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 studie s have primarily considered the effects of CO2 doubling expected to occur i n the future. We present a benchmark estimate of radiative forcing in the M LT due to the increase of CO2 mixing ratio from 313 ppm to about 360 ppm (o r by 15%) observed over the last four decades. The Spectral Mesosphere/Lowe r Thermosphere Model is employed for "retrocasting" the atmospheric respons e. As expected, the thermal response is predominantly negative. As a functi on of altitude, the cooling maximizes in the mesosphere at about 3 K, pract ically vanishes at 100-120 km, and grows to 10-15 K in the thermosphere. Al though this vertical shape is remarkably consistent with various sets of ob servations, 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, mi ght have contributed substantially to the negative temperature trend.