The atmospheric system and the greenhouse effect

By Matt Burdett, 29 January 2018.

On this page, we look at the atmospheric system, including the natural greenhouse effect and energy balance (incoming shortwave radiation and outgoing longwave radiation).

  • Boracay, Philippines: Stunning tropical sunsets are the result of the interaction of light and atmospheric particles. The red effect is caused by gases in the atmosphere which scatter red light less than other wavelengths, so the deep orange colours are what we are able to see after this remaining light hits clouds which reflect it towards us.

The atmospheric system

The atmosphere is not one continuous feature, but is in fact made up of several layers. These layers of the atmosphere are quite distinct from one another. Each layer is known as a ‘sphere’ while the boundary between the layers is known as a ‘pause’.

  • Layers of the atmosphere. Source: Gronstal, 2012. Reproduced under Creative Commons licence for reuse.

The lowest layer is the troposphere. This is the layer in which almost the entirety of human existence takes place. Above it is the tropopause, where there is a sudden stagnation of any temperature changes, before temperatures rise again in the stratosphere. This layer also contains the ozone layer in the upper part (which is why it gets warmer the higher you go – ozone absorbs energy). Above this is the mesosphere, where temperature decreases because there is almost nothing there to absorb energy. Still higher, the thermosphere contains lots of oxygen which absorbs solar radiation and therefore heats up, but the particles are few and far between – virtually a vacuum – so this higher ‘temperature’ has little meaning in human terms.

The atmosphere as a whole is ‘fatter’ around the equator since this is where it spins fastest, creating a waltzer like effect with it being pushed further out.

Environmental lapse rates

Put simply, the environmental lapse rate is the reduction in temperature as altitude increases. Within the troposphere, there is a drop in temperature the higher up you go for two main reasons:

  • The air is actually not heated from the sun, but from the ground. Therefore the higher up you are, the further away from heat you are, so you feel colder.
  • Also, the air is thinner at higher altitudes. This means that at low altitudes, there is more friction between air particles, because of the increased pressure at the ground level.

Globally the lapse rate is roughly 6.4 degrees Centigrade with every 1000 metres of altitude. It is the circulation of warm and cold air within the troposphere that causes the weather: as warm air tries to rise over the cold air and cools, it creates circulation currents. This zone contains most of the water and therefore most of the weather.

  • Temperature variation in the Earth’s atmosphere. Source: National Weather Service, n.d. Reproduced under Creative Commons licence for reuse.

The energy budget

The energy budget is the balance of energy coming in to the Earth’s atmosphere compared to the amount going out. The amount of energy coming in is usually expressed as 100 units, and in the natural greenhouse effect 100 units leaves the atmosphere (see ‘the greenhouse effect’ below).

  • The energy budget. Source: NASA, n.d. Reproduced under Creative Commons licence for reuse.

The vast majority of the energy that enters the Earth’s atmosphere is from the sun. As the sun is a very hot object, it emits radiation on a short wavelength. This energy is partly reflected by the atmosphere back out to space (for example from clouds which have a light reflective surface colour). Some makes its way through the atmosphere to the surface of the Earth, where darker surfaces absorb it before reradiating it. As these surfaces are relatively cool, they emit radiation on a long wavelength. This longer wavelength energy passes into the atmosphere where some of it is trapped by the ‘greenhouses gases’, while some escapes to space.

What is albedo?

The amount of energy that is absorbed compared to reflected depends on the ‘albedo’ of a surface. Albedo refers to the level of reflectivity of a surface. Bright surfaces such as snow, clouds and sandy deserts have a high reflectivity and therefore a high albedo. Dark surfaces such as asphalt, granite rocks and deep oceans (see photo below) have a low albedo as they don’t reflect much energy.

  • The darker surface of the sea absorbs more energy than the light surfaces of the African continent that are visible in the top right of the photograph. Source: NASA [Apollo 17 mission], 1972. Reproduced under Creative Commons licence for reuse.

The result is that there is roughly an equal amount of energy coming in and going out of the atmosphere, keeping the Earth’s climate at roughly the same temperature over the long term. However, changes in albedo could significantly affect this balance of energy.

The greenhouse effect

The surface of the earth is much cooler than the sun so the wavelength of the radiation is much longer, i.e. longwave radiation. It can’t pass through the atmosphere as easily, so a lot of it is absorbed and then reradiated back to earth. It gets stored long enough to raise the temperature of the atmosphere. This is the natural greenhouse effect and is shown in the diagram below.

