The global energy balance

Overview

Global energy balance, earth's planetary albedo and double role that clouds play in the energy balance.

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

   What is meant by the global energy balance?

   Model answer

   Over the planet as a whole, and averaged over a year, the energy the Earth receives as incoming short-wave solar radiation is equalled by the energy it loses back to space as reflected short-wave radiation plus outgoing long-wave terrestrial radiation. Because input equals output, average global temperature stays roughly constant. The balance only holds for the planet as a whole: individual latitudes are in surplus or deficit.

2. 2

   Why does a square metre of ground at the equator receive far more solar energy than a square metre near the poles?

   Model answer

   Near the equator the sun is high overhead, so a beam of a given width strikes a small area and its energy is concentrated. At high latitudes the same beam hits the curved surface obliquely and is spread over a much larger area, so each square metre receives less. The oblique beam also travels through a greater thickness of atmosphere, so more of it is scattered, absorbed and reflected before it arrives. High latitudes also have snow and ice cover, which reflects much of what does arrive.

3. 3

   What is albedo?

   Model answer

   Albedo is the proportion of incoming short-wave radiation that a surface reflects rather than absorbs, usually expressed as a percentage. A high-albedo surface reflects most of the energy and stays cool; a low-albedo surface absorbs most of it and warms.

4. 4

   Give approximate albedo values for fresh snow, thick cloud, desert sand, forest and open ocean.

   Model answer

   Fresh snow reflects about 80-90 per cent; thick cloud about 70-80 per cent; desert sand about 25-40 per cent; forest about 10-15 per cent; a calm open ocean with the sun high about 6-10 per cent. This is why ice-covered polar regions absorb so little and why oceans absorb so much.

5. 5

   What is the Earth's planetary albedo, and where does the rest of the incoming radiation go?

   Model answer

   Roughly 30 per cent of incoming solar radiation is reflected straight back to space by clouds, the atmosphere and bright surfaces such as ice; that is the planetary albedo. The remaining 70 per cent is absorbed: about 20 per cent of the incoming total within the atmosphere itself, mainly by ozone, water vapour, dust and cloud droplets, and about 50 per cent of the incoming total by the land and sea surface.

6. 6

   Between which latitudes is there a net radiation surplus, and where is there a net deficit?

   Model answer

   There is a net surplus, with more energy received than lost, between roughly 35 degrees North and 35 degrees South. Poleward of about 35 degrees in each hemisphere there is a net deficit, with more energy radiated away than received. The deficit grows steadily towards each pole.

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