What are the speakers mainly discussing?

W:(4)Doctor White, we recently learned that a large piece of the Larsen Ice shelf in Western Antarctica broke off in early January. Could you please tell us exactly what happened?
M: I’d be happy to. The piece that broke off was over a thousand square miles in area. More recently and quite alarming to scientists is that a forty-mile crack, some 30 feet wide in places has torn through the ice shelf.
W: I understand that the scientific community is quite concerned. What is the significance of these events?
M:(5)Well, some scientists believe that this is a clear sign of global warming. Back in 1978, some American researchers predicted that Antarctica would show early signs of global warming due to the green house effect.
W: But couldn’t crumbling ice shelves also be a result of the unusual weather Antarctica itself has experienced lately?
M: That’s certainly possible, but you’ll have to remember(6 that over the past thousands of years, ice shelves have been through a lot of weather changes without breaking up.
W: I think most people know that if the ice cap over Antarctica melts, the level of the oceans will rise. What sort of impact will this have?
M: Well, the ice shelves currently insulate the Antarctic continent from wind, which slows down the melting. (7)If the winds cause even a tenth of the continent’s ice to melt, the world’s oceans could rise as much as 30 feet.