before? Yes. In the longer term, say hundreds to thousands of years, there is considerable indirect, proxy evidence (not from thermometers) of both warming and cooling. Since humankind can't be responsible for these early events, this is evidence that nature can cause warming and cooling. If that is the case, it then opens up the possibility that some (or most) of the warming in the last 50 years has been natural, too.