  We investigate whether or not the decadal and multi-decadal climate
oscillations have an astronomical origin. Several global surface temperature
records since 1850 and records deduced from the orbits of the planets present
very similar power spectra. Eleven frequencies with period between 5 and 100
years closely correspond in the two records. Among them, large climate
oscillations with peak-to-trough amplitude of about 0.1 $^oC$ and 0.25 $^oC$,
and periods of about 20 and 60 years, respectively, are synchronized to the
orbital periods of Jupiter and Saturn. Schwabe and Hale solar cycles are also
visible in the temperature records. A 9.1-year cycle is synchronized to the
Moon's orbital cycles. A phenomenological model based on these astronomical
cycles can be used to well reconstruct the temperature oscillations since 1850
and to make partial forecasts for the 21$^{st}$ century. It is found that at
least 60\% of the global warming observed since 1970 has been induced by the
combined effect of the above natural climate oscillations. The partial forecast
indicates that climate may stabilize or cool until 2030-2040. Possible physical
mechanisms are qualitatively discussed with an emphasis on the phenomenon of
collective synchronization of coupled oscillators.
