The solid blue curve shows the observed northern Hemisphere temperatures
and the dashed blue curve is a smoothed version. The red curve is the
temperature history for a model that responds to the external forcing of
greenhouse gases and solar variability but not to natural climate
variations. The blue alternations about the red curve represent the
natural AMO oscillations. When the AMO decreases, as from 1950 to 1975,
global warming may appear to be reversed. When the AMO increases, as
from 1975 to the present, the global warming (red) is exaggerated.