  • The Greenhouse Effect. Souce: Trenberth, Fasullo and Kiehl, 2009. Reproduced under Creative Commons licence for reuse.

As the particles that absorb the long wave radiation heat up they will begin to emit radiation which explains why the earth doesn’t just keep getting hotter and hotter – the radiation that is emitted can pass out to space through the atmosphere. However, it is still relatively longwave, and counts towards the 69 units that leave the atmosphere as longwave radiation (the 31 units of short wave are reflected straight back out the atmosphere from the clouds and surface etc.).

The Earth is warmed during the day by incoming insolation and cooled at night (and a bit during the day) by outgoing infrared radiation. There is a balance between the incoming and outgoing radiations. However, some of the radiation is trapped for a time in the atmosphere by a blanket of gases. These gases are present only in trace amounts (meaning it would be very hard to measure the actual amount of them). The gases that trap the heat include water vapour, carbon dioxide, methane, nitrous oxide and CFC (chlorofluorocarbons) (NASA, n.d.). These gases allow short wave radiation to pass from the sun down to the surface to heat the ground and atmosphere but intercept some of long wave radiation given back out. Without these gases, life would be unable to exist as the Earth would be an average of 33 degrees Centigrade colder.

The role of water vapour and clouds on the greenhouse effect can be demonstrated by comparing temperatures on clear and cloudy nights. On cloudy nights, long wave energy is absorbed and re-radiated it back to Earth giving us warm temperatures (as well as preventing the warm surface air rising). On clear nights there is more escape of radiation.


Sources

Gronstal, A., 2012. Fresh Air for the Future. Published by NASA [National Aeronautics and Space Administration] Goddard Institute for Space Studies. https://www.giss.nasa.gov/research/features/201210_shindell/ Accessed 23 January 2018.

NASA, 1972. The Blue Marble. Available from https://www.nasa.gov/multimedia/imagegallery/image_feature_329.html Accessed 22 January 2018.

NASA, n.d. The Atmosphere’s Energy Budget. https://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php Accessed 23 January 2018.

NASA, n.d. Is the sun to blame? https://climate.nasa.gov/causes/ Accessed 29 January 2018.

National Weather Service, n.d. Layers of the Atmosphere. http://www.weather.gov/jetstream/layers Accessed 23 January 2018.

Trenberth, K. E., J. T. Fasullo, and J. Kiehl, 2009: Earth’s global energy budget (PDF). Bull. Amer. Meteor. Soc., 90, No. 3, 311-324. This image viewed via https://commons.wikimedia.org/wiki/File:The_green_house_effect.svg Accessed 23 January 2018.


The atmospheric system and the greenhouse effect: Learning activities

Questions

  1. Imagine you are in a space rocket. Starting with the atmospheric conditions of your launchpad, describe the changes you encounter as you blast into the exosphere. [8]
  2. Why do temperatures drop in the troposphere the higher you go? [4]
  3. How can temperatures rise in the upper layers of the atmosphere despite being further from the warming influence of the Earth’s surface? [4]
  4. Copy and complete the following passages [1 mark per space]:

The global atmospheric system is closed with energy being inputted and outputted. The evidence for this is that _____________ remains broadly the same year on year, suggesting that inputs and outputs are balanced.

This balance is called the ____________ ____________. The energy budget is the balance between energy (heat) going in and out of the atmospheric system.

Everything gives out a form of energy called r____________. Cold objects give out ____________ ___________radiation and hotter objects give out ________ _______ radiation.

The ____ provides the energy input to the Earth. When this s_________ w________ radiation reaches the surface of the earth, it is absorbed. The land then gives out energy as heat – so the atmosphere is heated from below!

Once the shortwave radiation has been absorbed by the surface and gases in the atmosphere, it is radiated back out as longwave radiation. This is the o________. This is because the surface is, relative to the temperature of e.g. the sun, a _______ surface.

5. What is albedo? [2]

6. Give examples of surfaces with high albedo and low albedo. [2]

7. Why might changes in albedo be important regarding global climate change?

8. Draw and annotate a diagram to show the greenhouse effect. Include the following labels:

  • Shortwave radiation
  • Longwave radiation
  • Greenhouse gases

9. Name four greenhouse gases.

Other tasks

Find three different diagrams showing the process of the greenhouse effect. Decide which is best, and explain your reasons.

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