More greenhouse gases in the atmosphere = increase of natural greenhouse effect. (slideplayer.com)
Perhaps you've misread Koonin's statement: For example, human additions to carbon dioxide in the atmosphere by the middle of the 21st century are expected to directly shift the atmosphere's natural greenhouse effect by only 1 % to 2 %. (judithcurry.com)
More greenhouse gases in the atmosphere = increase of natural greenhouse effect Greenhouse gases include water vapour, CO 2, methane, nitrous oxide, and CFCs. (slideplayer.com)