MADRID, Sept 12 (Reuters) - Climate change has become the prime cause of an accelerating spread of deserts which threatens the world's drylands, the United Nations' top climate official said on Wednesday.

Yvo de Boer said that, although growing populations in dry areas were putting serious stress on the environment through over-grazing, water demand, deforestation and other activities, climate change was the greater threat.

"Climate change will change weather patterns even more ... especially in Africa, rainfall is decreasing and climate change is exacerbating desertification. Climate change is probably the biggest factor," said de Boer, who was speaking on the sidelines of a U.N. desertification conference in Madrid.

Drylands, including many in crop producing areas, cover 41 percent of the world's land surface, of which 10-20 percent are already degraded.

De Boer added that desertification would, in turn, speed up climate change.

"You'll see a sort of feedback mechanism ... quite a lot of carbon is captured in soil, so with more desertification (exposing the soil), you also get more CO2 emissions. They are two halves of the same coin."

Major deserts like the Sahara, Gobi and Kalahari are all expanding and desertification has begun to threaten countries like Spain and Kazakhstan, where swathes of cropland have had to be abandoned in the last three decades.

The U.N. estimates that more than 250 million people are directly affected by desertification and approximately one billion in over 100 countries are at risk.

De Boer said he hoped the two-week conference would agree a 10-year strategy to fight desertification that better emphasised the link with climate change. He added that member states needed to pledged millions of additional dollars to fund the programme.

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