The relevant bit is where he says that an expected(imaginary) temperature rise from an average of 15c to 20c is a rise of 25%. By normal earth bound maths it is a rise of 33%, but wait, it gets worse. The rise should be measured from absolute zero (minus 273K) so should read 288/293=.983 which means a temp rise of 1.7%. Quite a big difference 33%(or 25%) and 1.7%.
That really is scary!!! You could say 59 deg F to 68 deg F which (with sufficient accuracy) is a 15% increase. But as you say, it's an absurdity using a scale which doesn't start at zero.
