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Little is known about climate change effects in the transition region between the Earth's atmosphere and space, roughly at 80-120 km. Some of the earliest observations in this region come from noctilucent clouds (NLC) at similar to 83-km altitude. There is a long-standing dispute whether NLC are indicators of climate change. We use model simulations for a time period of 138 years to study the impact of increasing CO2 and H2O on the development of NLC on centennial time scales. Since the beginning of industrialization the water vapor concentration mixing ratio at NLC heights has increased by similar to 40% (1 ppmv) due to methane increase, whereas temperatures are nearly constant. The H2O increase has led to a large enhancement of NLC brightness. NLC presumably existed centuries earlier, but the chance to observe them by the naked eye was extremely small before the twentieth century, whereas it is likely to see several NLC per season in the modern era.

Plain Language Summary In our paper we address a problem that is controversially disputed since several decades, namely, whether noctilucent clouds (NLC) in the middle atmosphere are indicators of climate change. NLC are a spectacular optical phenomenon in the summer season at midlatitudes. We show in our paper that (i) NLC are indeed indicators of anthropogenic activity, (ii) the reason for this is increasing water vapor (caused by methane increase), which significantly enhances the visibility of NLC; and (iii) contrary to common understanding, cooling of the middle atmosphere due to increased reduces(!) the visibility of NLC. NLC constitute the earliest observations in this height region. In our model we expose 40 million dust/ice particles to long-term changes in the middle atmosphere, namely, for 138 years starting with the beginning of industrialization. The model is nudged to the real world in the lower atmosphere. Since the beginning of industrialization,the chance to observe a bright NLC has increased from just one per several centuries(!) to a few per year. We conclude that NLC are indeed an indicator for climate change.