The Earth does naturally shift from warm to cold and vise versa. However, that process takes thousands of years to produce any really noticeable change. The issue is that the influx of carbon emissions is actually measurable and actually matches what humans are putting out. Global Warming isn't a theory anymore.
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This.
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Except the part that it has never been proven. Increased CO2 is meaningless as it does not drive temperature and there are many variables you're not considering.