Cooling and Contraction of the Mesosphere and Lower Thermosphere From 2002 to 2021

We examine the thermal structure of the mesosphere and lower thermosphere (MLT) using observations from 2002 through 2021 from the SABER instrument on the NASA TIMED satellite. These observations show that the MLT has significantly cooled and contracted between the years 2002 and 2019 (the year of the most recent solar minimum) due to a combination of a decline in the intensity of the 11-year solar cycle and increasing carbon dioxide (CO₂.) During this time the thickness of atmosphere between the 1  and 10^-4 hPa pressure surfaces (approximately 48 and 105 km) has contracted by 1,333 m, of which 342 m is attributed to increasing CO₂. All other pressure surfaces in the MLT have similarly contracted. We further postulate that the MLT in the two most recent solar minima (2008-2009 and 2019-2020) was very likely the coldest and thinnest since the beginning of the Industrial Age. The sensitivity of the MLT to a doubling of CO₂ is shown to be -7.5 K based on observed trends in temperature and growth rates of CO₂. Colder temperatures observed at 10^-4 hPa in 2019 than in the prior solar minimum in 2009 may be due to a decrease of 5% in solar irradiance in the Schumann-Runge band spectral region (175-200 nm).