Then 
about a hundred years ago we started to see an uptick of CO<INF>2</INF> 
levels starting to rise. Something happened about a hundred years ago 
or a little more than a hundred years ago causing carbon dioxide levels 
to rise.
  What happened is we started to burn coal and oil. When we burn coal 
it puts carbon dioxide in the atmosphere. So those levels of carbon 
dioxide started to go up, and they kept going up and now they are going 
up at a rate unprecedented in global history. As far as we know thi