Climate Change and Crops: Hindering Harvests
气候变化与农作物：妨害产量
Changes in the climate are already having an effect on crop yields – but not yet a very big one
气候变化已经对农作物产量造成影响——但是迄今影响不大
The problems climate change looks likely to bring in the future may increasingly be visible in the records of the past. Not just in the far-off ages of surging sea levels following ice-age thaws, spikes in prehistoric temperatures correlated with natural releases of greenhouse gas and ancient civilisations brought low by drought, but in records from living memory – which are based on reliable measurements made at the time. Using such data researchers have now compiled an estimate of global changes in crop yields which can be put down to recent increases in temperature and decreases in rainfall (the world as a whole is getting wetter, but the rain has stayed away from some agricultural plains). The bad news is that they find that climate change has lowered the amount of maize (or corn, if you prefer) and wheat produced in a given area. The good news is that the effect is so far reasonably small.
冰河融化后的远古时代，海平面急剧上升，由于温室气体的自然释放，气温处于史前时代最高峰，古代文明由于干旱而衰落。在记忆可及的近代，人们根据可靠的测量对气候变化做了记录。根据过去的这些记录，气候变化可能造成的影响，在未来也许会越来越明显。研究人员利用这些数据，现在已经对全球农作物产量的变化做出了估计。农作物产量变化的原因可能是近来气温的升高和降水的减少（世界总的来说越来越潮湿，但是雨水远离了好些农业平原地区）。消息有好有坏。坏消息是，他们发现，在上述地区气候变化使玉米（或叫包谷，如果您喜欢的话）和小麦减产。好消息是，迄今为止气候变化的影响相当小。
David Lobell and Justin Costa-Roberts, of Stanford University, and Wolfram Schlenker, of Columbia University, first put together temperature and precipitation figures for the parts of the world where four staple crops – wheat, maize, rice and soya – are grown. Those four crops, between them, account for about 75% of the calories people end up eating, although a lot of the soya is fed to animals first.
斯坦福大学的戴维德·罗贝尔、扎斯汀·考斯特-罗伯特和加利福尼亚大学的沃尔夫然姆·史灵克，首次将世界上以小麦、玉米、稻子和大豆作为主要农作物的部分地区的气温和降水量结合在一起。他们共同研究发现，尽管很多大豆首先被用来喂牲畜，但人们最终所吃的热量有75%是这四种作物提供的。
It turns out that during the seasons in which crops grow, these arable areas had on average become significantly warmer in the 29 years after 1979. Some bits of Europe that grow wheat, for example, have heated up by a couple of degrees since 1980. The researchers then assembled models of how the yields changed from year to year, and against the longer trend, to find changes linked to temperature and rainfall that are independent of improvements through better farming. Finally they compared today’s yields with what their models say yields would have been with today’s farming but in the 1980’s climate.
研究发现，从1979年以后的29年里，在农作物的生长季节，这些可耕作面积的气温普遍越来越暖。例如，自1980年起，一些种植小麦的欧洲地区气温上升了两度。为了发现与气温和降水有关的产量变化，并且是不依赖于农耕方法的改进引起的变化，研究人员接着建立了产量逐年同比变化的模型，并与更长的趋势相对照。模型显示，产量变化除了1980年与气候变化有关外，可能与现在的耕作方法有关。最后，他们将现在的产量与模型显示的产量相对照。
Popping corn
夸大得像爆玉米花
For both wheat and maize, the results, published this week by Science, were negative. Globally, wheat yields are down 5.5% compared with what they would have been with no climate change, and maize yields are down 3.8%. For soya, some places saw improvements, some saw damage, with no real net effect on the global scale. For rice, warming brought a clear benefit for crops at higher latitudes and some losses in warmer places. Temperature played a bigger role than precipitation. The results seem to fit with previous studies into such effects in individual countries.
研究发表在本周的《科学》杂志上，对小麦和玉米来说，结果是否定的。在全球范围内，与没有气候变化的条件下相比较，小麦产量下降5.5%，玉米产量下降3.8%。大豆则某些地方增产，某些地方减产，没有真正相同的全球范围的影响。对稻子来说，气温变化对较高维度地区明显有益，而对较温和的某些地区则有损害。气温比降水造成的影响更大。这一结果似乎符合以前做过的关于在单个国家里这种影响的研究。
But there are caveats. For example, the analysis does not track changes over time in the areas being farmed, using instead a crop map from around 2000. And many agronomists hold that relying on year-to-year yield changes for modelling exaggerates the damage due to longer climate shifts. Farmers will tend to adapt. That said, subtler effects of climate change such as more sudden rains and particularly hot days with disproportionate effects on yield are left out, which might mean the study underestimates the effects.
