Most of them still depend hugely on rain-fed agriculture and on commodities.

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Around 450m live off rain-fed agriculture, and this year's monsoon rains, which between June and September provide 80% of India's precipitation, have been the scantiest in decades.

ECONOMIST: India's water crisis: When the rains fail | The

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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The assessment argues that modest outlays on rain-fed agriculture, in particular, could drastically improve the productivity of farming in poor countries and so help both to raise farmers' incomes and also to cut the need for an expansion of agriculture elsewhere.

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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.

ECONOMIST: Climate change and civilisation