The sun is nearly 150 million kilometres away, but it seems to have Earth's rivers on a leash. The flow of a huge South American river - and thus the rainfall that feeds it - appears to rise and fall with the number of sunspots.

Though scientists reject the climate sceptics' assertion that the sun's activity can explain global warming, many have wondered whether it can affect rainfall. No one has been able to test this, though, as it has proved difficult to collate rainfall measurements over long timescales and areas large enough to rule out local variations.

Pablo Mauas of the University of Buenos Aires in Argentina and his colleagues decided to take a different tack by studying the 4000-kilometre-long Paraná river in South America. It has the fourth-largest streamflow in the world and so acts as an indirect indicator of rainfall right across the continent. A gauging station in Corrientes, Argentina, has been measuring the Paraná's streamflow for trade ships since 1904 - unlike on similarly large rivers such as the Amazon or Congo. That meant the team had access to a century's worth of daily measurements.

Mauas's team compared the streamflow with an indicator of solar activity: the number of sunspots seen each year on the sun's surface. The more there are, the greater the sun's activity. The researchers found that over a timescale of decades, the streamflow in the Paraná increased and decreased in accordance with the number of sunspots. "There is less than a 0.01 per cent chance that this correlation is by chance," says Mauas (Physical Review Letters, vol 101, p 168501).

Exactly how the sun might affect precipitation on Earth is not clear. Some researchers have suggested that galactic cosmic rays could seed clouds by ionising particles in the atmosphere and that the solar wind could modulate the number of rays reaching Earth. However, in this scenario rainfall would be higher at times of low solar activity, as this creates less solar wind and hence a greater flow of cloud-seeding cosmic rays.

Climate scientist Jonathan Overpeck of the University of Arizona in Tucson speculates that solar irradiance - the average incoming solar radiation - might be affecting the Paraná's streamflow by influencing the hydrological cycle in the tropics and sub-tropics. "More work is clearly needed to see if the link is real," he says.

Overpeck also cautions that the study should not be used to link any variability in the sun's activity to global warming, because it makes no link between the sun and global surface air temperature. "This is a common misconception that needs to be cleared up," he says. "There is no such corresponding trend in the solar irradiance data as measured with satellites."