Study : The New York Times William Ruddiman : People would slash , burn `` five times in a 20 year period '' Ken Caldeira , a climate scientist , says study exaggerating impact of early man .
Study : Ancient people were fewer in number , but burned 10 times as much land . William Ruddiman : People would slash , burn `` five times in a 20 year period '' Ken Caldeira , a climate scientist , says study exaggerating impact of early man .