This is an exercise for developing creativity.
Ask a question that starts with: 'What would happen if...' and see what responses you get. The first responses will probably be the obvious consequences of the situation, so encourage people to come up with more oblique, less obvious, counter-intuitive or seemingly contradictory suggestions.
You could use questions like: 'What would happen if the sea level rose by three metres?' or 'What would happen if the average world temperature increased by 3ºC?'
The first answers will be the ones that we have all heard about on television or have read in the newspapers, but it won't be long before you start getting responses like 'Some houses would increase in value because they would now be beach-front properties' or the 'The British Isles would have colder winters'
That last one I found particularly surprising it seems that an increase in the sea temperature of the Gulf of Mexico could cause the Gulf Stream, which keeps the British Isles wet and mild, to fail.