There is strong evidence of the influence that the variations in the
levels of solar activity have on the terrestrial climate. It has even
been proposed that Global Warming is not due to the increase in
greenhouse gases due to human activities, but to the increasing levels
of activity during the twentieth century. Here I will review some of
the most important evidences of this influence, and discuss the
implications. 
I will also present some new results, showing a strong correlation
between solar activity and the streamflow of the Paranà river, one of
the largest rivers of the world. This correlation is also seen with
other South American rivers. These results are a step toward flood
prediction, which might have great social and economic impacts. During
the last flood, in 1997, 180,000 km^2 of land were covered with water,
125,000 people have to be evacuated, and 25 people died. In all, the
three largest floods of the Paranà during the 20th century cause
economic losses for five thousand million dollars.