但是需要做一些说明。例如，这一分析没有追踪现已被耕种地区的产量随时间的变化，反而使用的是2000年前后的作物分布图。很多农学家认为，依靠逐年同比的产量变化建模，夸大了较长时间气候变化引起的损害。那样农民将会适应。这说明，诸如较为突然地降雨等气候变化的较微弱的影响，和特别热的天气对产量的很大的影响被忽视，这可能意味着研究低估了影响。
Some people will be surprised, even dismayed, that comparatively modest climate changes are already doing measurable damage. But in context, it is quite small. Yields have been going up around the world despite the warming climate – and over 29 years those increases swamp the estimated global reductions due to climate. The sort of loss that climate change inflicts in a decade is often the sort of gain that better farming brings in a year. What is more, for wheat the increasing level of carbon dioxide that is changing the climate also makes photosynthesis easier, which should have increased yields enough to wipe out perhaps half of the climate-related loss (maize, which photosynthesises in a different way, is thus the bigger loser of the two in net terms).
有些人将感到吃惊甚至惊愕，因为不太大的气候变化已经造成了重大的损害。但是在上文中，这种损害相当小。尽管气候变暖，但世界各地的农作物产量一直在增加——而且29年来的这种增产超过了估计的由于气候变化造成的全球减产。十年里气候变化造成的减产，经常与一年里由于耕种技术改进带来的增产相等。而且，对小麦来讲，一直影响气候的CO2的含量增加也使光合作用更为容易，这将使产量增加到足以抵消大约一半的与气候有关的损失（玉米的光合作用方式不同，因此按净额计算受损较大）。
Then there is a point made by Richard Tol of VU University Amsterdam: farm yields show the worst of the situation. Easily achievable improvements in roads, markets and other things can increase the availability of food a lot even if farm yields stop rising. And people can adapt, at least to lowish levels of change; indeed the study provides evidence to help them do so as it shows which crops in a given country are the most affected by global warming. So Malthus looks beatable even when he sits astride the apocalyptic horse of climate change.
接着阿姆斯特丹自由大学的理查德·托尔指出一个要点：农作产量指示着最坏的情形。即使农作物产量停止增长，在公路、市场和其它方面容易取得改进能够使食物的有效利用大大增加。而且人们至少能够适应相当小的变化；的确，当研究表明在一个被考察的国家里哪一种农作物受全球升温影响最大时，研究提供的证据有助于人们去适应。因此即使马尔萨斯骑在预示气候变化大灾难的高头大马上时，看起来他也被击败了。
Moreover, there is a simple way more or less to abolish the effect of climate change on yield to date. According to William Cline of the Peterson Institute for International Economics in Washington, DC, at least 4% of the world’s grain is used to make ethanol for fuel. Most of this is doing little good for the environment, and stopping subsidies for such fuels would boost the supply of grain for feeding people on a scale similar to the hit that the past three decades of warming have provided.
此外，有一个简单的方法或多或少能够消除迄今为止气候变化对农作物产量的影响。根据皮特森国际经济研究所（位于华盛顿哥伦比亚特区）的威廉·克莱恩的研究，世界粮食至少4%被用于制造酒精燃料。这大部分对环境完全没有好的影响，而且停止对这种燃料的补贴将增加食用粮食的供应，增加的程度相当于过去三十年来气温升高造成的损失。
Still, in a world where more than a billion people are undernourished and the population is still growing, every little hurts. The net effects of climate and carbon dioxide, Dr Lobell and his colleagues calculate, is to make grain prices about 5% higher than they would otherwise be. Dr Lobell reckons that means about $50 billion a year. In some countries the effects are particularly large: climate change means Russian wheat yields are more than 10% down. As Mr Cline points out, growth in yields has slowed recently. And climate effects could speed up. Earlier work by Dr Schlenker has showed that plants have skewed preferences: they dislike temperatures beyond the optimum more than they dislike temperatures below the optimum, so increasing warmth can have worsening effects beyond a certain point.
尽管如此，在一个有十多亿人营养不足的世界里，而且人口一直在增加，每一点变化都是有害的。罗贝尔和其同事们计算，气候和二氧化碳的最终影响造成的损失，在粮食价格上比其它方面高出5%。罗拜耳计算，这意味着每年损失大约500亿美元。在有些国家影响特别大：气候变化造成俄罗斯的小麦减产超过10%。正如克莱恩指出的，增产最近变慢了。但是气候影响可能加速。史灵克做的早期研究表明，假象歪曲了参数选择：与不喜欢温度低于最佳点相比，他们更讨厌温度高于最佳点，因此升温可能使影响夸大超过了某一程度。
That might be keenly felt if patterns of warming shift. One of the reasons that the climate effects Dr Lobell and his colleagues have dug out of the data are not worse is that, although the planet as a whole has warmed up during the past 30 years, growing seasons in the parts of America which produce 40% of the world’s maize and soybeans have failed to follow suit. No one is quite sure why this might be and no one knows if it will last. That climate change has not yet done very much harm may be cheering, but the past offers no firm guarantees for the future.
如果升温的模式发生变化，这种情况可能会被强烈地感觉到。罗贝尔和其同事们从这些数据中得出，气候变化不是更差的，原因之一是，尽管地球整体来讲在过去30年里气温升高，但是在美国的部分地区生长季节气温并没有升高，这些地区生产的小麦和大豆占世界产量的40%。没有人能完全确定这其中的原因，也没有人知道它是否将持续。气候和变化迄今为止还没有造成非常大的损害，这也许是件令人高兴的事，但是过去并不对未来提供可靠的保证。
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