Do we need to cry now that the climate wolf is at the door?
The little boy who cried "wolf" was finally proved right and was gobbled up as punishment for his earlier pranks. Malthusians have been crying wolf for a couple of centuries. But in global warming they may well have seen a real one. It would be wrong to say their warnings are being ignored. But they are certainly not receiving much practical attention. The Kyoto protocol now covers a mere 30 per cent of emissions, while its hard limits cover just 8 per cent. Is global warming a wolf at our door?
The argument that it is starts with the observation that some atmospheric gases generate a greenhouse - or warming - effect. This is an excellent thing since they make the earth's surface temperature about 30???oC warmer than it would otherwise be. But, as concentrations of greenhouse gases rise, so will the the temperature (albeit only if other things remain equal). This, goes the argument, is what is happening.
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Since the 18th century, there has been a big increase in the output of man-made (anthropogenic) CO2, largely as a by-product of the burning of fossil fuels discussed here two weeks ago. Currently, emissions are running at 7bn tonnes a year, measured as carbon (just over one tonne per person). Concentrations of CO{2 }in the atmosphere have risen from about 280 parts per million in the 18th century to 380 parts today. This is, say the scientists, higher than in the last 420,000 years and possibly the highest in 20m years (though levels have been higher still in the more distant past). The 10 warmest years on record have occurred since 1994, with an average global warming of 0.7???oC since 1900. (See charts.)
Concentrations of CO2 are headed much higher still. Under plausible assumptions, human-caused emissions of greenhouse gases (which include water vapour, methane, nitrous oxide, as well as CO2) will tend to rise over the next half century. This is not only because of the rising use of fossil fuels discussed here last week. Bigger populations in developing countries also mean further deforestation. Moreover, while anthropogenic emissions make up only about 5 per cent of the natural exchanges between atmosphere, sea and land, their impact is cumulative, since about60 per cent of our emissions stay there.
If trends of the past half century were to continue, concentrations would reach 550 parts per million by 2050. That level would itself raise global temperatures by between 2.2???oC and 3.6???oC by 2100. The continents might warm by between 2.2???oC and 6.2???oC and the Arctic by between 3.6???oC and 11.4???oC. Such changes could well be associated with extreme events: reversal of the oceanic currents (notably the Gulf Stream); and the melting of permafrost and subsequent release of huge quantities of methane. Feedback effects might push temperatures higher than at any time in the last 50m years. The world would be a different place, as humans returned to the atmosphere the carbon extracted by photosynthesis and fossilised over millions of years.
I have no intellectual difficulty with this argument, since it is grounded in scientific reasoning. Nevertheless, it raises several further questions. First, how certain are we of the magnitudes of potential warming? Second, how far is the warming itself a "bad thing"? Third, is there any chance that we will, in practice, find a workable way of dealing with it? Finally, what can and should we do about it, while taking into account both the benefits and the costs of any actions?
The answer to the first of these questions is that there remain substantial uncertainties in long-run climate forecasts, as can be seen from the ranges I have given. I sympathise with the climatologist here: forecasting the behaviour of complex systems is hard, as economists know. But the forecast direction of change at least seems plausible, to this scientific illiterate.
The answer to the second question is trickier, at least to the lay person. It is, after all, not obvious why a warmer world would be such a bad thing. Most people and, for that matter, most of the world's fauna and flora inhabit the warmer parts of the globe, for good reason. It is, by and large, easier to live there. There would certainly be beneficiaries of global warming, perhaps very many of them. But sudden changes impose huge costs of adjustment that would include the disappearance of habitats. Life would survive this, as it did the mass extinctions of the past, and so, almost certainly, would that most adaptable of species, humanity. But the adjustment would surely prove disruptive, with an overwhelming probability that the poor would suffer most, as Sir Nicholas Stern of the UK Treasury argues in his preliminary analysis of the economic issues.*
Now we come to the hard questions - what will, can and should be done? The answer to the first is already quite clear: next to nothing. Emissions continue to rise, Kyoto or no Kyoto: since 1990, aggregate human emissions from fossil fuel consumption (and cement production) have risen at1.1 per cent a year. Emissions per head may have stabilised, but world population is growing: it is forecast to reach 9bn by mid-century, up from 6.5bn today. Above all, emissions are above the level needed to stabilise concentrations of greenhouse gases (measured in CO2 equivalents) even at 550 parts per million.
