The greenhouse effect and the enhanced greenhouse effect

Overview

Natural greenhouse effect step by step, enhanced greenhouse effect and global warming potential.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Explain the natural greenhouse effect step by step.

   Model answer

   Incoming solar radiation is mostly shortwave, and the atmosphere is largely transparent to it, so about half of it passes through and is absorbed at the Earth's surface, warming the ground and the sea. The warmed surface then re-radiates that energy upwards, but because the surface is far cooler than the Sun it radiates at much longer wavelengths, in the infrared. Certain trace gases in the atmosphere - water vapour, carbon dioxide, methane, nitrous oxide and ozone - are transparent to shortwave radiation but absorb longwave radiation strongly. They absorb the outgoing infrared and re-emit it in all directions, including back down to the surface. The surface therefore receives energy from the atmosphere as well as from the Sun, and settles at a higher temperature than it otherwise would.

2. 2

   What difference does the natural greenhouse effect make to the Earth's temperature?

   Model answer

   Without any greenhouse gases the Earth's mean surface temperature, given the solar radiation it receives and the fraction it reflects, would be about minus 18 degrees Celsius. The observed mean is about 15 degrees Celsius. The natural greenhouse effect is therefore worth roughly 33 degrees of warming, and without it the planet would be frozen and effectively uninhabitable, with liquid water rare at the surface. It is entirely natural, it has operated throughout Earth history, and it is the reason life as we know it is possible. Venus and Mars illustrate the two extremes, one with a runaway greenhouse and one with almost none.

3. 3

   What is the enhanced greenhouse effect, and how does it differ from the greenhouse effect?

   Model answer

   The greenhouse effect is the natural process described above, which keeps the planet about 33 degrees warmer than it would otherwise be and makes it habitable. The enhanced greenhouse effect is the additional warming caused by human activity increasing the concentration of those same greenhouse gases, and adding some entirely new ones. A higher concentration absorbs a greater proportion of the outgoing longwave radiation, so less energy escapes to space and the surface must warm until the balance between incoming and outgoing energy is restored. It is an intensification of a natural process, not a new or separate one, and the human contribution is the enhancement, not the effect itself. Saying the greenhouse effect causes global warming is therefore wrong; the enhanced greenhouse effect does.

4. 4

   Explain why it is wrong to say that the greenhouse effect is causing global warming.

   Model answer

   Because the greenhouse effect is a permanent and beneficial feature of the planet, not a problem, and it was operating for billions of years before humans existed. Blaming it for warming implies that the goal should be to remove it, which would leave the Earth at about minus 18 degrees Celsius. The correct statement is that human emissions have enhanced the greenhouse effect by raising the concentration of greenhouse gases, and that it is this enhancement, the extra absorption of outgoing longwave radiation, which is causing the observed warming. Examiners treat the distinction as a test of whether a candidate understands the mechanism, and confusing the two is the single commonest error in this topic.

5. 5

   Why is it wrong to confuse the enhanced greenhouse effect with the hole in the ozone layer?

   Model answer

   Because they are separate problems with different gases, different mechanisms and different effects. Ozone depletion is the destruction of stratospheric ozone, mainly by chlorine released from chlorofluorocarbons, which allows more harmful ultraviolet radiation to reach the surface, increasing skin cancer and damaging ecosystems; it is worst over Antarctica in spring. The enhanced greenhouse effect is the trapping of more outgoing longwave radiation by higher concentrations of greenhouse gases, which raises surface temperature. A thinner ozone layer does not cause global warming, and it in fact cools the stratosphere. The two are connected only in that chlorofluorocarbons happen to be both ozone-depleting and extremely powerful greenhouse gases, so controlling them under the Montreal Protocol helped with both.

6. 6

   Demonstrate knowledge and understanding of the greenhouse gases that matter, and of the human sources of each.

   Model answer

   Carbon dioxide is much the most important overall, released by burning coal, oil and gas, by cement manufacture and by deforestation and land-use change. Methane comes from ruminant livestock, flooded rice paddies, landfill sites, and leakage from coal mines and from oil and gas production. Nitrous oxide comes chiefly from nitrogen fertilisers and manure on farmland, and from some industrial processes and vehicles. Halocarbons, including chlorofluorocarbons and their replacements, come from refrigeration, air conditioning, foam blowing and aerosols. Tropospheric ozone forms from vehicle and industrial pollution. Water vapour is the most abundant greenhouse gas but its concentration is set by temperature rather than directly by emissions.

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