Atmospheric System and the Global Heat Budget

An ever-improved understanding of atmospheric processes is required if popular interest in the weather and climatic change is to be satisfied. Satellites with a range of remote sensing devices can now give us an almost instantaneous picture of global weather. This vast increase in information has to be matched with improved computer capacity and software, in order to arrive at an understanding of the atmosphere’s behaviour.

Transcript

Atmospheric System and the Global Heat Budget

We are all aware of the importance of the atmosphere for all human activity. In a sense, we live in an ocean of air, as fish do in water, and the atmosphere is as important to us as water is to the fish. We are bombarded with information on the atmosphere’s activity, from news reports of winter blizzards to summer droughts, particularly the events which cause destruction and death.

An ever-improved understanding of atmospheric processes is required if popular interest in the weather and climatic change is to be satisfied. Satellites with a range of remote sensing devices can now give us an almost instantaneous picture of global weather. This vast increase in information has to be matched with improved computer capacity and software, in order to arrive at an understanding of the atmosphere’s behaviour.
Although our knowledge of atmospheric processes is increasing rapidly, unfortunately, action to remedy atmospheric damage is not so rapid. Even our ability to communicate crucial weather warnings has on many occasions been inadequate because our predictive abilities are not always matched by communication systems to warn the people affected. Particularly when they are in rural areas with poor communications.
International concern about global warming has highlighted the need for continuing research into the workings of our atmosphere. The results of scientific research reach a much larger audience, outside the scientific community, because the implications of global warming are so very significant. The very concept of an enhanced “greenhouse effect” was a debated theory some years ago and although it is widely accepted as a reality there are still some vested interests who do what they can to refute the evidence.
The earth and its atmosphere can be viewed as a closed system (earth-atmosphere system), dependent on continuing inputs of energy from the sun. There are also some very small energy contributions from the earth’s interior (geothermal energy) and tidal energy.

The Structure of the Atmosphere

The atmosphere consists of 4 identifiable layers, the troposphere, stratosphere, mesosphere, and thermosphere. The diagram shows the different layers of the atmosphere and how temperature changes with altitude, from the ground up to space.

The troposphere is the lowest layer of the atmosphere. This is the layer where we live and where weather occurs. The temperature in the troposphere generally decreases with altitude. The boundary between the stratosphere and the troposphere is called the tropopause. The jet stream, which I will cover in the section on Atmospheric Circulation, is located at this level and it marks the highest altitude which can influence the weather. The upper limit of the troposphere varies with location, being of a higher altitude over warmer areas and lower over colder areas. Above the tropopause lies the stratosphere. In this layer, the temperature increases with height. This is because the stratosphere houses the ozone layer. The ozone layer is warm because it absorbs ultraviolet (UV) rays from the sun. The third layer is the mesosphere. The temperature after an initial rise decreases with altitude, just as it does in the troposphere. This layer also contains ratios of nitrogen and oxygen, similar to the troposphere, except the concentrations are 1000 times less and there is little water vapour there. The thermosphere is the uppermost layer of the atmosphere. The temperature in this layer increases with altitude because it is directly heated by the sun.
The molecules that make up the atmosphere are pulled close to the earth's surface by gravity. This causes the atmosphere to be denser at the Earth's surface and thins rapidly with altitude. Air pressure is a measure of the weight of the molecules above you. As you move up in the atmosphere there are fewer molecules above you, so the air pressure is lower. 50% of all gases in the atmosphere are found below 6km and 99% of the gasses are found below 40km. On top of Himalayan mountain tops, the air pressure is 70% lower than it is at sea level. This explains when mountain climbers breathe air on top of the mountains, they are only inhaling 30% of the oxygen they would get at sea level. It is no surprise that most climbers use oxygen tanks when they climb Mount Everest or other Himalayan peaks.
Temperature decreases with altitude in the troposphere. This is true for a couple of different reasons. The sun's energy is mostly absorbed by the ground. The ground is constantly releasing this energy, as heat in infrared, so the atmosphere in the troposphere is heated, largely, from the ground up, causing it to be warmer near the earth’s surface and cooler higher up. Another reason is the decreasing air pressure with height. As air rises into areas of lower pressure it expands because there are fewer molecules around it to compress it. The molecules in the air use some of their energy to move apart from each other, causing the air temperature to decrease. The constantly decreasing air pressure in conjunction with ground-up heating means the temperature in the troposphere decreases with altitude.
The actual vertical temperature structure at any point on the earth’s surface is influenced by air masses with specific properties of temperature and humidity being blown into the area as well as effects of daytime heating. For example, if there is a layer of warm air on top of a cooler layer closer to the surface, that is called an "inversion". That warmer layer acting as a cap on the cooler air below
Energy Transfers in the Atmosphere

The solar energy which drives the earth-atmosphere system is received and transformed in a series of energy transfers, most of these within the atmosphere itself.
Some of the solar energy is reflected back into space (25%) while some is absorbed (23%), transformed into heat and is then emitted as long-wave radiation. 52% of the original solar energy finally reaches the earth’s surface where 46% is absorbed (and later radiated back into the atmosphere), and a small amount is reflected into space (6%). For every 100 units of energy provided by the sun, 31 are reflected back into space while 69 are absorbed by the earth and its atmosphere. The 31% lost by reflection is termed the earth’s albedo.

