On the Anthropogenic Impact on Long‐Term Evolution of Noctilucent Clouds

Abstract

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 ∼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 ∼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.

Updated Long‐Term Trends in Mesopause Temperature, Airglow Emissions, and Noctilucent Clouds

“…This is also in line with model simulations, which demonstrate about zero trend in the NLC occurrence rate and small positive trend in the NLC brightness at middle and subpolar latitudes (Berger & Lübken, ; Lübken & Berger, ). At the same time, the found small secular statistically insignificant NLC trends contradict large trends modeled by Lübken et al () for the period of 1960–2008 at 55–61°N. NLCs demonstrate statistically significant negative response to solar activity, as anticipated from theoretical consideration due to photodissociation of water vapor by the Lyman α flux. If ozone forcing is introduced to the MRA of the NLC data, then it produces a minor effect on long‐term changes in NLC occurrence frequency and brightness.…”

“…About the same result has been obtained by Berger and Lübken (2015), who have calculated near zero and even negative trends in NLC occurrence frequency at middle and subpolar latitudes (50-55°N and 55-60°N) based on model simulations for 1961-2013. At the same time, the most recently published model by Lübken et al (2018) have demonstrated large positive trends in the IWC at the mesopause, NLC occurrence frequency, and NLC brightness at middle and subpolar latitudes (55-61°N) for the period of 1960-2008. The authors have concluded that "NLC are indeed a longterm indicator for climate change due to the increase of H 2 O caused by the oxidation of methane, whereas the growth of carbon dioxide mixing ratio is of secondary importance."…”

“…Numerous ground‐based NLC observations (Dalin et al, ; Dubietis et al, ; Kirkwood et al, ; Kirkwood & Stebel, ; Pertsev et al, ; Romejko et al, ; Zalcik et al, , ) have clearly demonstrated about zero and/or small positive statistically insignificant trends in NLC occurrence frequency and brightness for the past five decades. Thus, large modeled long‐term trends in NLC occurrence frequency and brightness obtained by Lübken et al () contradict both PMC space and NLC ground‐based long‐term observations at middle and subpolar latitudes, in spite of the fact that these studies have addressed different PMC/NLC databases, different methods of NLC observations around the globe, and different analyzed periods for the past five decades. One should not ignore these well‐established observational results, when discussing long‐term trends in NLC/PMC parameters.…”

“…The authors have concluded that “NLC are indeed a long‐term indicator for climate change due to the increase of H 2 O caused by the oxidation of methane, whereas the growth of carbon dioxide mixing ratio is of secondary importance.” Unfortunately, the authors have not discussed whether their previous modeled results (Berger & Lübken, ; Lübken & Berger, ) on no long‐term trends in NLCs at middle and subpolar latitudes are still valid, but we could not find any disproof of their previous results in the literature so far. Also, Lübken et al () have not compared their modeled results with long‐term trends in actual PMC and NLC observations. Here such a comparison can be done in the following way.…”

“…Unfortunately, the authors have not discussed whether their previous modeled results (Berger & Lübken, 2015; on no long-term trends in NLCs at middle and subpolar latitudes are still valid, but we could not find any disproof of their previous results in the literature so far. Also, Lübken et al (2018) have not compared their modeled results with long-term trends in actual PMC and NLC observations. Here such a comparison can be done in the following way.…”