AS CLIMATE talks in Marrakech drew to a close last week, the UN Environment Programme issued a stark warning of what global warming could do. Harvests of vital crops like rice, wheat and corn could plummet by a third over the next hundred years, it said, leaving billions to starve.
The UN body rang the alarm bells as part of a last-ditch effort to persuade the US and others to ratify the Kyoto Protocol on limiting greenhouse gas emissions. But experts contacted by 麻豆传媒 say that whatever the other consequences of global warming, its effects on crops are much more complex than UNEP is claiming. And the US stuck to its position that the Kyoto deal is not the best way fix the planet鈥檚 climate.
Delegates from 178 countries attended the Marrakech talks to finalise the wording of the Kyoto Protocol. It was also seen as a golden opportunity to convince wavering nations to ratify the protocol and join a global coalition committed to fighting climate change.
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UNEP quoted alarming studies by the International Rice Research Institute (IRRI) in Manila, Philippines, showing that crop yields can drop by 10 per cent for every 1 掳C temperature rise. They also predicted a dramatic decrease in the land area suitable for growing cash crops of coffee and tea, including almost all of Uganda (see Graphic).
Those are stunning effects. But according to Mark Rosegrant of the Washington-based International Food Policy Research Institute, which studies food production in the developing world, these figures are contentious. 鈥淭he finding of a 10 per cent reduction in yield per 1 掳C increase in temperature is not necessarily a consensus among climate scientists,鈥� he says. 鈥淢any believe that to 2020, and maybe even 2050, the net effect of global warming on crop yields will be positive.鈥�
Increasing carbon levels in the atmosphere could help make crops more fertile, he says, more than counterbalancing the direct effects of global warming. A sophisticated study from the IRRI in 1995 concluded that while crop yields would fall by 4 per cent over the next century throughout Asia as a whole, they would probably go up in Indonesia, Malaysia, Taiwan and parts of India and China.
Whereas UNEP鈥檚 warnings were based on studies that just examined the effect of rising temperatures, the earlier IRRI study took into account a range of factors, including carbon dioxide levels, nitrogen recycling, rainfall and the possibility of planting two crops in a season instead of one.
UNEP鈥檚 forecasts for the fate of coffee and tea harvests are also suspect because they are based on data from a simple climate model. 鈥淭his is an awareness raising exercise,鈥� admits Otto Simonett, a researcher from GRID-Arendal, the UNEP centre in Norway that did the study. 鈥淲e鈥檙e not coming at it from the agricultural side.鈥� The figures presume a uniform warming of 2 掳C across the tropics, says Simonett. But he admits that this is probably an oversimplification. These predictions also ignored likely changes in precipitation and the possibility of growing different strains of coffee in the warmer areas.
John Sheehy, a climate researcher at the IRRI, says the prediction for rice quoted by UNEP relies on the observation that the crop begins to be damaged by heat at 30 掳C and becomes sterile at 40 掳C-that鈥檚 a 100 per cent reduction in yield over 10 掳C, or 10 per cent per degree. But this sensitivity only applies when the plants are flowering, which they do only once in their life cycle. In many parts of the world crops can be protected from rising temperatures by timing planting so that they flower during cooler weather.
However, that could prove impossible in the tropics. Richard Betts from the Hadley Centre for Climate Prediction in Britain says: 鈥淚f our simulation is correct, by the end of the century most tropical land would consistently be experiencing annual mean temperatures of 30 掳C or above-some places up to 35 掳C.鈥�
To beat the tropical heat, Sheehy is now looking for genes that make crops flower earlier in the morning. But that still leaves the problem of higher night-time temperatures. Hotter nights force plants to respire more, diverting energy which they could otherwise use to create their seeds. This effect hasn鈥檛 been quantified yet, says Sheehy, but his preliminary research indicates that fewer pollen grains are formed and the pollen tubes shrink, reducing the chance of fertilisation.
What is clear is that night-time temperatures are rising. Nights in the Philippines are now 2.5 掳C warmer than they were 50 years ago. But even if the UNEP claims are an oversimplification, there is still cause for worry. With the growing population creating a greater demand for food, Sheehy says, 鈥渆ven a small decline could be potentially devastating.鈥�
