Because we can analysis the earths weather cycles over millions of years through looking at things like ice core data which can tell us a lot about weather over a huge span of time. What is evident is that we are being 'kicked' out of a a natural cycle of climate through accelerated climatic change caused mainly through unprecedented levels of CO2 in the air. The isotopic signature of the extra levels of CO2 in the atmosphere are from fossil fuels. This is scientific fact as can be measured. What is also clear is that CO2 levels almost always lag behind climate change over thousands/millions of years, what is happening now is that CO2 levels are leading the change in the climate. We have departed the earths natural climatic cycle, and something like 97% of scientists agree on this.
Those skeptical about this established science are not 'skeptics', they are 'denialists' - because they are simply refusing to believe the evidence before them. Skeptic insinuates someone who is willing to question and is open to be convinced. A denialist is someone who simply will not accept the evidence.
Do you agree that climate can change naturally? For example going into and out of an ice age?