http://www.ncdc.noaa.gov/oa/climate/...arming.html#Q9






From the US Government site

Top of Page Can the observed changes be explained by natural
variability, including changes in solar output?

Since our entire climate system is fundamentally driven by energy
from the sun, it stands to reason that if the sun's energy output were
to change, then so would the climate. Since the advent of space-borne
measurements in the late 1970s, solar output has indeed been shown to
vary. There appears to be confirmation of earlier suggestions of an 11
(and 22) year cycle of irradiance. With only 20 years of reliable
measurements however, it is difficult to deduce a trend. But, from the
short record we have so far, the trend in solar irradiance is estimated
at ~0.09 W/m2 compared to 0.4 W/m2 from well-mixed greenhouse gases.
There are many indications that the sun also has a longer-term variation
which has potentially contributed to the century-scale forcing to a
greater degree. There is though, a great deal of uncertainty in
estimates of solar irradiance beyond what can be measured by satellites,
and still the contribution of direct solar irradiance forcing is small
compared to the greenhouse gas component. However, our understanding of
the indirect effects of changes in solar output and feedbacks in the
climate system is minimal. There is much need to refine our
understanding of key natural forcing mechanisms of the climate,
including solar irradiance changes, in order to reduce uncertainty in
our projections of future climate change.

In addition to changes in energy from the sun itself, the Earth's
position and orientation relative to the sun (our orbit) also varies
slightly, thereby bringing us closer and further away from the sun in
predictable cycles (called Milankovitch cycles). Variations in these
cycles are believed to be the cause of Earth's ice-ages (glacials).
Particularly important for the development of glacials is the radiation
receipt at high northern latitudes. Diminishing radiation at these
latitudes during the summer months would have enabled winter snow and
ice cover to persist throughout the year, eventually leading to a
permanent snow- or icepack. While Milankovitch cycles have tremendous
value as a theory to explain ice-ages and long-term changes in the
climate, they are unlikely to have very much impact on the
decade-century timescale. Over several centuries, it may be possible to
observe the effect of these orbital parameters, however for the
prediction of climate change in the 21st century, these changes will be
far less important than radiative forcing from greenhouse gases.