Study : 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 . 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 : 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 .