The last Year Without a Summer lasted 40 months. That very cold summer was caused by volcanoes churning up ash into the skies. But there isn't any noteworthy volcanism going on now.
Which means this must be caused by something else.
You guessed it, Global Warming. Global warming is causing the frigid temperatures in Europe.
Note: I got this story from a French newspaper (Le Monde). I looked for an English-media source picking up on the story. Shock -- there isn't one. The press in England and the US has decided they don't want to talk about the possible Year Without a Summer.
Thus, the crappy translation.
(Note, though: The generally poor (cold) weather of spring is discussed at different places. It's attributed to global warming.)
Corrected: I first called 1816 the start of "the Little Ice Age." That's wrong. The Little Ice Age ran from around 1350 to 1850 -- so it contained the especially-cold 1816-1819 period, but it didn't kick it off and wasn't caused by the cold snap of 1816.
That was just what we call a "complete mistake."
[Update JohnE.]: Possible explanation for the Little Ice Age below.