Is Earth Getting Darker?

地球变得越来越暗了吗？

The sun is not growing weaker, yet its light appears to be dimming. Between 1960 and 1990, some scientists believe, the amount of solar radiation reaching Earth's surface may have declined as much as 10%--and in some places, Hong Kong, for example, more than 35%.

太阳不是变得越来越弱，但太阳光似乎正变得越来越黯淡。有些科学家认为，从1960年到1990年，到达地球表面的太阳辐射量可能下降了10%，而有些地方, 例如香港，下降了35%以上。

What was going on? Well, it appears that increased air pollution during those 30 years--over Asia, in particular--with the help, perhaps, of some increased cloudiness, may have exerted a cooling influence on the surface of the planet even as carbon dioxide and other greenhouse gases were encouraging the atmosphere to warm. The impacts of that tug-of-war on the climate system could be devilishly difficult to untangle. At the same time, no task could be more urgent. For if global pollution has helped keep global warming in check, says Veerabhadran Ramanathan, an atmospheric scientist at the University of California at San Diego, then the full impact of the buildup of greenhouse gases has yet to be felt. This week, at the American Geophysical Union meeting in Montreal, Ramanathan and others will be presenting the latest data on the solar-dimming problem and pondering its implications for the climate system as a whole.

这是怎么了？在过去30中，全球——特别是亚洲——不断加剧的大气污染，借助数量有所增加的云度可能对整个地球表面起了一种冷却作用，同时二氧化碳和其它温室气体却在促使大气变暖。要摆脱这种拔河式效应对气候系统的影响是极其困难的。但同时，没有别的什么任务能比这更迫切的了。加州大学圣地亚哥分校的大气科学家Veerabhadran Ramanathan说，如果全球性污染有助于控制全球气候转暖这一现象，那么，世人必将遭受温室气体堆积造成的全面影响。本周，在蒙特利尔召开的美国地球物理联盟会议上，Ramanathan和其它人士将向大会展示有关太阳变黯问题的数据，期望与会人士能认真考虑太阳变黯对整个气候系统的实质性影响。

Many scenarios for global warming, for example, invoke a speedup in the hydrological cycle by which water evaporates and then comes down as rain. The cooling produced by solar dimming, however, may slow the rate of evaporation, while higher up in the atmosphere the pollutants responsible for absorbing and reflecting sunlight are likely to interfere with the process that produces rain.

比如，全球变暖的许多情况加速了水借以蒸发、随后变成雨降下的水文循环周期。然而，太阳变黯所产生的冷却作用也许会降低水的蒸发速度，而大气层上部吸收和反射太阳光的污染物很可能对雨产生的过程进行了干预。

Why? These pollutants, which take the form of tiny, airborne particles called aerosols, act as nuclei around which cloud droplets form. The problem is, there are too many aerosols in the atmosphere competing for water molecules, so the cloud droplets that form are too small and never become weighty enough to fall to the ground. As a result, says Beate Liepert, an atmospheric physicist at Columbia University's Lamont-Doherty Earth Observatory, the atmosphere could be filled with moisture while Earth's surface thirsts for rain.

为什么？这些表现为极其微小的气载微粒(叫做悬浮微粒)形式的污染物扮演核心角色，在它周围形成微小的云状水滴。问题是，大气层中有太多的争抢水分子的悬浮微粒，因此，所形成的云状水滴显得又太小，永远达不到足以落到地面的重量。哥伦比亚大学拉蒙-多尔蒂地球观测所的大气物理学家比特•利伯特说，因此，尽管大气层可能充满了湿气，但地球表面却干燥无比，渴望下雨。

Many questions remain, including the true extent of the dimming. One analysis pegs the average worldwide darkening to be about 4% over three decades, while another computes it to be more than twice that much. There are also questions about the reliability of the devices that measure the sunlight reaching Earth's surface. Known as radiometers, these instruments are nothing more than flat, black solar collectors capped with glass. They are sometimes finicky; a smudge of dirt or a speck of dust can cause bogus readings and change the calculated results.

许多问题——包括太阳变黯的真实程度——尚无解决方案。一位分析人员认定，在过去约三十年中，世界范围太阳平均变暗的程度为4%，而另一位分析人员估计变暗的程度超过此数的两倍。测量到达地球表面阳光的设备是否可靠也是一个问题。测量阳光的仪器叫做射线测定器。这些仪器无非是一些罩着玻璃的黑色平板太阳能收集器。这些太阳能收集器有时过分挑剔：一块小小的污迹或一点点灰尘都可能造成虚假读数，改变计算结果。

Solar dimming, in other words, is a problem still in the process of being defined, and as its dimensions become clearer, so will the nature of the challenge the world faces. Although scientists have done a lot of thinking about global warming, they are just beginning to grapple with the problem of how global warming and solar dimming interact. As Ramanathan puts it, "It's like we have a new gorilla sitting down at the table"--and it could turn out to be a very big gorilla indeed.

换句话说，太阳变黯仍然是一个尚处在定义过程中的问题，因此，随着其轮廓变得越来越清晰,世界所面临挑战的性质也会变得愈加清晰可见。虽然科学家们对全球气候变暖进行了大量的研究和思考，但他们也只是刚刚开始着手解决全球气候变暖与太阳变黯如何相互作用的问题。正如Ramanathan所说，"这就像我们让一个新来的大猩猩在餐桌旁就座的情形一样。" ——人们发现它确实是一个非常大的大猩猩。