Nacreous cloud
Nacreous cloud (Photo: Rob Cullen)

Looking up: atmospheric sciences

The Earth's atmosphere extends above us thousands of kilometers and it's physical properties and composition change dramatically at that distance. To simplify the study of the atmosphere, it is split up into regions according to these properties and the names of the regions end in ‘sphere’. For example the troposphere, from the ground to the height of the tops of thunderstorms, cools as you go up. The stratosphere, which is the next region, warms as you get higher because the ozone up there is absorbing the energy from the sun's cancer-producing UV rays. Next is the mesosphere where it gets colder as you go up, and finally the thermosphere, where it gets very hot.

Atmospheric scientists at the Australian Antarctic Division are looking at the mesosphere and the mesopause (the transition region between the mesosphere and the thermosphere) to see if things are changing due to natural or human made effects. You may have heard that the introduction of chemicals into the atmosphere, like the gases that used to be in refrigerators, has brought about a huge change in the ozone layer. In a similar way, small changes in the atmosphere are likely to have a large effect on the mesosphere, so it is a good place to keep an eye on.

Solar variation

All energy that drives the Earth comes ultimately from the Sun. This solar energy is not constant but varies over an eleven-year cycle. Any changes that occur in the atmosphere may be caused by variations in the Sun's energy. These changes are very small but need to be taken into account.

Troposphere variation

Fluctuations in the troposphere have been brought about by humans and their activities. From the Industrial Revolution onwards, humans have been burning fossil fuels to provide energy for our machines, and this has increased the level of carbon dioxide in the atmosphere. Carbon dioxide is a “greenhouse” gas, that is, it contributes to keeping the energy that is normally radiated into space, trapped in the lower atmosphere. Methane, a waste product from food digestion, is another of the problem greenhouse gases. Heat trapped in the lower atmosphere has the potential to raise the temperature of the lower atmosphere and to alter weather patterns. This can lead to hardship for millions of people.

It is vitally important that we have accurate information on the atmosphere to give us warning of any changes that might be occurring.

Gravity waves

Gravity waves provide transport for energy from the lower atmosphere to the middle atmosphere. Gravity waves are generated by weather systems, air flow over mountains, and many other tropospheric sources. You can sometime see their effects in the sky, collections of bands in clouds are a typical trademark of these waves. The breaking of these gravity waves transfers energy from one area to another.

Antarctic mesopause

The Antarctic summer mesopause is the coldest part of the atmosphere (−150°C to −130°C), and physicists suspect that it is very sensitive to climate change. In winter the mesopause is hot and in summer cold. This is because of dynamic transport of energy. Climate change variations are warming the troposphere. This is thought to be causing further cooling in the mesosphere. The very low temperatures in the polar mesopause permit the formation of polar mesospheric clouds known previously as 'noctilucent' or night-luminous clouds. These clouds are seen by observers on the ground when the sun has set on the lower atmosphere, but is still shining on the clouds at mesospheric heights. Polar mesospheric clouds are not well understood. They are composed of water-ice crystals. The first sightings of these clouds were recorded in 1885, soon after the massive Krakatoa volcanic eruption, which is thought to have influenced the cloud formation by injecting particles into the upper stratosphere and mesosphere. In recent years the occurrence and brilliance of polar mesospheric clouds have increased. Scientists have attributed this to a decrease in mesopause temperature and an increase in the gas methane in the middle atmosphere.

Polar mesospheric clouds occur only in summer when temperatures fall low enough for ice crystals to form. There are suggestions that polar mesospheric clouds were unknown prior to the late 19th century. The historical record of the clouds themselves may, when cloud physics is better understood, provide information on long-term climate change in the atmosphere.