90 Kilometres above the Antarctic – lowest temperatures since measurements began

The temperatures in the boundary layer between the atmosphere and outer space are measured with a spectrometer. It can detect the very faint glow that is generated at an altitude of 90 kilometres. Here, hydroxyl molecules (OH), a compound of atomic oxygen and atomic hydrogen, emit infrared radiation, which is invisible to the human eye.

Scientists can interpret like a fingerprint the wavelength mixture around the 1.5 micrometre range. They are thus able to infer the concentrations of trace substances and the temperature at this altitude. The GRIPS spectrometer (GRound Based Infrared P-Branch Spectrometer) used for the purpose is being employed by EOC at several locations worldwide. Measurements at the Neumayer III Antarctic station were made possible by a cooperation agreement concluded in 2013 with the station’s operator, the Alfred Wegener Institute (AWI). Once a year the AWI team that will spend the coming winter there undertakes training in Oberpfaffenhofen on the use of this instrument before they set out for the Antarctic. The nocturnal measurements are made there automatically every 15 seconds. They are transmitted directly to Oberpfaffenhofen, automatically interpreted, and published.

The nocturnal mean temperature values (Figure 1) show that at 90 kilometres altitude in winter it is 50°C warmer on average than in the summer months. During summer temperatures down to minus 120°C can occur here – Earth coldest location! Temperatures on the ground and at 90 km altitude counter-correlate. No measurements are made In Arctic summer when the sun remains in the sky around the clock since the instrument cannot detect the weak infrared signal during the day.

The diagram of annual mean temperature values (Figure 2) shows the lowest value in 2020 and a conspicuous deviation in 2017. In 2017 atmospheric temperature dropped  by about 4°C to 5°C. There were also unusual anomalies observed at ground level: the average temperature in the Antarctic was up to +1.5°C above the multiyear mean, and as a result the extent of the Antarctic ice sheet decreased enormously by about 106 km2 compared to the previous year.

One reason for the observed high altitude cooling at high elevations is the presence of the greenhouse gas CO2. When colliding with other molecules, CO2 absorbs energy and radiates it into space. Thus, the atmosphere loses energy and the temperature drops. The situation in the lower atmosphere is quite different. Three-atom molecules like CO2 and methane ensure that less heat is radiated into space there, so Earth warms up. Nevertheless, the processes at 90 kilometres altitude are in no way fully understood. For example, the contraction of the atmosphere resulting from cooling influences the spectrometer measurements. A multitude of unanswered questions remain.

The atmosphere is a linked system. Occurrences at its different elevations interact. In some cases they can, however, be observed earlier and better in the upper levels of the atmosphere. Air density there is only about one millionth of that at ground level. For that reason temperature trends in the so-called mesopause turn out to be much more prominent and thus recognizable earlier.

EOC exploits this characteristic of the mesosphere also to develop applications and is working on procedures for early recognition of occurrences like tsunami and volcanic activity. EOC scientists are also investigating whether the path of heavy storms can be more easily predicted with the help of the mesopause, and what influence global warming has on atmospheric currents. The global Network for the Detection of Mesospheric Change, NDMC, which includes the Antarctic monitoring station, helps to elucidate the complex processes in the climate structure of our Earth System

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