Activity: Investigating Arctic Sea Ice
The data in Energy budget figure in the textbook can be used to set up a spreadsheet to investigate the effect of changing one of the parameters, in this case the amount of Arctic Sea Ice and the amount of energy 'reflected by surface'.
Th spreadsheet below has been set up with these data values. They are in column B.
A simplified climate model has been constructed in the remaining columns. How was it constructed and how can it be interpreted?
Physics behind the model
If the amount of energy reflected by sea and land ice decreases from 30 to 25 Wm–2, then 5 Wm–2 of energy has been absorbed by sea water. This increases the total amount of energy absorbed by the earth's surface, which is greater than the amount of energy emitted, and so the surface temperature increases. This leads to more energy being emitted. These changes are in column D.
But before the changes can be calculated, the model needs to make an assumption. How is the extra energy being emitted spread across the different forms of thermal, evapo-transpiration and surface radiation? This model assumes that the proportions stay unchanged. These values have been calculated in column C.
The extra energy emitted by the surface is mostly absorbed by the atmosphere, with a small amount going straight out into space. The atmosphere then emits this extra energy; some out into space, but some back down to the surface. Once again, the proportions are kept the same. These values have been calculated in column E.
The consequence of the changes in column D is to shift the whole earth away from energy equilibrium as you can see from the Energy balance values in column D compared with the starting values in column B. The changes in column E make some amends, but by the atmosphere emitting a small amount of energy back to the surface, the whole process repeats itself but with smaller values. With repeated iterations, the values in the energy balance get closer to agreement, when the surface will stabilise at a higher temperature and a higher energy output.
Questions
1. If the energy 'reflected by surface' changed from 30 Wm–2 to 25 Wm–2, how much more energy would the surface be emitting after the new equilibrium is reached?
2. The energy out for the whole earth is after the new equilibrium is reached back to the value it started at. Why is this the case?
3. Use the Stefan-Boltzmann relationship to determine the factor by which the temperature of the surface in Kelvin has increased. If the average temp is 15 °C, what will be the new temperature?
4. If sea ice melts, the sea level does not rise. Is the melting of sea ice an environmental concern or a symptom of climate change elsewhere?
5. Try different values for the energy reflected by the surface and then graph 'Increase in energy emitted by the surface' against ' Energy reflected'. *
A second spreadsheet looks at the effect of increased concentration of carbon dioxide in the atmosphere. In this investigation, the energy reflected by the surface is reset to 30, but the proportion of the radiation from the surface that escapes to outer space has been reduced to 5% from 8.1%
Complete questions 1–3 and 5 from above, but applied to this scenario.