气候变化 呈"钟形曲线"变化的天气

ARE heatwaves more common than they used to be? That is the question addressed by James Hansen and his colleagues in a paper just published in the Proceedings of the National Academy of Sciences. Their conclusion is that they are-and the data they draw on do not even include the current scorcher that is drying up much of North America and threatening its harvest. The team's method of presentation, however, has caused a stir among those who feel that scientific papers should be dispassionate in their delivery of the evidence. For the paper, interesting though the evidence it delivers is, is far from dispassionate.

热浪来袭真的比以前更加频繁了吗？詹姆斯•汉森与他的同事试图在《美国国家科学院院报》上发表的一篇新文章中回答这一问题。他们得出的结论是，情况的确如此，而且得出这一结论的依据这还不包括最近北美的酷热天气。现在那里的大部分地区已经干旱无比，当地农作物的收成也因此受到了威胁。然而，詹姆斯•汉森与他的同事所采用的展示方案却在一群人中引起了骚动。这些人认为科技论文在表述证据时应客观公正，但有意思的是，该文章在表述证据时却远未做到不偏不倚。

Dr Hansen, who is head of the Goddard Institute for Space Studies, a branch of NASA that is based in New York, is a polemicist of the risks of man-made global warming. Despite his job running a government laboratory, he has managed to get himself arrested on three occasions for protesting against those he thinks are causing such climate change. He clearly states in the paper's introduction that he was looking for a way of conveying his fears to a sceptical public.

总部在纽约的戈达德太空研究所(GISS)是美国国家航空航天局的一个分支部门，该研究所的负责人汉森博士能言善辩，他声称人为因素导致的全球变暖将给人类带来危害。尽管汉森博士主管政府实验室，他却参加过抗议活动，以此来反对那些他认为将造成此类气候变化的行为，而他也因此"如愿"被警察逮捕过三次。汉森博士在他的文章序言中明确指出，他正在寻找一种方法，以此让那些对他的观点持怀疑态度的公众体会到他的焦虑。

Some of that scepticism is connected with the fact that although changes in the climate will inevitably result in changes in the weather, ascribing any given event-such as a local heatwave-to climate change is impossible. Dr Hansen has therefore tried to go beyond the study of individual causes by demonstrating that what was once unusual is now common.

尽管气候变化将不可避免地导致天气变化，但不能将所有特定事件（如地方性的热浪）都归因于气候变化，而公众的怀疑态度中有一部分就与这一事实有关。因此，汉森博士通过论证过去人们眼中的异常天气如今已是十分常见，试图使他的研究不仅仅局限于个别原因。

Longer, hotter summers

夏天愈加漫长炎热了

To do so, he and his colleagues took 60 years' worth of data (the period from 1951 to 2011) from the Goddard Institute's surface air-temperature analysis. This analysis divides the planet's surface into cells 250km (about 150 miles) across and records the average temperature in each cell. The researchers broke their data into six decade-long blocks and compared those blocks' statistical properties.

为此，汉森博士与其同事从戈达德研究所收集了60年（从1951年至2011年）地表气温分析数据。这些分析材料将地球表面分成了一个个宽250公里（约150英里）的区块，并记录了每个区域内的平均温度。研究者将这些数据按十年一组分成了六个时间段，并对这些时间段内的统计特性进行了比较。

They looked in particular at the three months which constitute summer in the northern hemisphere (June, July and August). First, they created a reference value for each cell. This was its average temperature over those three months from 1951-80. Then they calculated how much the temperature in each cell deviated from the cell's reference value in any given summer. That done, they plotted a series of curves, one for each decade, that showed how frequently each deviant value occurred.

研究者特别关注了北半球夏季三个月（六月、七月、八月）的温度。首先，他们计算出了1951年至1980年每一个区块夏季三个月的平均温度，并将这些数据作为每个区块的参考值。接着，研究者又计算在任意指定的夏季内每个区块内的温度分别与各自的参考值存在多少偏差。这一步完成后，他们以十年为单位绘制了六条曲线，这些曲线显示出了每一个偏差值发生的频率。

Since small deviations are common and large ones are rare, the result of plotting data in this way is a curve shaped somewhat like the cross-section of a bell. Such distributions can be modelled by a mathematical function known as the normal distribution-or "bell curve".

