Due to abnormally low solar activity. Telegraph:

The Earth could enter a new ‘Little Ice Age’ in the coming years due to low solar activity, astronomers believe.
Sunspot activity, which follows an 11-year cycle, is due to peak in 2013 after which it will start to wane slightly.
But astronomers think the next upswing will be less intensive than normal, or could fail to happen at all.
That could affect weather on Earth because low solar activity has been linked to low global temperatures in the past.
Between 1645 and 1715 almost no sunspots were observed, a solar period which came to be called the Maunder Minimum.
During those decades Europe suffered frequent unusually harsh winters, and the time was later termed the Little Ice Age.

Or, there’s nothing to worry about. From the “Dot Earth” blog at NYTimes.com:

Douglas Biesecker, a scientist at the Space Weather Prediction Center of the National Oceanic and Atmospheric Administration and the leader of an international effort to chart and forecast trends in sunspots, has offered a detailed new critique of three lines of analysis predicting a long and deep lull in sunspot activity, with substantial potential impacts on Earth’s climate (and climate policy). [June 16, 10:09 a.m. | Update Dibyendu Nandi, an Indian solar physicist, has agreed with Biesecker, saying basic solar behavior precludes predictability beyond one solar cycle.]
The last such hiatus in the sunspot cycle, the Maunder Minimum from 1645 to 1715, coincided with a prolonged stretch of cool temperatures, particularly in winter in the Northern Hemisphere.

A power-point and other backup to the claim here.