Lesson 22 What is the Relationship between Climate Change and Weather

第 22 课 气候变化与天气之间的关系是什么

Climate is generally defined as average weather, and as such, climate change and weather are intertwined. Observations can show that there have been changes in weather, and it is the statistics of changes in weather over time that identify climate change. While weather and climate are closely related, there are important differences. A common confusion between weather and climate arises when scientists are asked how they can predict climate 50 years from now when they cannot predict the weather a few weeks from now. The chaotic nature of weather makes it unpredictable beyond a few days. Projecting changes in climate (i.e., long-term average weather) due to changes in atmospheric composition or other factors is a very different and much more manageable issue. As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence that the average age of death for men in industrialised countries is about 75. Another common confusion of these issues is thinking that a cold winter or a cooling spot on the globe is evidence against global warming. There are always extremes of hot and cold, although their frequency and intensity change as climate changes. But when weather is averaged over space and time, the fact that the globe is warming emerges clearly from the data.

气候通常被定义为平均天气，因此气候变化与天气密不可分。观测可以显示天气发生了变化，而通过时间上的天气变化统计数据可以识别气候变化。虽然天气和气候密切相关，但二者之间存在重要差异。当科学家被问到如何能预测 50 年后的气候，而几周后的天气却无法预测时，往往会引发对天气和气候的混淆。天气的混沌特性使其在几天之外无法预测。而预测因大气成分变化或其他因素引起的气候变化（即长期平均天气变化）则是一个非常不同且更易处理的问题。作为类比，虽然无法预测某一特定男子的具体寿命，但我们可以相当有把握地说，工业化国家男性的平均寿命大约为 75 岁。另一个常见的误解是认为寒冷的冬季或地球上的某个降温区域是反对全球变暖的证据。 虽然极端的高温和低温始终存在，但它们的频率和强度会随着气候的变化而变化。然而，当天气在空间和时间上取平均值后，全球变暖的事实在数据中清晰显现。

Meteorologists put a great deal of effort into observing, understanding and predicting the day-to-day evolution of weather systems. Using physics-based concepts that govern how the atmosphere moves, warms, cools, rains, snows, and evaporates water, meteorologists are typically able to predict the weather successfully several days into the future. A major limiting factor to the predictability of weather beyond several days is a fundamental dynamical property of the atmosphere. In the 1960s, meteorologist Edward Lorenz discovered that very slight differences in initial conditions can produce very different forecast results. This is the so called butterfly effect: a butterfly flapping its wings (or some other small phenomenon) in one place can, in principle, alter the subsequent weather pattern in a distant place. At the core of this effect is chaos theory, which deals with how small changes in certain variables can cause apparent randomness in complex systems.

气象学家在观察、理解和预测天气系统的日常演变方面投入了大量精力。利用控制大气运动、升温、降温、降雨、降雪和水蒸发的物理概念，气象学家通常能够成功预测未来几天的天气。超过几天的天气可预测性的一个主要限制因素是大气的基本动力学特性。在 20 世纪 60 年代，气象学家爱德华·洛伦兹发现，初始条件的微小差异可以产生非常不同的预报结果。这就是所谓的蝴蝶效应：一只蝴蝶在某处扇动翅膀（或其他小现象）原则上可以改变远处的后续天气模式。这个效应的核心是混沌理论，它研究某些变量的微小变化如何导致复杂系统中的表面随机性。

Nevertheless, chaos theory does not imply a total lack of order. For example, slightly different conditions early in its history might alter the day a storm system would arrive or the exact path it would take, but the average temperature and precipitation (that is, climate) would still be about the same for that region and that period of time. Because a significant problem facing weather forecasting is knowing all the conditions at the start of the forecast period, it can be useful to think of climate as dealing with the background conditions for weather. More precisely, climate can be viewed as concerning the status of the entire Earth system, including the atmosphere, land, oceans, snow, ice and living things that serve as the global background conditions that determine weather patterns. An example of this would be an El Niño affecting the weather in coastal Peru. The EI Niño sets limits on the probable evolution of weather patterns that random effects can produce. A La Niña would set different limits.

然而，混沌理论并不意味着完全没有秩序。例如，历史早期略微不同的条件可能会改变风暴系统到达的日期或它将采取的确切路径，但该地区和该时间段的平均温度和降水量（即气候）仍然大致相同。由于天气预报面临的一个重大问题是了解预报期开始时的所有条件，因此将气候视为与天气的背景条件相关是有用的。更准确地说，气候可以被视为涉及整个地球系统的状态，包括大气、陆地、海洋、雪、冰和作为全球背景条件的生物，这些条件决定了天气模式。一个例子是厄尔尼诺现象影响秘鲁沿海的天气。厄尔尼诺现象限制了随机效应可能产生的天气模式的可能演变。拉尼娜现象则会设定不同的限制。

Another example is found in the familiar contrast between summer and winter. The march of the seasons is due to changes in the geographical patterns of energy absorbed and radiated away by the Earth system. Likewise, projections of future climate are shaped by fundamental changes in heat energy in the Earth system, in particular the increasing intensity of the greenhouse effect that traps heat near Earth’s surface, determined by the amount of carbon dioxide and other greenhouse gases in the atmosphere. Projecting changes in climate due to changes in greenhouse gases 50 years from now is a very different and much more easily solved problem than forecasting weather patterns just weeks from now. To put it another way, long-term variations brought about by changes in the composition of the atmosphere are much more predictable than individual weather events. As an example, while we cannot predict the outcome of a single coin toss or roll of the dice, we can predict the statistical behavior of a large number of such trials.

另一个例子可以在夏季和冬季之间的明显对比中找到。季节的变化是由于地球系统吸收和辐射能量的地理模式变化造成的。同样，对未来气候的预测是由地球系统中热能的根本性变化形成的，特别是由大气中二氧化碳和其他温室气体的量决定的温室效应强度增加所导致。在 50 年后由于温室气体变化造成的气候变化预测是一个非常不同且更容易解决的问题，而不是预测仅几周后的天气模式。换句话说，由于大气组成变化带来的长期变化要比单个天气事件更加可预测。例如，虽然我们无法预测一次硬币抛掷或掷骰子的结果，但我们可以预测大量此类试验的统计行为。

While many factors continue to influence climate, scientists have determined that human activities have become a dominant force, and are responsible for most of the warming observed over the past 50 years. Human-caused climate change has resulted primarily from changes in the amounts of greenhouse gases in the atmosphere, but also from changes in small particles (aerosols), as well as from changes in land use, for example. As climate changes, the probabilities of certain types of weather events are affected. For example, as Earth’s average temperature has increased, some weather phenomena have become more frequent and intense (e.g., heat waves and heavy downpours), while others have become less frequent and intense (e.g., extreme cold events).

虽然许多因素仍在影响气候，但科学家已确定人类活动已成为主导力量，并且是过去 50 年观察到的大部分变暖现象的主要原因。人类引起的气候变化主要是由于大气中温室气体含量的变化，但也受到小颗粒物（气溶胶）变化以及土地利用变化等因素的影响。随着气候变化，某些类型天气事件的概率也受到影响。例如，随着地球平均气温的升高，一些天气现象变得更加频繁且强烈（如热浪和强降雨），而其他现象则变得不那么频繁和强烈（如极端寒冷事件）。