New Model of the 15-micrometer Cooling of the Mesosphere and Lower Thermosphere

The year 2026 marks the 56th anniversary of Paul Crutzen’s (1995 Nobel Prize in Chemistry) hypothesis that collisions of CO₂ molecules with O(3P) atoms play a dominant role in the excitation of CO₂ in the mesosphere and lower thermosphere (MLT) and its 15-µm emission. The CO₂ + O(3P) problem has remained open for the past five decades due to unacceptably large discrepancies between (1) the small laboratory-measured rate coefficient, kO, estimated between (1.3–2.7) × 10−12 cm3 s−1 for this process, (2) the large kO ≃ (5-9) × 10−12 cm3 s−1 values inferred from space-based observations, and (3) the median kO = 3.0 × 10−12 cm3 s−1, routinely used in general circulation models (GCMs) to estimate CO₂ cooling of the MLT.
The current non-LTE models in CO₂ are inconsistent with observational facts. Using a large kO to fit the observed 15-µm signals contradicts the small, laboratory-confirmed kO. Using a small value for kO contradicts atmospheric observations. Using a median kO in current GCMs simultaneously contradicts both atmospheric observations and laboratory measurements.
We have proposed a new model for the non-LTE 15-µm cooling of the MLT, which is consistent with both laboratory experiments and space-based remote-sensing observations [1]. The model uses two different values for kO. First, the large value of kO = 6.0 × 10−12 cm3 s−1 is used to calculate the extent of CO₂ excitation that is consistent with the observed 15-µm emission. Second, the small, laboratory-established, kO = 1.5 × 10−12 cm3 s−1 is applied to determine the rate of energy exchange between the 15-µm radiation and the heat reservoir—the true CO₂ radiative cooling of the MLT.
We show that the standard non-LTE models, which utilize a single value for kO, strongly overestimate the total CO2 15-µm cooling of the MLT: for kO = 3.0 × 10−12 cm3 s−1, which is routinely applied in the GCMs, this excess radiative cooling can surpass 30 K/day. For kO = 6.0 × 10−12 cm3 s−1, the excess cooling reaches values up to 60 K/day.
Our results cast serious doubt on the widespread belief that the primary cooling mechanism of the MLT is the 15-µm infrared emission of CO₂. Other cooling mechanisms must compensate for the greatly reduced 15-µm cooling. One such mechanism is dynamical cooling, which involves the dissipation and/or breaking of gravity waves (GWs).

This research was supported by NSF (AGS-2312191/92, AGS-2125760, AGS-1301762) and NASA (80NSSC21K0664).

Kutepov, A.; Feofilov, A.; Rezac, L.; Kalogerakis, K.S. Remote Sensing in the 15-µm CO2 Band: Key Concepts and Implications for the Heat Balance of Mesosphere and Thermosphere. Remote Sens. 2025, 17, 1896. https://doi.org/10.3390/ rs17111896