The radiation received by the earth’s surface causes its temperature to rise and, like any other hot body, it radiates energy. This energy is emitted as long-wave radiation into the atmosphere and here certain trace gases (carbon dioxide, methane and water vapour) play an important part in absorbing it. These gases are heated and emit radiation in all directions. Much of this energy is returned to the surface while clouds effectively block some from escaping into space. This return of energy to the earth’s surface maintains warm temperatures which would be somewhat 30°C colder without this effect.
This maintenance of high temperatures by the atmosphere, allowing in short-wave radiation and trapping the bulk of the outgoing longwave radiation, is sometimes called the “greenhouse effect”. The enhancement of this greenhouse effect from increases in carbon dioxide and other greenhouse gasses, which trap the outgoing heat, is what causes so much concern as global temperature increases.

The Global Heat Budget
If there was no human interference with the atmosphere, the earth as a planet would not be getting appreciably hotter or colder. It would neither lose energy nor store energy and therefore the total output of energy from the earth-atmosphere system would equal its input from the sun. 31% of the energy from the sun is reflected back into space (albedo) while the remaining 69% is lost to space via other routes such as latent heat transfer and eventually radiated by the atmosphere back into space.
Of the 46% (let’s call these 46 units for ease of reference) of the solar energy which reaches the earth’s surface, 14 units are re-radiated as long-wave radiation (7 into space directly and 7 into the atmosphere), 10 units are returned to the atmosphere by conduction and 22 units are transferred by latent heat. Conduction is the transfer of heat between the earth’s surface and the atmosphere it is in contact with. However, only the lowest portion of the atmosphere is warmed in this way, as air is a poor conductor. Transfer by latent heat is where water is evaporated at the earth’s surface, rises in the atmosphere and transfers heat energy as it condenses. Of the 46 units of energy received by the earth’s surface, 39 units are transferred to the atmosphere. As was mentioned earlier the atmosphere only manages to absorb 23 units of short-wave solar radiation; therefore, the atmosphere receives most of its energy from the earth’s surface. The most significant aspect of the earth’s heat budget is that the atmosphere is largely heated from below.
Over 99% of all energy in the earth-atmosphere system comes from the sun, 46% of the energy warms the sea and land while 22% powers the global hydrological cycle.
Changes in the composition of the atmosphere will have a significant effect on the cycle of energy in the earth-atmosphere system. The atmosphere's role in transmitting, absorbing and reflecting radiation is influenced by its chemical composition. The build-up of carbon dioxide, CFC's, other pollutants and clouds alters the atmosphere's role in this cycle, increasing global temperatures. Trace gases such as carbon dioxide absorb energy in the form of long-wave radiation (infra-red) emitted by the earth's surface. Increased temperatures allow the atmosphere to contain more water vapour (hence clouds) and this can store more heat, raising temperatures still further.

Global Insolation
Energy from the sun heats the atmosphere and the oceans. Most energy is received by equatorial regions. The wind and ocean circulation redistribute some of this heat from equatorial to polar regions.
Because the earth is spherical the sun’s radiation (insolation) only strikes the earth’s surface at the perpendicular between the Tropics of Cancer and Capricorn. The further away from the Tropics, the larger the surface area hit by the solar energy and therefore surface heating is much less. Also, the sun’s radiation must pass through more of the atmosphere before reaching the surface, because of the oblique angle, and hence reduces the heating still further. This difference in global insolation helps to explain the variation and distribution of temperatures over the earth.

The inclination of the earth’s axis causes the angle of incidence (the angle that the sun’s rays strike any point on the earth’s surface) to change throughout the year and therefore the insolation changes with the seasons. At the poles, where there are six months of light and six months of night, the variation in insolation is most extreme. At the equator, there are two maxima, in terms of insolation, (when the sun is directly overhead) at the equinoxes and two minima (when the sun is overhead at either of the Tropics) at the solstices. At the Arctic Circle (66.5°N) insolation is zero on the winter solstice (22nd December) and this period of zero insolation lengthens with increasing latitude. The measurements of insolation are taken as measurements of the sun’s energy received at the outer limits of the atmosphere.