A model estimate of cooling in the mesosphere and lower thermosphere due to the CO2 Increase over the last 3–4 decades

Long‐term observations indicate a substantial cooling in the mesosphere and lower thermosphere (MLT) over the last 3–4 decades. Available model studies have primarily considered the effects of CO2 doubling expected to occur in the future. We present a benchmark estimate of radiative forcing in the MLT due to the increase of CO2 mixing ratio from 313 ppm to about 360 ppm (or by 15%) observed over the last four decades. The Spectral Mesosphere/Lower Thermosphere Model is employed for “retrocasting” the atmospheric response. As expected, the thermal response is predominantly negative. As a function of altitude, the cooling maximizes in the mesosphere at about 3 K, practically vanishes at 100–120 km, and grows to 10–15 K in the thermosphere. Although this vertical shape is remarkably consistent with various sets of observations, the magnitude of the cooling rate is smaller by about a factor of 2–10. This suggests that other mechanisms, e.g., the ozone depletion, might have contributed substantially to the negative temperature trend.