Greenhouse gases -- Cause atmosphere to trap more heat.
Sun's output -- Can vary.
Ice age -- Peaked 20,000 years ago.
Explanation:
The human activity has created a large amount of greenhouse gases by the burning of fossil fuel and this fossil fuel is non-recyclable and causes the atmosphere to trap more heat and rise in global climate average. The ice ages have peaked in the past 10 - 20,000 years ago during the periods of interglaciation.