由于小的偏差时有发生，而大的偏差则较为少见，因此用这一方法对数据进行描绘，得出的曲线形状就像一个钟的横截面。这样的分布状态可以用数学上的一个函数来表示，即正态分布，又称"钟形曲线"。

Whether based on data or a mathematical ideal, such a curve always has two parameters. These are its mean (the value of all of the data points added together and divided by the number of points; this is also the peak of an ideal curve) and its standard deviation, which measures how wide the bell is. The standard deviation is calculated from all of the individual deviations of the data points.

无论是基于数据还是理想数学模型，钟形曲线都有两个参数。一个是平均值（将所有数据的值相加，除以总数所得；同时也是这条理想曲线峰值的横坐标值），另一个是它的标准差（代表钟形曲线的"胖瘦"）。标准差可以通过计算所有数值的方差（再开方）得到。

To see what was going on, Dr Hansen superimposed the actual curves for each decade from the fifties to the noughties on a normal distribution, which acted as a reference curve. To make all the curves comparable, he expressed the values of the actual deviations as fractions of a standard deviation, and their frequencies as proportions of their total number.

为了弄清气候变化，汉森博士以正态分布曲线为参照曲线，将其与六十年中每十年一条的实际曲线相叠加。为了使所有这些曲线具有可比性，汉森博士将实际偏差值用其与所属样本标准差的比例来衡量，用数据发生偏差数与数据总数的比例表述其频率。

As the chart (right) shows, there are two trends. First, the peaks of the data-based curves move right, over time, with respect to the reference curve. In other words, the average temperature is rising. Second, more recent curves are flatter. A flatter curve means a bigger standard deviation and a wider spread of results.

如（右）图所示，钟形曲线表现出了两种趋势。第一，对照参考曲线，随着时间的推移，这种数据型曲线的峰值会向右移，也就是说地球上的平均温度在上升。第二，时间距离现在越近，曲线越"矮胖"，这也就意味着标准差越大，温度数据分布越广。

If the mean of each curve were the same, such flattening would imply both more cold periods and more hot ones. But because the mean is rising, the effect at the cold end of the curves is diminished, while that at the hot end is enhanced. The upshot is more hot periods of local weather.

如果每一条曲线的平均值相同，那么这种平滑过程也就意味着地球上的寒冷期和炎热期的出现次数都会增加。但由于这些曲线的平均值在增加，因此地球上寒冷期的出现次数会减少，而炎热期的出现次数则会增加。最终导致的结果就是各地出现炎热天气的次数越来越多。

Moreover, the bell-curve method makes it possible to say just how much more hot weather there is. Dr Hansen defined extreme conditions as those occurring more than three standard deviations from the mean of his reference curve. In that curve, this would be an eighth of a percent at each end, which is more or less the value in the curve for 1951-61. Nowadays, though, extreme conditions (or, at least, those that would have been considered extreme half a century ago) can be found at any given time in about 8% of the world.

此外，有了钟形曲线，人们还能判断炎热天气的出现频率比过去增加了多少。汉森博士下了定义：如果在他的参考曲线中某一数值偏离其平均值达到或超过三个标准差，那么这就是极端天气。在参考曲线中（标准的正态分布曲线），左右两端数据中偏离均值超过三个标准差的极端数据应该恰好分别为0.125%(百分之一的八分之一)，而1951至1961年这一时间段的极端数据分布情况大致上也符合这一特征。但现在世界上约有8%的地区在特定时间内都有极端天气（或者，至少在半个世纪前会被认为是极端天气）的出现。

Local weather patterns do, of course, have local causes. To that extent, they are accidental. But Dr Hansen's analysis suggests that claims there is more hot weather around than there used to be have substance, too.

当然，各地天气模式的出现的确包含着地方性的原因。在这种程度上说，它们的发生是具有偶然性的。但汉森博士的分析表明，各地的炎热天气的确比以往多这种说法也是有依据的。

Nothing in his analysis speaks of the cause of that substance. That is deliberate. As he says in the paper, he wants the data to speak for themselves-though he is personally convinced that the cause is human-generated emissions of greenhouse gases such as carbon dioxide. But as the United States bakes in what may turn out to be a record heatwave, he hopes he might now persuade those for whom global warming is, as it were, on the back burner, to agree that it is real, and to think about the consequences.

但他的分析并没有涉及到这种依据的来源，而这也是汉森博士有意为之。正如他在文章中所说，他希望数据能说明问题——尽管他本人相信气候变暖是因为人类排放二氧化碳等温室气体造成的。但现在，一股或许是有史以来最猛烈的热浪正在炙烤着美国。他希望他现在能够说服那些跟过去一样搁置考虑全球变暖问题的人，使他们认同全球变暖现象的确存在并考虑其后果。