By Dr David Whitehouse
BBC News Online science editor
A new analysis shows that the Sun is more active now than it has been at anytime in the previous 1,000 years.
Sunspots are plentiful nowadays
Scientists based at the Institute for Astronomy in Zurich used ice cores from Greenland to construct a picture of our star's activity in the past.
They say that over the last century the number of sunspots rose at the same time that the Earth's climate became steadily warmer.
The warming is being amplified by gases from fossil fuel burning, they argue.
'Little Ice Age'
Sunspots have been monitored on the Sun since 1610, shortly after the invention of the telescope. They provide the longest-running direct measurement of our star's activity.
The variation in sunspot numbers has revealed the Sun's 11-year cycle of activity as well as other, longer-term changes.
In particular, it has been noted that between about 1645 and 1715, few sunspots were seen on the Sun's surface.
This period is called the Maunder Minimum after the English astronomer who studied it.
It coincided with a spell of prolonged cold weather often referred to as the "Little Ice Age". Solar scientists strongly suspect there is a link between the two events - but the exact mechanism remains elusive.
Ice cores record climate trends back beyond human measurements
Over the past few thousand years there is evidence of earlier Maunder-like coolings in the Earth's climate - indicated by tree-ring measurements that show slow growth due to prolonged cold.
In an attempt to determine what happened to sunspots during these other cold periods, Dr Sami Solanki and colleagues have looked at concentrations of a form, or isotope, of beryllium in ice cores from Greenland.
The isotope is created by cosmic rays - high-energy particles from the depths of the galaxy.
The flux of cosmic rays reaching the Earth's surface is modulated by the strength of the solar wind, the charged particles that stream away from the Sun's surface.
And since the strength of the solar wind varies over the sunspot cycle, the amount of beryllium in the ice at a time in the past can therefore be used to infer the state of the Sun and, roughly, the number of sunspots.
Dr Solanki is presenting a paper on the reconstruction of past solar activity at Cool Stars, Stellar Systems And The Sun, a conference in Hamburg, Germany.
He says that the reconstruction shows the Maunder Minimum and the other minima that are known in the past thousand years.
But the most striking feature, he says, is that looking at the past 1,150 years the Sun has never been as active as it has been during the past 60 years.
Over the past few hundred years, there has been a steady increase in the numbers of sunspots, a trend that has accelerated in the past century, just at the time when the Earth has been getting warmer.
The data suggests that changing solar activity is influencing in some way the global climate causing the world to get warmer.
Over the past 20 years, however, the number of sunspots has remained roughly constant, yet the average temperature of the Earth has continued to increase.
This is put down to a human-produced greenhouse effect caused by the combustion of fossil fuels.
This latest analysis shows that the Sun has had a considerable indirect influence on the global climate in the past, causing the Earth to warm or chill, and that mankind is amplifying the Sun's latest attempt to warm the Earth.