Two steps forward, one step back: four years of progress and setbacks on invisible ship tracks

The idea of invisible ship tracks for the study of aerosol-cloud interactions sounds promising: We have been studying the effects of aerosols on clouds for many years, among others by investigating the bright lines of clouds left in low marine clouds by ships. However, only a small fraction of ships leaves behind visible tracks. This means we can only study aerosol-cloud interactions under certain meteorological conditions, biasing our understanding. Instead, by studying all clouds polluted by ships ('invisible ship tracks') with a methodology we developed, we should be able to get a full picture of aerosol-cloud interactions. A number of interesting and impactful results have come out of this research, along with several setbacks and corrections to initial results. Here, we examine them in order, showing how correcting for one identified bias can introduce two new ones. Unexpected glitches arise from sources as varied as: choices regarding ship track definition, retrieval geometry, specific weather systems biasing results, and mathematical subtleties. What can we conclude after four years of progress on this methodology? While some results still stand, others had to be significantly corrected. This makes us see invisible ship tracks as an example of research that is closer to a method of 'tinkering' than to a 'magnificent discovery'.