  The two main drivers of climate change on sub-Milankovic time scales are
re-assessed by means of a multiple regression analysis. Evaluating linear
combinations of the logarithm of carbon dioxide concentration and the
geomagnetic aa-index as a proxy for solar activity, we reproduce the sea
surface temperature (HadSST) since the middle of the 19th century with an
adjusted $R^2$ value of around 87 per cent for a climate sensitivity (of TCR
type) in the range of 0.6 K until 1.6 K per doubling of CO$_2$. The solution of
the regression is quite sensitive: when including data from the last decade,
the simultaneous occurrence of a strong El Ni\~no on one side and low aa-values
on the other side lead to a preponderance of solutions with relatively high
climate sensitivities around 1.6 K. If those later data are excluded, the
regression leads to a significantly higher weight of the aa-index and a
correspondingly lower climate sensitivity going down to 0.6 K. The plausibility
of such low values is discussed in view of recent experimental and
satellite-borne measurements. We argue that a further decade of data collection
will be needed to allow for a reliable distinction between low and high
sensitivity values. Based on recent ideas about a quasi-deterministic planetary
synchronization of the solar dynamo, we make a first attempt to predict the
aa-index and the resulting temperature anomaly for various typical CO$_2$
scenarios. Even for the highest climate sensitivities, and an unabated linear
CO$_2$ increase, we predict only a mild additional temperature rise of around 1
K until the end of the century, while for the lower values an imminent
temperature drop in the near future, followed by a rather flat temperature
curve, is prognosticated.
