While the world's leaders bicker at international conferences over who is to blame for global warming, farmers are scrambling to adapt to an uncharted future of harsh weather that could shrivel their fruit and wipe out their wheat.

Already apple farmers around Grabouw and Elgin in the Western Cape, who need cold winters for optimal production, are struggling to produce export-quality fruit because of rising temperatures.

"If you just have a couple of days that go above a critical temperature, you get sunburn in these export apples," said Guy Midgley, head of the climate-change unit at the SA National Biodiversity Institute.

Midgley said scientists knew "with a high degree of confidence" that temperatures have warmed in much of South Africa over the past 30 years.

"The Western Cape, much of the coastal regions and, to a certain extent, the interior have warmed significantly."

In Durbanville on the outskirts of Cape Town, wine and grain farmers are counting the cost of a storm that pelted their crops with hailstones the size of golf balls late last month.

One of the farmers, Kosie Loubser, said the storm was unprecedented in the 80 years his family had owned the farm.

"We've been having very strange weather recently. It rains at the wrong time, and our grain doesn't ripen properly."

Porchia Adams, spokesman for the farmers' organisation Agri Western Cape, said: "The drought in the southern Cape is strange as well.

"They haven't had enough rain in a couple of years, which is very uncommon. And the Karoo drought has lasted for about 30 years."

Farmers have contributed to global warming through the use of nitrogen fertilisers that produce greenhouse gases as they break down.

Now many of them accept that they will have to change their ways as temperatures rise, evaporation increases and rain becomes more unpredictable.

"All of us should be concerned about it and do what we can," said Flip Nel, who farms 300ha of vegetables on the banks of the Limpopo.

Nel sells his produce to Woolworths, which this week launched a "farming for the future" initiative aimed at persuading the 500 or so farmers who supply it to cut the use of chemicals and water by building healthy soils through compost and earth-friendly cultivation methods.

"It's quite shrewd," said Mark Botha, an ecologist with the environmental group WWF. "Woolworths are going to be massively reducing their risk in 10 years' time by having a supply base that's got far more sustainable farms that are less vulnerable to the vagaries of climate change and pest outbreaks."

According to Botha, agriculture generated 18% of greenhouse gases and farm irrigation accounted for 70% of South Africa's water consumption.