The peak of her green interest was in the late 80s - a time of bubbling media coverage of acid rain, climate, waste and rainforest destruction.

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The bad news is that more of this sort of rain is expected as the climate changes.

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The Mediterranean coastal fringes have a Mediterranean climate with some rain between October and March, but the vast area of desert inland has very little rain at all and here some of the highest temperatures in the world have been recorded.

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It is unfair to categorise Syrian plans for irrigation as a waste of water when Turkey plans much more, yet has a climate better suited to rain-fed agriculture than Syria's or Iraq's.

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While southern California is a dry Mediterranean climate, one the highest rain falls in North America was recorded in the San Gabriel River watershed in 1943, at staggering 26.12 inches fell in 24 hours at Hoegees Camp.

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Of course, rain-fed agriculture is even more vulnerable to climate change than the irrigated variety, as Ms Parcak's Cambridge colleague Lauren Ristvet showed the conference with her study of northern Syria during the same period as the fall of Egypt's Old Kingdom.

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The earlier loss of carbon dioxide will occur because as the climate gets hotter and wetter, more rock is weathered by rain.

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Ferretti, vice president at the Chicago Climate Exchange, extolled emissions trading, noting that the practice had helped bring sulfur dioxide and acid rain under control much less expensively than traditional command-and-control regulation.

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