The decline comes at a time when there is an urgent need to raise yields to feed as growing, global, population.
The scientists have found evidence that rising temperatures, linked with emissions of greenhouse gases, can damage the ability of vital crops such as rice, maize and wheat, to flower and set seed.
New studies indicate that for every one degree C rise in areas such as the Tropics, yields could tumble by as much as 10 per cent. The Intergovernmental Panel on Climate Change (IPCC), the team of scientists
that advise governments, estimate that average, global, temperatures in the Tropics could climb by as much as three degrees C by 2100.
Meanwhile a second group have found that key cash crops such as coffee and tea in some of the major growing regions will also be vulnerable over the coming decades to global warming.
They fear that desperate farmers will be forced into higher, cooler, mountainous areas intensifying pressure on sensitive forests and threatening wildlife and the quality and quantity of water supplies.
The findings on staple food crops have come from researchers at the International Rice Research Institute (IRRI) based in Manila, Philippines, which is part of the Consultative Group on International Agricultural
Research. The United Nations Environment Programme (UNEP) has worked with members of this Group on issues such as agro-forestry over recent years.
The findings on cash crops have come from GRID Arendal, a UNEP collaborative centre based in southern Norway, with internationally-renowned skills in mapping.
Klaus Toepfer, Executive Director of UNEP, said: "Billions of people across the tropics depend on crops such as rice, maize and wheat, for their very survival. These new findings indicate that large numbers are facing acute hunger and malnutrition unless the world acts to reduce emissions of carbon dioxide and other greenhouse gases".
Speaking at the latest round of climate change negotiations taking place in Marrakech, Morocco, he added: "A similar threat to cash crops is also emerging in areas such as East Africa. Poor farmers here face declining yields and incomes in the traditional coffee and tea growing areas pushing them into even more biting poverty. Just to survive, they will be forced to clear forests in higher, cooler, areas. This can only add to environmental damage which in turn can lead to increased poverty, hunger and ill-health".
"I would urge governments and delegates at this week's 7th Conference of the Parties to the United Nations Framework Convention on Climate Change to remember the billions of people living at or near the poverty line whose lives face ruin as a result of global warming. Delegates are here to agree the operational rule book for fighting climate change. This must not only be agreed and be effective, but brought into force as matter of urgency," said Mr Toepfer.
The work by IRRI is being spearheaded by Dr John Sheehy, a crop ecologist. He said that many food crops grown in the tropics are at or near their thermal limits making it difficult for them to withstand further rises in temperature.
The Tropics are between the Tropic of Cancer and the Tropic of Capricon and include large swathes of Africa, Asia and Latin America.
"In rice, wheat and maize, grain yields are likely to decline by 10 per cent for every one degree C increase. This effect appears to occur when temperatures in the tropics climb over 30 degrees C during flowering. I
would say we are at or close to this threshold where damage appears to occur. Heat damage has been seen in Cambodia and India. We are certainly seeing significant temperature rises with average night time temperatures at our own centre in the Philippines now 2.5 degrees higher than they were 50 years ago," said Dr Sheehy.
He said preliminary studies indicated that other functions of the plant could also be damaged by high temperatures. But flowering is critical because it is a one-off event from which there is no possibility of
recovery from failure. "One possible research solution is to find genes which will make flowering occur during the cool of the early morning," said Dr Sheey.
The scientists are poised to launch the Global Challenge Program to more precisely chart the likely effects of climate change on a wide range of crops.
"Initial results indicate that yields in the Tropics might fall as much as 30 per cent over the next 50 years, " said Dr Sheehy.
He said the fall was likely to be bigger than that caused simply by day time temperature rises. The scientists, who have carried out preliminary field and laboratory trials, believe night time temperatures may play a
part in damaging a plant's ability to produce pollen and pollinate itself. (see notes to editors)
"There are still great uncertainties, the actual fall might be less or it might be more. But even a small decline could be potentially devastating, "said Dr Sheehy.
He said their forecasts did not include other potentially damaging developments as a result of global warming including a rise in agricultural pests, impacts on pollinating insects and declines in rain fall.
Under scenarios developed by the Intergovernmental Panel on Climate Change, global warming could benefit agricultural production in some areas of the globe such as Canada and Siberia.
But Dr Sheehy said these gains are unlikely to offset the losses in the Tropics even if food surpluses in one region could be effectively distributed to those suffering shortages.
"The population of Asia is expected to increase by 44 per cent in the next 50 years and yields must at least match that growth rate if famine is to be avoided Currently more than half the people in South East Asia have a calorie intake inadequate for an active life, and ten million children die annually from diseases related to malnutrition. So any decline in yields as a result of climate change will have alarming consequences," added the scientist whose team are urgently trying to develop new strains of key crops that are more heat tolerant.
"We need to reduce emissions of greenhouse gases but we also need to invest in research and we need to do it now so we have the plants that can survivethe harshness of the coming climate," said Dr Sheehy.
The impacts on cash crops such as tea and coffee in traditional growing areas will also be stark, if the scientific forecasts of climate change prove sound
Svien Tveitdal, Managing Director of GRID Arendal, said: "The findings cover Uganda and Kenya, but the have implications for the Caribbean, Latin America and Asia where coffee and tea are also economically important produce".
In Uganda the total area for growing Robusta coffee would be dramatically reduced with an average temperature rise of 2 degree C.
"Only higher areas, the Ruwenzoris, South western Uganda and Mount Elgon, would remain as the rest would become too hot to grow coffee, according to our model," said Mr Tveitdal.
The overall area suitable for tea in Kenya would not be reduced by a warming of 2 degrees C, but existing plantations around Mount Kenya and the Aberdares would now lie outside the tea-growing temperature range.
"In these areas the tea belt would move upwards, where there are forests today, which indicates another potential future conflict," says the GRID Arendal study.
The impact on the economies of such countries could be serious. Agriculture earns Kenya an estimated 675 million US dollars a year in exports. Of this, 515 million comes from tea and coffee exports.
For Uganda, annual agricultural exports are worth around 434 million with tea and coffee worth $422 million.
Mr Toepfer said such impacts could be even more devastating to livelihoods if the current decline in coffee and tea commodity prices continues over the coming decades.
The coffee maps are part of a new web-based series of Vital Climate Graphics produced by GRID Arendal and accessible via the "climate portal " linked with the UNEP.net site.
The graphics include easily understandable maps and charts covering the latest reports on greenhouse gas emissions. They clearly demonstrate that despite a small reduction in emissions from countries who are party to the Kyoto Protocol, emissions are on the rise again.
The precise way in which rising day and night-time temperatures damage flowering and the setting of seed awaits more detailed research. However several effects are likely to be happening.
The number of pollen grains produced by the plant appears to decline and the pollen tubes or anthers appear to be also damaged. They need to extend to the area of the plant containing the female ova. Rising temperatures appear to reduce the distance to which the pollen tubes extend thus reducing the chances of fertilisation. Another effect appears to be a reduction in the carbohydrate found in the seeds.
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902 5034, E-mail: email@example.com or Dr John Sheehy on Tel: 63 2 845
0563 ext 711, E-Mail firstname.lastname@example.org
Vital Climate Graphics and UNEP's Climate Change Portal contact Svein
Tveitdal of GRID Arendal on Tel: 47 37 035730, Mobile: 47 90 589032
UNEP News Release 01/107