Natural Experiments, Bomber Raids, and Economics
The spring morning of May 11, 1944, dawned mostly clear over southern England. There were a few high clouds, but it was a bright and breezy day with a high of 20 C. A good day for an Allied bombing raid, albeit one that turned out to matter decades later.
Shortly after 10 am, 364 B‐24s and 536 fighter aircraft of the Eighth Air Force, second and third Bomb Divisions, took off for Germany from their bases. And a second raid, involving 609 B‐17s and 471 fighter aircraft of the first and third Bomb Divisions, took off just before 3 pm. And, a 2011 paper argues, the two massive waves of bombers had an unexpected effect: they changed the weather.
To understand why, it helps to keep a few things in mind.
There was no other air traffic at the time, mostly because of the war, but also because the commercial air travel industry was in its infancy. As a result, contrails—those dense and persistent condensation trails in the sky induced by hot jet exhaust meeting ice crystals —were non-existent. There is a large and growing literature about the effect of air travel on climate, and, more specifically, on the effect of contrails on the weather. The general view in the climate research community has been that contrails have an effect, but, until recently, it’s been hard to find sufficiently well-designed studies to make that definitive.
That 2011 study helped give an answer. It showed, via a kind of natural experiment, that waves of airplane-caused cloudiness (i.e., contrails) changed the weather. Specifically, one paper argues, the absence of contrails induced an almost 1° reduction in the daily high/low temperature range, caused, it seems clear, by those waves of bombers.
We saw something similar after 9/11 in the United States. With more than 36,000 canceled flights, contrails disappeared from the sky, which was usually gridded with the things as this NASA satellite picture from 2004 shows. And just like in 1944, there was a change in the daily range of temperatures: During the three-day aircraft grounding, the average daily temperature range in the US increased by 1.1° C.
We are in the middle of a similar experiment today as a result of the coronavirus outbreak. There have already been more (mostly trans-Pacific) flights canceled than during 9/11, and outbound China shipping (another source of cloud) has contracted more than 30%. The impacts will be manifold. For example, as happened after 9/11 and after the May 11, 1944 bomber raids, we will likely see reduced cloud cover, at least over the Pacific, which will mean greater temperature ranges and more unusual weather, albeit on the US west coast.
We will also see a sharp reduction in CO2 emissions from China, as has already been noted by energy observers. Estimates vary, but the drop could be 25% or more. Daily coal consumption from six major Chinese power firms has gone down dramatically, with no sign of recovery.
There will be other effects too, most of which will be unanticipated. We are in the midst of a massive natural experiment in turning back time, an unprecedented one in terms of scale and scope, and it is impossible to know the outcomes and consequences.
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