Cosmic impacts have occurred in the past and will happen again, the only question is when? They have even caused drastic changes in the Earth's history as in the extinction of the dinosaurs. Earth is hit every year with 500 plus meteorites hit the earth each year but few are significant enough to be noticed. The real danger of a large impact is the dust and debris ejected into the atmosphere blocking sunshine and causing food shortages for years.
A Supervolcano is a volcano with an ejecta volume of great than 1,000 sqkm, or 1,000 times more than the average volcano. A Supervolcano registered as an 8 on the VEI (volcanic explosivity index) and generally rare, the last Supervolcano to erupt was at Lake Taupo in New Zealand over 26,000 years ago. Many scientist believe we are overdue for an eruption of a Supervolcano.
A pandemic is a global disease outbreak which is determined by how the disease spreads and not how many victims it kills. The most recent pandemic was the 1918 flu in which approximately 20% to 40% of the world was infected. This resulted in around 50 million deaths. The use of antiviral drugs and modern medical practices have helped to prevent another pandemic but as we have seen with the H1N1 and other influenza strains the easy of global travel has increased the risk of the pandemic happening soon.
Blackout – EMP
A black out is in a lot of people mind the most realistic disaster scenario. A black out caused by an EMP rather natural or a nuclear blast could shut down the power grid for years leaving survivors to revert back to the pre electric days. Billions would starve or die from lack of medicine and medical treatments.