carbon dioxide and 400 parts per million

• What could explain the changes in carbon dioxide concentration over millennial time scales?

We are now at 400 parts per million of carbon dioxide in the earth's atmosphere.  This is staggering given that for 400 000  it seemed that carbon dioxide was just following a fairly up and down pattern between 180 ppm and 300 ppm.

The reasons for the fluctuation are still a little over my head.  First you have the ways in which the earth has been circling the sun (eliptical or circular), tilting towards or away from the sun and "wobbling" on its access which from reading some of the comments from the other participants of the course, affected solar radiation coming in, warming the planet and then whether carbon dioxide could be released from the oceans).  Then there is volcanic activity also releasing huge amounts into the atmosphere at different times throughout history (which then may have acted like a cooling agent preventing solar radiation from getting in and changing the course of carbon dioxide being stored rather than released from the oceans.

• What could explain the changes in carbon dioxide concentration over recent time scales?

Understanding the science behind the fluctuations over the last 400 000 years is important as it seems it would allow scientists to look at all the complex factors and determine the main causes of the earth's rising temperature.  And since that cause is clearly increased green house gas concentrations with carbon dioxide increasing from 280 ppm to 412 ppm in the last 150 years (according to NASA's global climate change site), we, as humans have to take responsibility in recognizing the burning of oil and gas as the main cause and commit to ways stop our reliance on it.