rill a core into the ice and they get those little air 
bubbles and they put them on a devices that measures the concentration 
of CO<INF>2</INF> and you know exactly how much CO<INF>2</INF> there 
was. So this is a very precise measurement.

  So we know a thousand years ago CO<INF>2</INF> levels were 278 or 
280. You will see the next hundred years are about the same. The next 
hundred about the same. The next hundred there are some minor 
deviations 500 or 600 years ago. They are staying about the same. 