Global radiative forcing from contrail cirrus : Nature Climate Change : Nature Publishing Group

Globally, the long-wave radiative forcing due to contrail cirrus (after correcting the scattering component of the long-wave forcing from the model³⁰) amounts to 47.1 mW m⁻² and short-wave radiative forcing to −9.6 mW m⁻², resulting in a net radiative forcing of 37.5 mW m⁻².

...Assuming that the decrease in natural-cirrus coverage amounts to approximately one fifth of the global contrail-cirrus coverage (as we have found a maximum of 2% (1.5%) decrease of natural-cirrus coverage downstream of the areas of 10% (6%) contrail-cirrus coverage over Europe (US)), the feedback due to this change in natural-cirrus cloudiness would induce a cooling of approximately a fifth of contrail-cirrus radiative forcing, that is −7 mW m⁻². This estimate is very uncertain and further work is needed to more reliably quantify the feedback.

That leaves about 30 mW m⁻². Compared to the grand total of 1.6 W/m², that's still miniscule, though the Spiegel article says that it exceeds air traffic contribution via CO2 emissions. (Diagram below from Wikipedia, with "linear contrails" from data preceding this study.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}