Night shining clouds and climate change
Long-term measurements show that the mesosphere has cooled during the last fifty years. This decreasing temperature is probably caused by an increase in the amount of carbon dioxide (CO2) due to human activities.
Although carbon dioxide heats up the bottom layers of the atmosphere by absorption of infrared radiation, carbon dioxide emits infrared radiation in the mesosphere, of which half escapes the Earth's atmosphere. This loss of energy causes a decreasing temperature.
Clouds of ice crystals at around 80 km
These perturbations in the mesosphere have faster and more severe consequences than those in the bottom layers of the atmosphere, due to the lower air density in the mesosphere. The perturbations can be observed with the naked eye in the form of polar mesospheric clouds. These clouds condense at very high altitudes, around 80 km. Ordinary clouds only appear in the troposphere. Polar mesospheric clouds are probably clouds of ice crystals becoming visible in the skimming light of the setting sun.
It is only with extreme low temperatures that the moistness in the rarefied air of the mesosphere can condense into ice crystals. In northern countries these clouds are seen more and more frequently. This is an additional indication for a global change in the thermal condition of our atmosphere. The considerable sensitivity of the mesosphere to external perturbations makes it an outstanding place to test our knowledge of physics and chemistry in the atmosphere and to warn us for future changes in the lower parts of the atmosphere.