For that to happen, emissions in 2050 would need to be no greater than they were in 1990 (which was 12 per cent below 2002 levels) and as much as 50 to 60 per cent below levels forecast for 2050, on a business as usual, "fossil-fuel-first" scenario. It is important to note, moreover, that small reductions in emissions slow the speed at which the concentrations "travel", but not the destination they reach: as Steve Koonin, BP's chief scientist, notes, even cutting CO2 emissions by 20 per cent below current trends only postpones the date at which we reach 550 parts per million by 15 years. The cuts have to be bigger than this because what we put up stays up for 200 to 300 years.**
A betting person who accepts the growing scientific consensus would wager that global warming is going, like the wolf, to gobble us up. But what could and should we do, instead? Are there technological fixes? Is there a policy regime that might be adopted and (unlike Kyoto) make a difference? Do the benefits of action outweigh the likely costs, or should we merely try to adapt? These are the questions to which I will turn in the final column in this series, next week.
全球变暖是“坏事”吗？
高
喊“狼来了”的小男孩最终证明是正确的，但由于他此前的恶作剧，他被狼吃掉了。马尔萨斯论者喊狼来了已有几个世纪。但在全球变暖这个问题上，他们可能看到了一只真正的狼。说他们的警告被忽视，也许并不正确。但可以肯定的是，他们没有得到太多切实的关注。《京都议定书》(Kyoto protocol)目前仅涵盖了30%的温室气体排放，而其中硬性限制的仅为8%。全球变暖是我们门口的狼吗？
认为它就是狼的观点，始于这样一项观测结果：大气层中某些气体产生温室效应（或变暖效应）。这是一件好事，因为它使地球表面温度比没有温室气体时高出约30摄氏度。但随着温室气体的浓度上升，气温也将随之上升（假定其它条件保持不变）。按照该论点，这就是目前发生的情况。
自18世纪以来，人类制造的（人为的）二氧化碳排放量大幅增加，很大程度上是化石燃料燃烧的副产品，两周前本专栏讨论过化石燃料问题。当今，全球每年碳排放量为70亿吨（人均排放量略高于1吨）。大气中的二氧化碳浓度从18世纪约280ppm(百万分之一)，上升到目前的380ppm。科学家们称，这高于过去42万年、甚至2000万年以来的最高值（尽管在更遥远的远古时期，二氧化碳浓度更高）。有记录以来最热的10个年份，均发生在1994年以后，自1900年起，全球平均气温上升了0.7摄氏度（见图表）。
二氧化碳浓度正朝远为更高的水平迈进。根据可信的假设，人为温室气体（包括水蒸气、甲烷、一氧化二氮、以及二氧化碳）排放在未来半个世纪将趋于上升。这不仅因为化石燃料使用增加（上周本专栏讨论过这个问题）。发展中国家的人口增长，也意味着森林将遭到进一步砍伐。此外，尽管人为排放仅占大气、海洋和陆地间自然交换的约5%，但其影响不断累加，因为我们排放的温室气体中，约有60%留在大气中。
