To answer this question you need to revisit the computer-based activity ‘Global Warming and Cooling’ (Book 1, Activity 8.1, p. 131).
(i) Go to the section called ‘Balancing sensitivities 2’ (Screen 17). Run the model with the initial values, which include an albedo of 0.087. This should result in a GMST of approximately 15.08 °C displayed under the graph (you may obtain a slightly different value). Which factors would lead toafall in GMST if they were to increase? (Guideline: four bullet points.)
(ii) Decrease the albedo to a value of 0.077 and run the model again. What is the resulting value of GMST? (It is sometimes difficult to set the albedo on exactly 0.077 so set the value as close as you can to 0.077 and state the value that you use.)
(b) You should have observed that if the albedo is decreased on its own in the simple model (Screen 17), then the GMST increases. However, if the albedo is decreased in the coupled model (Screen 21), then the water content of the atmosphere increases, the ice and snow cover decreases very slightly and the GMST increases more than in the simple model. Explain why the GMST increases in the simple model when the albedo is decreased. Then explain why, in the coupled model, the other factors change too and give a greater change in the GMST. (Hint: Your explanation should refer to the processes involved in the water cycle.) State whether this is an example of positive or negative feedback. Your answer should be written in complete sentences as continuous prose, rather than as a list. (Guideline: up to 140 words.)