OK, negative feedback. In the early days of Planet Earth, the sun was about 20-30% less bright than today, but the early earth was actually warmer than now. How come? The early earth was blanketed by carbon dioxide (from volcanoes) at much higher levels than today and water vapour. This is an example of negative feedback - a cooler sun but a warmer earth. Negative feedback tends to lead to stability, by one process or effect being counteracted by another.
I've already described the process whereby at some points in the earth's story, a 'snowball' earth was caused - when carbon dioxide was removed from the atmosphere by chemical weathering (falling as carbonic acid rain, combining with silicates in rocks, with the carbon ending up in the oceans, in the shells of sea-creatures and ultimately forming sedimentary carbonate rocks - limestone. A further 'negative feedback occurred when the lack of water vapour in the atmosphere - caused by cooling - meant a new build-up of atmospheric carbon dioxide from volcanoes, ultimately warming the earth and melting the ice enough for the sun to warm the oceans.
Still not sure I've got that, exactly. More research needed!
On to 'aerosols', both natural (volcanic ash, desert dust, salt from sea spray, ash from forest fires) and man-made (soot and smoke from incinerators, dust from over-grazing and agricultural 'dust-bowls' etc) - these can have a cooling effect - the natural ones reflect back about 25% of the sunlight. Some polliutioon, however, like soot and smoke, mainly carbon particulates, can absorb heat and so have a warming effect (I think!).
More tasks to do, but I did the test and got 12/15 so not bad. And one of those incorrect answers was caused by plain misreading of the question, so I need to pay closer attention.
The course description said to allow about 3 hrs per week for this course, but I'm finding I need a lot more than that... and I'm only on week two. Need to set aside more time!
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