Since you are looking for a short term event, then in all probability you will need a geological event with some sort of positive feedback cycle.
The only one which comes to mind for me is to somehow trigger a massive release of methane clathrate hydrates: i.e. methane that is trapped in ice crystals and held in suspension below the permafrost and in deep oceans.
Methane clathrate hydrate is suspected to exist in huge quantities (billions of cubic feet of methane if released, the sort of quantities which would make it worthwhile to mine or extract for energy production), but methane is also one of the truly effective greenhouse gasses (far superior to CO2 and not too far behind water vapour; the number one greenhouse gas). Since it is in out of the way places and stabilized by cold and pressure, you will somehow need to remove the cold and pressure keeping it trapped.
Given that much of it is trapped in the oceanic beds, a geological trigger such as an undersea volcano or the action of the Earth's plates disturbing large beds might start the process. An asteroid striking the Earth will likely land in the oceans (70% of the Earth's surface is covered by ocean, so the odds are stacked in favour of an ocean strike), and that too could trigger a massive release of methane into the atmosphere.
The methane will absorb the sunlight and start raising the temperature, creating conditions that will trigger more methane clathrate hydrates melting in shallower waters, pumping more methane into the atmosphere and creating a positive feedback loop, eventually creating a massive methane release and changing the character of the Earth's atmosphere. The huge quantity of methane in the atmosphere will certainly amplify combustion events, and will cause great strain on terrestrial ecosystems, both through the change in temperatures and the new composition of atmospheric gasses. Bacteria which thrive on methane will rapidly expand into many ecological niches (think of the world being overrun with toxic pond scum), and the ecology will change in ways that will tend to stabilize in the new configuration. Less efficient types of photosynthesis would probably be pushed from the stage, resulting in the extinction of may different types of plants (up to 95% if this adversely affects C3 photosynthesis), and massive extinction in the animal kingdom will result as well.