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　　Marshall and Winona are in Vancouver, planning their vacation.

　　M:Let's go somewhere warm and sunny. How about Miami? There are great seat sales on right now.

　　W:But just because the flight is cheap doesn't mean we should jet off to Florida. M:Why not?

　　W:Because jet aircraft aren't exactly environmentally friendly, and I don't want to contribute to the pollution problem.

　　M:I can understand that: apparently, a jumbo jet releases as much CO2sintosthe atmosphere in one cross-Atlantic flight as the average British motorist does in an entire year.

　　W:And it's greenhouse gases such as carbon dioxode that cause global warming. M:But are we sure it's really a problem? I mean, the earth's climate has always fluctuated.

　　W: But it's never warmed this fast--by some estimates,the planet's average temperature could be five degrees warmer by the year 2100.

　　M:That's a lot: the last ice age ended because of a similar-sized shift.

　　W:Ice caps are already melting, and the consequent rising sea levels could mean that low-lying coastal areas will one day be history.

　　M:Including cities such as Miami. So we should check it out now--while it's still there.