OK, you may think the title is a bit dramatic, but from what I've just been reading, some unprecedented stuff has been happening to the jet stream in June 2016, and we really don't know what the end result will be.
As you may know, the jet stream circles the Earth in the upper atmosphere in both the northern and southern hemispheres. Due to the poles warming up, the jet stream has been more erratic in recent years, and been meandering around a bit, sending weird weather further south across Europe and other places. However, heat at the equator causes air to rise, effectively forming a barrier which the jet stream cannot cross, separating the two hemispheres. But not this summer, when a climate-change-boosted El Nino has now transitioned into the related La Nina. The centre of the image below image (taken from here) shows the jet stream crossing the equator from North to South.
There's more info in Robert Scribbler's blog here, but the general idea is that this is allowing summer and winter air to mix, and it has never happened during the time we've been watching the jet stream. The impact is unknown, but it could in theory reduce the differences between summer and winter, and potentially make our climate and weather even more unstable.
If you're interested in further background, there's an interesting 15 minute video here, but be warned it is quite technical!