There are a few different doomsday scenarios out there. Some aren't even global-scale. Global Warming, pole-shift, continental dustbowl, Mayan doomsday prophecy, and the ever-popular global nuclear war. I often wish for these to happen, but only global warming and the pole-shift have any evidence of happening so far.
I won't bother explaining global warming, because everyone knows what it is, unless you've been living in a cave for the past 5 years. Or a vault....
The mega-dustbowl is only theory, based on the timeline of dustbowls throughout history (measured by studying age-lines in trees). Scientists say if there were to be another dustbowl, it would be ten times as massive and much longer-lasting, but won't come any time soon.
A pole-shift means the current of Earth's magnetic field (created by the spinning of the molten-metal core) reverses itself. When lava hardens, it leaves an imprint in itself of its magnetic condition at the moment of cooling. By studying this, geologists have found that since the 1700's, the position of the negative flows of the magnetic field (normally located on the southern hemisphere) have faded and/or moved north, while the positive flow in the north has done the inverse. The effects of a pole reversal are unknown, since humankind was not around for the last one.
My favorite, the Mayan Doomsday Prophecy, is derived from the ending of the Mayan calendar on December 21st, 2012, because the Mayans (along with many other prophets and oracles throughout history and the world) believed that on this date, global catasrophe would occur. The Mayans' calendar has been able to accurately predict weather and eclipses far into modern times. It is not stated just what the cataclysm will be, and that is where theories differ. Some believe it will be impact from another celestial body, and some believe it will be severe global climate change. There is a lot of very interesting material on this matter, so I recommend The History Channel's Youtube channel; they have two 1-hour specials on this subject broken into 5 parts each.