Why Global Warming Portends a Food Crisis

　　It can be difficult in the middle of winter — especially if you live in the frigid Northeastern United States, like I do — to remain convinced that global warming will be such a bad thing. Beyond the fact that people prefer the warmth to the cold, there's a reason the world's population is clustered in the tropics and sub-tropics: warmer climates usually mean longer and richer growing seasons. So it's easy to imagine that on a warmer globe, the damage inflicted by more frequent and severe heat waves might be balanced by the agricultural benefits of warmer temperatures.

　　A comforting thought, except for one thing: it's not true. A study published in the Jan. 9 issue of Science shows that far from compensating for the other damages associated with climate change (heavier and more frequent storms, increasing desertification, sea level rise), hotter temperatures will seriously diminish the world's ability to feed itself. David Battisti, an atmospheric scientist at the University of Washington, and Rosamond Naylor, director of the Program for Food Security and the Environment at Stanford University, analyzed data from 23 different climate models and found a more than 90% chance that by the end of the century, average growing season temperatures would be hotter than the most extreme levels recorded in the past.

　　That means that barring a swift and sudden reduction in greenhouse gas emissions, by the end of the century an average July day will almost certainly be hotter than the hottest heat waves we experience now. And the extreme heat will wilt our crops. Battisti and Naylor looked at the effect that major heat waves had on agriculture in the past — like the ruthless heat in Western Europe during the summer of 2003 — and found that crop yields had suffered deeply. In Italy maize yields fell by 36% in 2003, compared with the previous year, and in France they fell by 30%. Similar impacts were seen during a major heat wave in 1972, which decimated farmers in the former Soviet Union, helping to push grain prices to worryingly high levels. If those trends hold in the future, the researchers estimate that half the world's population could face a climate-induced food crisis by 2100. "I'm very concerned," says Naylor. "How are we going to feed a world of eight or nine billion, with the effects of climate change?"

　　It's true that as temperatures warm, there is likely to be a temporary, beneficial effect on agriculture. (Like people, plants generally prefer the warmth to the cold, and they may flourish with rising levels of CO2.) But as research from Wolfram Schlenker at Columbia University shows, as average temperatures continue to warm, those benefits dwindle and then eventually reverse, and crop yields begin to decline. "It simply becomes too hot for the growing plants," says Naylor. "The heat damages the crops' ability to produce enough yield."

　　What's more, Battisti and Naylor are looking only at the impact of higher temperatures in their study — not at the possible impact of changing precipitation patterns. Yet many climatologists believe that global warming will make dry areas drier and further damage farming, which is especially dire news for sub-Saharan Africa, a region that already struggles with heat waves, droughts and famines, even as population continues to grow. "Climate change is going to be a major concern for Africa," says Nteranya Sanginga, the director of the Tropical Soil Biology and Fertility Institute of the International Center for Tropical Agriculture, in Nairobi. "We could lose whole growing seasons."

　　With these frightening predictions in mind, we need to try to heat-proof our agriculture. That can be accomplished by using crops that have proven resistant to extreme heat — like sorghum or millet — to breed hybrid crop varieties that are more capable of withstanding higher temperatures. We'll need to drop any squeamishness about consuming genetically modified crops — unless we can tap the power of genetics, we'll never feed ourselves in a warmer world. But we'll need to act quickly — it can take years to breed more heat-resistant species, and investment in agricultural research has shriveled in recent years.

　　We also need to focus on improving the agricultural productivity of those parts of the world that have been left behind by the Green Revolution — such as Africa, where average crop yields per acre remain well below those in Asia or the West. One simple way is to increase the amount of fertilizer available to African farmers. Sanginga notes that about 440 lbs. (200 kg) of nitrogen fertilizer is generally needed to grow five tons (5,000 kg) of maize, but the average African farmer can afford only 8 lbs. of fertilizer. We can also work on safeguarding the degraded soils of Africa, where almost 55% of the land is unsuitable for any kind of cultivated agriculture. Help is on the way: the African Soil Information Service is launching a real-time, digital map of sub-Saharan Africa's soils, which should allow farmers and policymakers to make better use of the continent's agricultural resources. "Farmers need to know when to invest, and when to hold back," says Sanginga, who is involved with the mapping project.

　　There's a limit, however, to our ability to adapt to climate change — we still need to reduce carbon emissions, sharply and soon. If we fail, a warmer future won't just be uncomfortable, it will be downright frightening. "We need to wake up and take care of this," says Naylor. "We won't have enough food to feed the world today, let alone tomorrow."

