“温室效应”是指地球大气层上的一种物理特性。假若没有大气层，地球表面的平均温度不会是现在合宜的15℃，而是十分低的-18℃。这种温度上的差别是由温室气体导致的，这些气体吸收红外线辐射而影响到地球整体的能量平衡。受到温室气体的 影响，大气层吸收红外线辐射的份量比它释放到太空外的垛，这使地球表面温度上升，这个过程就是“天然的温室效应”。但由于人类活动释放出大量的温室气体，结果让更多红外线辐射被折射到地面上，加强了“温室效应”的作用。

全球变暖是目前全球环境研究的一个主要议题。根据对100多份全球温度变化资料的系统分析，发现全球平均温度已升高0.3～0.6摄氏度。其中11个最暖的年份发生在80年代中期以后，因而全球变暖是一个毋庸置疑的事实。全球变暖将带来非常严重的后果，如冰川消退、海平面上升、荒漠化等等，还给生态系统、农业生产带来严重影响。全球变暖现象最终可能会导致全球变冷，形成另一次冰河时代。就像我们在美国电影《后天》里面看到的可怕情景。

因此，探求全球变暖的起因成为重要的研究课题。分析表明，虽然地球演化史上曾经多次发生变暖--变冷的气候波动，但人类活动引起的大气温室效应增长可能是主要因素。

各国政府、企业和科学家们逐步达成共识，大家必须迅速行动起来，与气候变化作斗争。目前人们已经采取措施限制二氧化碳的排放量。许多公司正积极应对二氧化碳限排时代的到来。

Part 8 El Nino and La Nina 厄尔尼诺和拉妮娜

El Nino and La Nina are opposite phases of the El Nino-Southern Oscillation (ENSO) cycle, with La Nina sometimes referred to as the cold phase of ENSO and El Nino as the warm phase of ENSO.

El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe. Among these consequences are increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding, and drought in the West Pacific, sometimes associated with devastating brush fires in Australia.

In normal, non-El Nino conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.

The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.

During El Nino, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region.

Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

Global climate La Nina impacts tend to be opposite those of El Ninno impacts. In the tropics, ocean temperature variations in La Nina tend to be opposite those of El Nino. At higher latitudes, El Nino and La Nina are among a number of factors that influence climate. However, the impacts of El Nino and La Nina at these latitudes are most clearly seen in wintertime.

In the continental US, during El Nino years, temperatures in the winter are warmer than normal in the North Central States, and cooler than normal in the Southeast and the Southwest. During a La Nina year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.

The twists and turns in the ongoing dialogue between ocean and atmosphere in the Pacific can have a ripple effect on climatic conditions in far flung regions of the globe. This worldwide message is conveyed by shifts in tropical rainfall, which affect wind patterns over much of the globe.

Imagine a rushing stream flowing over and around a series of large boulders. The boulders create a train of waves that extend downstream, with crests and troughs that show up in fixed positions. If one of the boulders were to shift, the shape of the wave train would also change and the crests and troughs might occur in different places.

Dense tropical rainclouds distort the air flow aloft 5-10 miles above sea level much as rocks distort the flow of a stream, or islands distort the winds that blow over them, but on a horizontal scale of thousands of miles. The waves in the air flow, in turn, determine the positions of the monsoons, and the storm tracks and belts of strong winds aloft, commonly referred to as jet streams, which separate warm and cold regions at the Earth's surface.

In El Nino years, when the rain area that is usually centered over Indonesia and the far western Pacific moves eastward into the central Pacific, as shown on p. 17, the waves in the flow aloft are affected, causing unseasonable weather over many regions of the globe.

近年来，各类媒体越来越关注这样一个气候学名词：厄尔尼诺。众多气候现象与灾难都被归结到厄尔尼诺的肆虐上，例如印尼的森林大火、巴西的暴雨、 北美的洪水及暴雪、非洲的干旱等等。它几乎成了灾难的代名词！

可是厄尔尼诺究竟是什么呢？用一句话来说：厄尔尼诺是热带大气和海洋相互作用的产物，它原是指赤道海面的一种异常增温，现在其定义为在全球范围内，海气相互作用下造成的气候异常。

厄尔尼诺现象是发生在大气环流和海洋环流之间的强耦合事件的例子。大气环流（风）施加于海洋的应力是海洋环流的主要驱动力，同时，正如我们所看到的那样，来自海洋的热量，特别是蒸发作用对大气环流有极大的影响。

厄尔尼诺现象的基本特征是赤道太平洋中东海域海水温度异常增高。由于热带太平洋是一个控制大气运动的巨大热源和水气源，因此厄尔尼诺现象的发生将引起全球气候异常。

拉尼娜是指赤道太平洋东部和中部海面温度持续异常偏冷的现象（与厄尔尼诺现象正好相反）。是气象和海洋界使用的一个新名词。意为“小女孩”，正好与意为“圣婴”的厄尔尼诺相反，也称为“反厄尔尼诺”或“冷事件”。

厄尔尼诺与赤道中、东太平洋海温的增暖、信风的减弱相联系，而拉尼娜却与赤道中、东太平洋海温度变冷、信风的增强相关联。因此，实际上拉尼娜是热带海洋和大气共同作用的产物。

拉尼娜同样对气候有影响。拉尼娜与厄尔尼诺性格相反，随着厄尔尼诺的消失，拉尼娜的到来，全球许多地区的天气与气候灾害也将发生转变。总体说来，拉尼娜并非性情十分温和，它也将可能给全球许多地区带来灾害，其气候影响与厄尔尼诺大致相反，但其强度和影响程度不如厄尔尼诺。

Global environmental collapse is not inevitable. But the developed world must work with the developing world to ensure that new industrialized economies do not add to the world's environmental problems. Politicians must think of sustainable development rather than economic expansion.

Conservation strategies have to become more widely accepted, and people must learn that energy use can be dramatically diminished without sacrificing comfort. In short, with the technology that currently exists, the years of global environmental mistreatment can begin to be reversed.

科学技术以前所未有的速度和规模迅猛发展，增强了人类改造自然的能力，给人类社会带来空前的繁荣，也为今后的进一步发展准备了必要的物质技术条件。对此，人们产生了盲目乐观情绪，好象自己已经成为大自然的主人，可以长期掠夺资源而不会受到大自然的惩罚。然而，这种掠夺式生产已经造成了生态和生活的破坏，大自然向人类亮起了红灯。

人们承认面临的严重危机，但是可以通过共同的努力战胜它，寻求新的发展道路。可持续发展战略的核心是经济发展与保护资源、保护生态环境的协调一致，让人类子孙后代能够享有充分的资源和良好的自然环境。可持续发展是一个长期的战略目标，需要人类世世代代的共同奋斗。现在是从传统增长到可持续发展的转变时期，因而最近几代人的努力是成功的关键。