t shows is for the last thousand years what the carbon 
dioxide levels were on this red line, and that is expressed in parts 
per million, how many molecules of CO<INF>2</INF> per million there are 
in the atmosphere.
  If you see a thousand years earlier it was about 280, 278 parts per 
million on the left side of the chart. We know this because it is 
pretty amazing. We have air from that period, because there are trapped 
air bubbles that were trapped by these glaciers a thousand years ago. 
Scientists d