Of course the Earth's climate is a million times more complex than a greenhouse. Yes, human activity accounts for a tiny part of the greenhouse gases in the atmosphere, but here's a good analogy: Let's say you start with a glass of water that's half full. Then you add a teaspoon of water to represent the CO2 released by nature through the decay of organic matter. And then remove a teaspoon of water to represent the CO2 absorbed by oceans, trees, etc. You can repeat this many times and the water level will stay roughly unchanged. But then say you start adding a drop more than a teaspoon, and start removing a drop less than a teaspoon. The drops (+ and -) correspond to how much human activity adds CO2 to the atmosphere through the burning of fossil fuels, and reduces the CO2 absorbed from the atmosphere due to the cutting down of trees. The water leveI will rise slowly until eventually the glass overflows. It adds up.

The sun's energy (insolation), volcanoes, and greenhouse gases are the main factors in forcing climate changes. It's not just one or the other. All three are constantly in play, changing and interacting over time. It's just a question of how much of each factor is involved. For example, if the Earth's orbit moves it further from the sun, the loss of insolation might force a climate change, resulting in an ice age. CO2 levels and volcanic activity, even if very high, have little effect because the loss of insolation is overpowering.

The current warming period is marked by low insolation and volcanic activity, but with atmospheric CO2 rising to levels unseen in 1000's of years. This CO2 retains some of the sun's heat radiating off the Earth's surface and so it affects global surface temps. AGW started around 1950 and it's been rising steadily since then. In the 1980's, AGW had risen high enough to affect the Earth's climate, a process that's continuing to worsen. That's why the term "climate change" came into vogue. It's a different meaning, but it's often used interchangeably with AGW.