如果过去半个世纪的趋势继续下去，那么二氧化碳的浓度到2050年将达550ppm。这一水平本身将使全球温度到2100年升高2.2至3.6摄氏度。各大陆的温度可能上升2.2至6.2摄氏度，而北极的温度上升3.6至11.4摄氏度。这些变化很可能导致极端事件的发生：洋流逆转（特别是墨西哥湾流），永久冻土层融化，以及随之释放巨量甲烷。反馈效应可能使温度高于过去5000万年来的任何时候。随着人类把光合作用萃取、然后经数百万年变为化石的碳“返还”大气，世界将会面目全非。
对于这一主张，我无意提出任何质疑，因为它基于科学推理之上。但是，它引出了几个更深层次的问题。第一，我们对可能的全球变暖幅度有多大把握？第二，变暖本身在多大程度上是“坏事”？第三，我们能否在实践中找到应对这一局面的可行办法？最后，兼顾行动的效益与成本两方面因素，我们能够并且应当做些什么，来应对这一局面？
第一个问题的答案是，长期气候预测仍然存在很大的不确定性，这一点从我上文给出的数字区间就可看出。对此我对气象学家表示理解：预测复杂系统的行为并非易事，经济学家知道这一点。但对我这个“科学盲”来说，气候变化的预测方向至少看起来是可信的。
第二个问题比较难回答，至少对普通人来说是这样。毕竟，全球变暖是“坏事”的理由并不显而易见。多数人，乃至世界上多数动植物，都栖息在地球上比较温暖的地带，这是有很好原因的。这些地方显然更容易生存。全球变暖肯定会有受益者，也许数量巨大。但突然变化会带来巨大的适应成本，包括栖息地的消失。生命将得以延续，正如过去的生物大绝灭一样，而且几乎可以肯定的是，作为适应能力最强的物种，人类将得以生存。但适应肯定会造成冲击，而且几乎可以肯定的是，穷人将最遭罪，正如英国财政部尼古拉斯?斯特恩爵士(Sir Nicholas Stern)在其经济问题初步分析中所提出的那样*。
现在我们来看看最难的问题――我们将会做什么，能够并且应该做什么呢？“会做什么”的答案已相当明显：几乎什么也不会做。不管有没有《京都议定书》，温室气体排放都将继续上升：自1990年以来，来自化石燃料消耗（和水泥生产）的人为总排放量以每年1.1%的速度上升。人均排放量或许已稳定，但世界人口在日益增长。据预计，到本世纪中叶，世界人口将从目前的65亿增长到90亿。最重要的是，排放超过使温室气体（以二氧化碳当量衡量）浓度趋稳所需的水平，即使该浓度已高达550ppm。
在一切照常、“化石燃料作为首选”的情景中，要稳定温室气体浓度，2050年的排放量需低于1990年的水平（比2002年低12%），并且需比2050年的预测水平低50%至60% 。此外必须指出的是，排放的小幅减少能够放缓二氧化碳浓度的“攀升”速度，但并不能阻止其达到的最终水平：正如英国石油(BP)首席科学家史蒂夫?库宁(Steve Koonin)指出的，即使把二氧化碳排放量在当前趋势上削减20%，也只能把浓度达到550ppm的日期推迟15年。降幅必须要高于这个水平，因为我们所排放的，将在大气中停留200到300年时间**。
一个打赌的人如果接受了这一日趋普遍的科学共识，那他将打赌说，全球变暖将像狼一样，把我们吞食掉。但我们能够并且应该做些什么呢？存在技术上的解决方法吗？是否有一种比《京都议定书》管用的政策框架，能使情况有所改观？行动的收益，是否会超过可能的成本？还是我们应该只是试着去适应？在本系列下周最后一篇专栏文章中，我将探讨这些问题。
*《气候变化的经济学原理是什么？》(What is the Economics of Climate Change?)，2006年1月，
www.hm-treasury.gov.uk；
**《日益增长的碳排放》(Carbon on the rise)，2006年4月《前沿》(Frontiers)，
www.bp.com。
作者简介：马丁?沃尔夫 (Martin Wolf)是英国《金融时报》的资深编辑 (associate editor)和首席经济评论员。他对全球经济有着精辟的深刻分析，获得了国际上各界广泛普遍的承认赞赏。在 2003 年度“最佳商务记者奖”评奖中，他荣获“十年杰出成就奖 ”。沃尔夫先生1971年毕业于牛津大学，获经济学硕士。然后，他到世界银行任职工作，并于1974年出任世行资深经济学家。1999年以来，他一直是每年一度的 “世界经济论坛 ”的特邀评委成员。
