The weather may not always have been kind to cocoa farmers in West Africa, but until recently it was at least broadly predictable.

Temperature always hovered between 22 and 29 degrees Celsius, rains fell between April and July -- plus another short period between October and mid-November -- and the sun shone the rest of the time, fattening up cocoa beans and enabling drying.

Scientists say climate change may be altering these once reliable weather patterns in West Africa, which is the source of some two thirds of the world's cocoa.

Survival in a warmer world for the millions of smallholders who depend on cocoa may depend on moving to higher, cooler places or breeding new varieties, experts say.

Lately, farmers and agronomists say, weather has become hotter and more erratic. Temperatures often reach 32C and rains come too early or too late. Dry spells are harsher or the skies are overcast when they're meant to be sunny.

It's much hotter than it used to be, even two decades ago, said 71-year old cocoa planter Souleymane Drabre on his three hectare plantation in Ivory Coast, a country feeding a third of the world market.

That wouldn't matter if the cocoa tree was less sensitive, but yields suffer without the right mix of rain and sun at the appropriate times.

Production has definitely gone down. Because of the heat, the trees die. I used to get almost two tonnes out of my three hectares, now I'm lucky if I get a tonne, Drabre said, poking a shrivelled pod on his farm in Abengourou, near the Ghana border.

HEAT SENSITIVE COCOA

A report by the International Center for Tropical Agriculture (CIAT), an agricultural research institute in southwest Colombia, said in September that some growing zones in the two biggest growers Ivory Coast and Ghana may start to get too hot for cocoa by 2030, and that many more areas will by 2050.

A rise of 2.3 degrees Celsius by 2050 -- possible within consensus forecasts by climatologists -- would be enough to hurt production quite seriously in lowland regions, it said, although more elevated areas may become more suitable.

Already we are seeing the effects of rising temperatures on cocoa crops currently produced in marginal areas, and with climate change these areas are certain to spread, it said.

At a time when global demand for chocolate is rising fast, particularly in China, there is already upward pressure on prices ... This, combined with the impact of climate change, could cause chocolate prices to increase sharply.

The Organisation for Economic Cooperation and Development (OECD) said last week global average temperatures could rise by 3-6 degrees Celsius by the end of the century if governments fail to contain greenhouse gas emissions, outstripping the internationally agreed target of 2 degrees.

Delegates from nearly 200 countries will meet in South Africa from November 28 for climate talks with the most likely outcome modest steps towards a broader deal to cut greenhouse gas emissions to fight climate change.

But few expect to avoid a significant rise in temperatures.

A report by the U.N. Intergovernmental Panel on Climate Change (IPCC) this month urged countries to make disaster management plans to adapt to growing risks of extreme weather.

In the village of Sankadiokro, against a tropical landscape of cocoa trees and jungle, field workers were sitting by piles of cocoa pods, breaking them open and tipping out the goo-covered beans. Many pods are rotten with black pod disease.

There's been too much rain in too short a time. Fifteen-odd years ago, the rains used to be spread evenly across the season. Now it's so sporadic and too heavy when it comes, Lambert Aka, a 45-year-old farmer said.

That has increased black pod over the years, he said, by creating abnormally wet conditions in the wrong times.

The fact that the rains are so irregular, which is linked to climate change, threatens cocoa, said Ivorian agronomist Essien Paul. When it rains too hard it smothers the roots, then it gets too hot and the dry season becomes too harsh.

UNPREDICTABLE

There are still bumper years when the weather boosts the crop -- last season Ivory Coast managed a record 1.5 million tonnes thanks partly to favourable weather, but the long term trend does not look good.

Climate is still cyclical, so there are years that inflate production, there are also new plantations coming on tap, which has increased supply, said Ivorian agronomist Lassene Traore.

N'Gouandi N'Dri, 82, remembers when rainfall could be relied upon to be even and steady, some 40 years ago. Now his crops suffer from alternate floods and drought. He planted more to keep his output up, but his yields per hectare are falling.

Much will depend on how quickly West Africa adapts.

Higher altitude areas, like Ivory Coast's 18 Montagnes region, may take up some of the slack from lower output in lowlands, the CIAT report said.

But those are very few in coastal West Africa.

Ivorian authorities are trying to develop varieties that can resist black pod and other diseases when conditions are too wet.

Meanwhile, in neighbouring Ghana, scientists are working on techniques for replanting trees in cooler places or zones less prone to flooding -- as well as developing drought resistant strains that can deal with higher temperatures, said Francis K. Padi, a Ghanaian researcher involved in the projects.

We take individual traits from families that have drought tolerance and multiply them here rapidly, then we are able to wean them off and take them back to the field, he said.

(Writing by Tim Cocks; Editing by Anthony Barker)