Study : Ancient people were fewer in number , but burned five times as much land . Ruddiman and study co-author Erle Ellis , an ecologist with UMBC , acknowledge that some models of past land use show it 's only been in the past 150 years -- with a huge population explosion , the onset of the Industrial Age and the rise of fossil-fuel burning -- that global warming has accelerated . Ken Caldeira : People would slash , burn `` 10 times in thousands of years ago period '' William Ruddiman , a climate scientist , says study exaggerating impact of early man .
Study : Ancient people were fewer in number , but burned five times as much land . Ken Caldeira : People would slash , burn `` 10 times in thousands of years ago period '' William Ruddiman , a climate scientist , says study exaggerating impact of early man .