  We compare the performance of a recently proposed empirical climate model
based on astronomical harmonics against all available general circulation
climate models (GCM) used by the IPCC (2007) to interpret the 20th century
global surface temperature. The proposed model assumes that the climate is
resonating with, or synchronized to a set of natural harmonics that have been
associated to the solar system planetary motion, mostly determined by Jupiter
and Saturn. We show that the GCMs fail to reproduce the major decadal and
multidecadal oscillations found in the global surface temperature record from
1850 to 2011. On the contrary, the proposed harmonic model is found to well
reconstruct the observed climate oscillations from 1850 to 2011, and it is able
to forecast the climate oscillations from 1950 to 2011 using the data covering
the period 1850-1950, and vice versa. The 9.1-year cycle is shown to be likely
related to a decadal Soli/Lunar tidal oscillation, while the 10-10.5, 20-21 and
60-62 year cycles are synchronous to solar and heliospheric planetary
oscillations. Finally, we show how the presence of these large natural cycles
can be used to correct the IPCC projected anthropogenic warming trend for the
21st century. By combining this corrected trend with the natural cycles, we
show that the temperature may not significantly increase during the next 30
years mostly because of the negative phase of the 60-year cycle. The same IPCC
projected anthropogenic emissions would imply a global warming by about 0.3-1.2
K by 2100, contrary to the IPCC 1.0-3.6 K projected warming. The results of
this paper reinforce previous claims that the relevant physical mechanisms that
explain the detected climatic cycles are still missing in the current GCMs and
that climate variations at the multidecadal scales are astronomically induced
and, in first approximation, can be forecast.