　　如果你像我一样住在美国寒冷的东北部，严冬可能是很难过的——全球变暖将是一件坏事是令人信服的。事实上人们喜欢温暖胜过寒冷，原因之一，世界人口大部分集中在热带和亚热带地区：温暖气候较长且季节富于生长。所以很容易想象全球变暖由更频繁更严重的热浪所致的损害可能获益于农业的热温而保持平衡。

　　除一种情况外，一个舒适思想是不真实的。在一月九号出版的自然科学杂志上发布的一项研究显示：与气候相关的其它损害是无法补偿的(更严重更频繁的风暴，沙漠化增加，海平面上升)，高温将严重减少世界供养自己的能力。华盛顿大学大气科学家大卫巴提斯蒂及斯坦福大学食品安全与环境项目总裁罗萨曼那依勒分析发析23个不同气候模式资料发现：到本世纪末，平均季节温度增长将高于史上所记载的最高水平。

　　那意味着阻止温室气体排放的快速和突然减少。到本世纪末，平均七月天气温几乎肯定高于我们现在所经历的最热的热浪温度且炎热会使我们的庄稼枯萎。巴提斯蒂和那依勒观察过去主要热浪队农业的影响——像2003年夏西欧的无情热浪——发现粮食产量大大减低，2003年意大利玉米产量与前一年相比降低了36%，法国降低了30%。同样影响可见在1972年的热浪中，前苏联十分之一的农民帮助把粮食价格推上了峰巅。如果那些趋向将来停止的话，研究人员估计：到2100年世界上一半人口将面临气候导致的粮食危机。“我很关心，”那依勒说，“由于气候变化的影响，我们将怎样养活世界八九十亿人口呢?”

　　全球变暖可能是暂时的是获益于农业的结果，这是真的。(一般来说，植物和人一样喜欢温暖胜过寒冷，它们可能随着CO2的升高而更加茂盛)像哥伦布大学瓦尔福兰姆斯克伦克的研究显示一样，平均气温持续升高，那些益处减少而且最终改变，粮食产量开始下滑。“这对植物生长来说仅仅是天气太热的缘故”那依勒说，“高温损坏庄稼产粮能力。”

　　另外，巴提斯蒂和那依勒的研究仅仅看到了高温的影响力——而不是正在改变的快速模式可能影响。然而许多气候学家认为，全球变暖将使干燥的土地变得更干，进一步损害耕种，这对非洲撒哈拉以南地区是特别可怕的消息，这地区总是跟热浪，干旱及饥饿甚至人口持续增长抗争。“气候改变将成为非洲的主要问题，”那罗比热带农业国际中心热带土壤生物与繁殖学会会长纳特兰亚桑锦格说，“我们可能会失去所有的生长季节。”

　　由于这些可怕的预测在心，我们必需要尝试我们的抗热农业。这可能通过利用已知的耐高温农作物来完成——像高粱或小米——培育杂交能耐高温的多种农作物。我们不必惊讶食用遗传性改变了的粮食——除非我们能利用遗传学的力量，否则全球变暖我们将无法养活我们自己。我们要快速行动——培育更多的抗热植物要花费几年的时间且农业研究的投资近几年也减少了。

　　我们还要集中精力提高落在绿色革命后面部分地区农业生产——像非洲，平均每英亩粮食产量持续低于亚洲和西部地区。一个简单的方法就是增加非洲农民化肥的使用量。桑锦格指出，一般情况下生产五吨(5000kg)玉米大约需要440lbs.(200kg)氮肥，而非洲农民只能担负8lbs.的化肥。我们也能保护退化了的非洲土壤，那里几乎55%的土地不适合任何耕种的农业。援助在进行：非洲土地信息服务社发布了一个非洲撒哈拉以南土地的真实时间，数码地图，它允许农民和官员充分利用这块大陆的农业资源。“农民要知道何时投资何时停止。”参与制图项目的桑锦格说。

　　然而我们适应气候变化的能力是有限的——我们还要急剧且快速减少碳的排放量。如果我们做不到的话，全球变暖的将来不仅仅是不舒适而且是非常可怕的。“我们要觉醒并关心此事，”那依勒说，"我们没有足够的粮食养活当今世界上的人口，更何况明天呢。"