It's never stated...

and, to some extent, it doesn't matter.

The movie (quality aside) isn't about the apocalyptic event(s) themselves but rather how the people in the movie react to and behave due to the events around them.

In the process they tend to reveal who and what they truly are.

Here's part of a review which expresses this...

But there’s a restraint at work here, with Rosenthal and McLaren allowing the details of this apocalyptic event to drip out slowly, often from unreliable sources. Voices on the radio scream about North Korea and China, while others accuse the president of some kind of “cover-up.” When a bug-eyed Jeremiah (Mark O’Brien) emerges in the third act, he articulates these myriad strains of news in cryptic fashion, offering plenty of hints about the event without stripping away its mystery. Best of all, we’re never treated to one singular version of the truth. As in 1996’s The Trigger Effect and the original Cloverfield, the specific cause of the event is eclipsed by the chaos of trying to navigate it. This allows for a greater sense of disorientation, as well as a truer depiction of what such a situation might be like for those swept up in it.

^Source

I am currently watching this film and an idea came to me. I am a physicist and, although i haven’t studied this phenomenon, I am aware of some of the effects. I believe that what is causing the apocalypse is the reversal of the Earth’s magnetic field. It would explain the increase in heat (climate change), the compass going haywire, the birds to fly in strange patterns (they rely on the magnetic field), the earthquakes, the seismic activity and the fires (as there will be weaknesses in the field).

The events can be explained by a gamma ray burst from a supernova in our neighborhood in space. In the beginning of the movie, the scenes were that of "star factories" first seen by the Hubble Telescope a couple of decades ago. The gamma ray burst would cause ionization of the upper atmosphere, causing communication disruption (like cell phones), local magnetic anomalies (compasses going wild, birds flying in crazy formations) and aurora borealis phenomenon. It is theorized that there can be accompanying "gravity waves" too, so I suppose this could cause world-wide disruption of the earth's tectonic plates and result in the seismic activity scene in the movie.

The Earth's magnetic field reversing doesn't fit. Firstly "During this change the strength of the magnetic field weakened to 5% of its present strength." (wiki) means there wouldn't be prominent auroras at the lower longitudes we see in the movie or that the reversal had already happened but the earth's magnetic field is so weak it having already flipped wouldn't directly result in the disasters, and finally if the field reversed in a few days I'm not sure that would be long enough for the complete collapse of the ozone layer letting enough UV to create the heat effects. Super volcano is more probable though I don't see the effects having reached far enough east, especially the ash cloud.

I'm going to then vote:

• Massive Solar Flare
• In addition to a near-super volcanic eruption

Little late to the party, but I'd like to provide my theory.

I would agree with Toni. A Gamma Ray Burst could explain all what happened in the movie. First of all, the event doesn't seem to affect every place on Earth the same; the city where the movie starts is relatively unharmed apart the power outage. The news reel on the airport television states a fatal heat wave is affecting Europe.

These differences could be explained by Earth's orientation in relation to the GRB.

One thing I also noticed was that GPS wasn't functioning. This could indicate something was wrong with the GPS satellites and therefore hint that whatever was causing the event, wasn't necessarily only happening on Earth but also in it's athmosphere. A GRB could quite easily fry anything on it's path, including satellites.

I noticed 2 things not mentioned in this thread: When Will is at the airport in Chicago, the CNN report he starts watching right as all the flights get cancelled...scrolling along the bottom, the headline reads that the death toll rises in Europe due to power outage (or maybe it said heat - I can’t remember, and I’m not about to watch it again). Secondly, the Aurora Borealis can be seen from outside Seattle toward the end of the movie, right before Will sits down with the whacko “neighbor” and Sam. I would agree with Toni’s theory that the Earth’s magnetic field shifted. Oh, one more thing, I’m no lunar expert, but did y’all notice how during the long drive west, they kept showing the various moon shots? Since the moon orbits Earth, if the magnetic field were shifted and/or the axis skewed, that might have an effect on the ol ball o cheese.

My theory: an electromagnetic shift caused a seismic chain of earthquakes measuring 10+ on the Richter Scale along the entire continental shelf. This could result in extreme tectonic and possibly volcanic activity, as well as the tsunami described in the film Problems with "the air" could arise from gases from lava pouring in masse into the ocean as seen during the recent eruptions in Hawaii OR, as shown in the film, ash clouds from firestorms from sweeping wildfires, as we've seen in California repeatedly. The electromagnetic interference would cause power outages, etc. as you all have stated above.The destabilization of the tectonic plates surrounding the Pacific area could cause further instabilities across the globe. The only problem I have with my theory is the heatwaves in Europe, though this has happened recently and through out history. Another thought is the displacement of water across the globe due to the melting icecaps, the shifting weight of which could cause releases of pressure and distortions in the Earth's crust. Californian faultlines, and others as well, would be likely weak points that would lead to massive seismic disturbances. Electromagnetic interference has been shown to be a side effect of earthquakes but this does not explain the total blackout of all communication. Side note: cars still run, wouldn't fluctuating electromagnetic fields not cause failures in machinery?

I feel a bit compelled to answer after watching the movie. The following is obviously just my opinion. Everything I saw in this movie is what would and I think will happen in the event of a pole shift. Although a pole shift is not quite the correct cause and explanation. Rather than trying to explain it I can direct you to google The Adam and Eve Story by Chan Thomas.

From what I saw I'd go with super-volcano explosion as at least one of big contributing reasons. There's a common agreement among scientists that the effects of such explosions are tremendously more serious rather than "regular" volcano explosions, which as we know also can be quite catastrophic.

Particularly in California (where presumably everything started) there's a Long Valley Caldera, one of the biggest supervolcanos on Earth. While it's explosion is estimated highly unlikely, we are talking about movie here.

Well one of my theories is the earths core has stopped rotating that's why the birds, and thunderstorms were acting wierd, airplane crashes is caused through emp I'm guessing from the earths magnetic field this is a very real cause and may someday happen without our magnetic field and not enough o zone space on the southern hemisphere the waves will rise and tsunamis can happen and the cloud of smoke is from a super volcano erupting and that's the after dust cloud from the volcano all these parts of the movie match up to the earths core affecting magnetic field and giving abnormal negative and positive formation in our clouds and also damaging anything electronic the compass couldn't even read west to east .simple theory thoe

It appears the globe's axis angle has shifted and is continuing moreso. I believe this scenario explains each of the identifiable radical natural behaviors (changes) and the tragic results. Thus the scene at the end of the movie as more destruction results.