because in the geological record there is massive asteroid sediment at the same time the dinosaurs died.
(In 1980, a team of researchers consisting of Nobel prize–winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous–Tertiary boundary contain a concentration of iridium many times greater than normal (30, 160 and 20 times in three sections originally studied).Iridium is extremely rare in the earth's crust because it is a siderophile element, and therefore most of it travelled with the iron as it sank into the earth's core during planetary differentiation. As iridium remains abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the earth at the time of the K–T boundary. There were other earlier speculations on the possibility of an impact event, but this was the first evidence uncovered.)
Besides that, the only people who think the world will end if the poles shift is the woo crowd. As long as the field doesn't collapse, there probably wouldn't be any effect. If it did, we would all die of radiation poisoning and asphyxiation in, oh, a billion years or so.
Seems kind of a waste of time to go looking for a knife, when you got a smoking gun right in from of you.
On edit: after thinking about, we might not even loose the atmosphere without the magnetic field. Venus and mars have atmospheres, but no magnetic field.
The greenhouse effect on Venus is NOT because of the magnetic field.