Between the 1950s and the early 1990s the level of solar energy reaching the earth's surface had dropped 9% in Antarctica, 10% in the USA, by almost 30% in Russia. And by 16% in parts of the British Isles.
From the ground contrails appear spidery and insignificant. From space, however, we can see just how much light they can block. The following images are courtesy of NASA's Visible Earth site:
South East US
Lake Superior and Lake Michigan
When the volcanic eruption of Eyjafjallajökull grounded flights across much of Europe in April there was some speculation that temperatures would rise once more in the absence of contrails. On the other hand the volcano itself was pumping vast quantities of ash and gas into the atmosphere at the same time, which might mess with the data a little, as Meteorological Officer Brian Delaney pointed out:
Volcanic ash should, if anything, have a cooling effect due to its reflectance of solar radiation, however over such a short time period and such a small geographic area (Ireland), temperatures will be determined by general circulation patterns in the North Atlantic more than anything else.
Below are maximum and minimum temperatures for Knock Airport in the west of Ireland for every day from April 13th to May 18th. The data is confused further because different countries restricted their airspaces over different periods, but broadly speaking from the 15th-23rd April there was a major international shutdown, with shorter closures on May 4th-5th.With the main shutdown periods over Ireland highlighted in yellow, the graph shows no clear correlation between the two at all. If anything there seems to be slightly lower temperatures during this period. In truth the graph is a disappointment. This blog post was written some months ago but that initial research yielded such a disappointingly vague result that it was left aside. Sometimes reality decides not to match the simple graphs and narratives we like to apply to it, leaving us to admit failure and bewilderment. The post will peter out here...