What is one to make of a recent press release and submitted preprint blaming global warming on the Tunguska meteor event in 1908? Well, although it is not unknown for impact events to affect climate (the K/T boundary event springs to mind) there are a number of hurdles for any such theory to overcome before it moves into the mainstream from the wilder shores of unsubstantiated speculation.
Firstly, one would anticipate that immediate effects of the impact on climate would be strongest near the time of the impact (allowing for some inertia in the system) and decay away subsequently. Secondly, the timescales for any mechanism associated with the impact (in this case disruption of the atmopsheric water vapour) would need to be in line with the change one hopes to explain. And thirdly, one has to show that this explanation is better than the alternatives. Unfortunately, none of of these requirements are met by this hypothesis.
An impact hypothesis is usefully contrasted to the impacts of a large volcanic eruption like Pinatubo in 1991. There was a very clear dip in temperatures a year or so after the eruption and a subsequent relaxation back to normal. No such event (warming or cooling) is recorded in 1908 to 1910. The timescales for water vapour in the lower atmosphere is on the order of days (see our previous post on the subject), while in the stratosphere it is a a few years. But there are no reservoirs of climatically important water vapour amounts that could still be causing the impact effect to be felt (and to accelerate!) almost 100 years later. And finally, current theories based on greenhouse gas increases, changes in solar, volcanic, ozone , land use and aerosol forcing do a pretty good job of explaining the temperature changes over the 20th Century. It’s very hard to see what this idea has to add to that.
In an additional twist, it is suggested that atmospheric nuclear tests from 1940s to the 1970s masked out the effects of the impact due to the supposed mixing up of tropospheric water vapour into the stratosphere after every explosion. This is even odder since stratospheric water vapour is actually quite a significant greenhouse gas, and had this occured to any large extent, it would have been a warming factor, not a cooling one.
So while the physics being invoked here is barely worth discussing, a more interesting question might be why the University of Leicester thought that this was worthy of a press release in the first place, and why this got any traction in the media at all. True, it didn’t get much attention, so maybe there is some hope for science journalism after all…