My only skepticism with global warming is that every activist I have ever seen can't explain the ice age. It's as if they believe everything with the earth is figured out and we know everything. How do we know that the earth isn't a constant loop of ice age when needed? How do we know the earth doesn't evolve as things such as CO2 emissions rise?
The Earth has been in an Ice Age since the beginning of the Pleistocene, about 2.5 million years ago. In formal science, an Ice Age is defined as a period where the Earth still has ice year round at the pole(s). Ice ages are punctuated by glacial minimums and maximums, where the extent of the ice waxes and wanes. Consensus among climate researchers is that variations in the Earth's orbit are the major *long-term* cause of Ice Ages and glacial growth/decay, these variations on cycles from 20,000 to 300,000 years. We are currently in a glacial minimum, the last maximum ended about ~20,000 BC and was followed by the dawn of civilization.
The earth has been both much warmer millions of years in the past, and also much colder. Life persisted. Nobody is really worried about the effects of man-made climate changes on life itself - it's the effect of these changes on ourselves, on society, on international politics. Coming out of the last glacial maximum, global climate has been relatively stable with a few minor fluxes like the medieval cold period or some volcanic eruptions. An unstable climate is what worries scientists, and the data is strongly suggesting that man made developments are warming the climate.
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