Will biggest danger from global warming be the change in diets?
来源：未知 作者：蔡垡感 时间：2019-03-08 03:15:05
Mark Pearson/Alamy Stock Photo By Michael Le Page Global warming will make it harder for people to eat more fruit and vegetables, leading to 500,000 deaths in 2050 that would otherwise have been avoided, according to a modelling study. That means the biggest health impact of climate change in the short term could result from its effect on how much fruit and vegetables we eat, rather than from heat waves, starvation or the spread of tropical diseases. Can this possibly be true? Well, maybe, but such findings come with many caveats. Studying how climate change will affect the food we eat is far from easy. For starters, we don’t know how much greenhouse gas will be emitted, so a range of possibilities need to be considered. Next, the resulting changes in the climate have to be projected, yet different models and model runs produce slightly different results. Then these results have to be used to predict changes in food production and ultimately consumption, which is anything but simple. For instance, if it becomes harder to grow a particular crop, prices will rise and farmers will plant more of it. Peter Scarborough of the University of Oxford, whose team carried out the latest study, relied on an existing model called the International Model for Policy Analysis of Agricultural Commodities and Trade for this part. Last but not least, the health effects of the changes in diet have to be estimated, on the basis of on meta-analyses of the relevant studies. The conclusion of Scarborough and his colleagues is that fruit and vegetable consumption will rise compared with today’s levels but not by as much as it would if without climate change, which means many more deaths in a world with climate change. “The climate change scenario results in 500,000 extra deaths in 2050 compared to a no-climate change scenario,” says Scarborough. “It is roughly equivalent to an annual increase of 500,000 deaths.” It’s a surprising result because not only is the number of estimated deaths higher than previous studies, the biggest cause of deaths is different too. A recent report by the World Health Organization, for instance, concluded that by 2050 climate change would lead to 95,000 extra deaths per year because of heat and 85,000 because of starvation, with another 70,000 people dying of things such as increased risk of malaria and diarrhoeal diseases. Scarborough’s team also concluded that many people would be forced to eat less food overall. The group calculates, however, that the 250,000 or so extra deaths attributable to undernutrition will be balanced out by fewer deaths caused by overeating. There will also be 30,000 fewer deaths because of lower consumption of red meat, they found. But this will be far outweighed by the 530,000 extra deaths due to reduced fruit and vegetable consumption relative to what would happen if there was no warming. Half a million is an awful lot of deaths, but by 2050 there will be about 80 million deaths annually. So warming will only cause a small fraction of global mortality, Scarborough says, as would be expected given that the dietary changes are modest. Because of the complexity of this study, there are many reasons for questioning its results. Climate models may underestimate the number of extreme weather events, such as the 2010 Russian heat wave that caused a food price spike, for instance. Or maybe our estimates of the health benefits of fruit and vegetables are wrong. Another factor is bioenergy carbon capture and storage (BECCS) – growing plants to soak up carbon dioxide. Our only hope of limiting global warming to 2 °C is to suck lots of CO2 out of the atmosphere, and BECCS is the least implausible way of doing this. But by competing for growing land with crops it could have a huge impact on food production – a factor that Scarborough’s team did not assess. And it would be very difficult to assess, says Rachel Warren of the University of East Anglia, UK, because it would require making many assumptions about what kind of biomass crops are grown and where, and so on. On the flip side, the study did not factor in the CO2 fertilisation effect. “CO2 effects would help yields of crops, particularly fruits and vegetables,” says David Lobell of Stanford University. But just how much it would help is not clear, because so many other factors influence plant growth – and food prices – too. And to complicate matters even further, higher CO2 levels could make plants less nutritious. Perhaps the biggest limitation of this study, though, is that it looks only at the next 30 or so years. Many studies suggest that climate change will boost food production initially, with serious losses kicking in in more and more regions as temperatures soar in the second half of the 21st century. Journal reference: The Lancet, DOI: 10.1016/S0140-6736(15)01156-3 Read more: NHS game-changers: