If you want to go on eating regularly in a rapidly warming world, live in a place that's high in latitude or high in altitude. Or be rich, because the rich never starve. But otherwise, prepare to be hungry.

That's the real message of the Intergovernmental Panel on Climate Change's report on the effect of warming on human beings, released this week. The main effect is on the food supply. Of course, everybody who was paying attention has known that for years, including the scientists. It's just that scientists are professionally cautious, and will not say anything that they cannot prove beyond any shadow of a doubt.

But the World Bank, for example, has long known how much food production every major country will lose when the average global temperature has risen 2C. At least seven years ago it gave contracts to think-tanks in every major capital to answer that question.

What the think-tanks told the World Bank was that India would lose 25 per cent of its food production. China, I have been told by somebody who saw the Beijing think-tank's report, would lose a catastrophic 38 per cent.

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But these results have never been published, because the governments concerned did not want such alarming numbers out in public and were able to restrain the World Bank from releasing them.

The armed forces of many countries have been incorporating predictions of this sort into their scenarios of the future for at least five years. The Pentagon and the Central Intelligence Agency in the United States and the British armed forces have been doing it openly, and I have seen strong indications that the Russian, Indian, Pakistani, Chinese and Japanese armed forces are also doing so.

The scenarios do not only predict serious food shortages in most tropical and sub-tropical countries (which contain about 70 per cent of the world's population). They predict waves of refugees fleeing from these countries, a proliferation of failed states in the sub-tropics, and even wars between countries sharing the same river system when there's not enough water to go around.

As for what will happen to crop yields by 2050, assuming an average global temperature increase of 3C by then, you have to go elsewhere for information. The military doesn't plan that far ahead.

But the World Resources Institute published a map recently that estimated the losses country by country by 2050, and according to the WRI's calculations they are really bad. Crop yields will be down everywhere in the Middle East and the Mediterranean countries. In Morocco, Egypt and Saudi Arabia, they will be down by 50 per cent.

All of Africa will be down except Lesotho, Rwanda and Kenya, which are all or mostly above 1000m in altitude.

Food production will be down in almost all of South America except Chile, also very high, where it will be up.

North American crop yields will be down, too, except in Canada and a few US states along the Canadian border. High latitude is even better than high altitude.

In Europe and Asia, latitude is decisive. Countries far away from the equator will still be doing well; countries even a bit closer to the equator will be hammered.

Russia, Scandinavia, Germany and Poland will be producing more food than ever, but southern Europe will have lost production.

India, China, and all of Southeast Asia will be sharply down, as will Australia - but Japan will be only a bit down and New Zealand will be sharply up. It pays to be an island, too.

But this is not a "mixed" result, in the sense that it all works out about even. The total population of the countries where food production will be stable or higher in 2050 will be less than half a billion.

At least 8.5 or 9 billion will live in countries where food production has fallen, sometimes very steeply. It will be a very hungry world.

Gwynne Dyer is an independent journalist whose articles are published in 45 countries.