The sun has become increasingly active since 2008, a new NASA study shows. Solar activity is known to fluctuate in cycles of 11 years, but there are longer-term variations that can last decades. Case in point: Since the 1980s, the amount of solar activity had been steadily decreasing all the way up to 2008, when solar activity was the weakest on record. At that point, scientists expected the sun to be entering a period of historically low activity.

But then the sun reversed course and started to become increasingly active, as documented in the study, which appears in The Astrophysical Journal Letters. It's a trend that researchers said could lead to an uptick in space weather events, such as solar storms, flares, and coronal mass ejections.

"All signs were pointing to the sun going into a prolonged phase of low activity," said Jamie Jasinski of NASA's Jet Propulsion Laboratory in Southern California, lead author of the new study. "So it was a surprise to see that trend reversed. The sun is slowly waking up."

The earliest recorded tracking of solar activity began in the early 1600s, when astronomers, including Galileo, counted sunspots and documented their changes.