Recently evidence was found, our ozone layer has been, and will be destroyed again by volcanic eruptions.
The amount of ozone depletion all depends on the size and location of the eruption.
A theory exists about an ice age, that will develop almost over night.
Due to the warming of the oceans and climate, the following might happen. The possibility is quite real, as it happened several times before.
When to much fresh water ends up in the artic seas, there will be a point where colder/salter water will no longer sink ( warmer water will rise to
the surface, but salt gives the water the extra weight to keep a steady flow going.)
This process which is part of a system that wraps the planet, is called the oceanic conveyor belt or Thermohaline circulation
Basically, when the flow stops at one end, the whole thing comes to a halt, for an unknown period of time.
When this happens, the U.S. , Canada and Europe will freeze over from the moment winter kicks in. This will also happen a few months earlier, as the
warm air that travels along the gulf current, will no longer end up in this part of the world. It is the gulf current which has made the last
10.000 years of temperate climate possible, for what we call "Western society".
Shutdown of thermohaline circulation
Of course these are all theories.
I don't think you'll have to be afraid of a nuclear holocaust. The atomic bomb is pretty destructive, yet no one is foolish enough push that
button, unless they are fundamentalist or something... Hiroshima and Nagasaki are both still populated areas. taking in consideration that there have
been 2053 nuclear tests, since they build the first atomic bomb.
Where you might get worried is Fukushima which is spreading massive amounts of radioactive material into our environment. However... Tsernobile shows
that nature doesn't really give a damn about radioactive polution.
The climate has been pretty unstable all through the Earths history, and periods of stable climate like we've seen and experienced the last 10.000
years, are rare...
The Sun seems to affect our climate way more then we thought, and so does our cosmic neighborhood.
The biggest concern I have is about humanity as a species, and the way we are destroying the natural habitats of countless of species, and the way we
are killing of whats left in the oceans at enormous rates.
We are actually behaving like every mammal that is left without natural predators. We are increasing the amount of people around by breeding like
bunnies. We make it worse by being able to make people survive and grow older then what would be possible in nature without modern medicine.
We eat and destroy everything along the way. There will be a point when we realize there is nothing left.
Unless we lower the population there isn't much hope I guess... although the west to blame for all of the mess we are in.
A volcano or meteor impact... That's what could hit us hard... Like Yellowstone or something. Or an Italian caldera.
It's save to say, the possibilities are endless.