“每一个ppm的二氧化碳含量对应21亿吨大气层中的碳。目前的水平是380ppm，即8000亿吨碳。”这是个非常可以的估计。

在您随后引用过的斯特恩报告的第196页上写着“在过去的两个世纪，将近2000Gt的二氧化碳（Gt=10亿吨）通过人类活动而排放进大气层（主要通过燃烧化石燃料和土地使用上的变革）。地球上的土壤、植物及海洋估计吸收了这些排放量的60%，剩下积存在大气层中的二氧化碳约有800Gt。这对应着大气二氧化碳浓度的100ppm的增加，也就是说，每8Gt的二氧化碳对应着1ppm的浓度增量。”您的结果和以上数字有天壤之别，您的2.1Gt=1ppm和斯特恩的8Gt=1ppm. 这是因为您的8000亿吨碳只包括了过去两个世纪中人类活动的排放量，并不包括自然反应导致的二氧化碳排放及其带来的环境浓度的升高。800Gt的二氧化碳排放量该对100ppm的二氧化碳浓度增量负责，而不是全部380ppm的大气二氧化碳浓度。

这将导致将来对二氧化碳排放的不同估计。在《斯特恩报告》的同一页上还说，550ppm的二氧化碳基本相当于3700Gt的二氧化碳。经过植物土壤和海洋的60%的吸收，相当于1.5万亿吨，远远超过您估计的0.4万亿吨。气温上升将会减少植物、土壤和海洋的吸收，这固然正确，但是他们将继续吸收二氧化碳，除非发生毁灭性的变化。《斯特恩报告》中给出的数字和其它研究相比是偏乐观的。IPCC(1996)发布了不同CO2浓度对应的排放量。其中用来稳定550ppmCO2浓度的CO2排放量数值在870-990Gt之间，与450ppm对应的是640Gt。

根据人口测算的中国份额则是比较客观公正的。但是当我们想到究竟有多少中国生产的商品是中国人自己享用这个问题的时候，你就可以测算出有多少与他人的消费和福利挂钩的碳排放量被转移进了中国的公平份额。尽管如此，中国仍必须马上积极控制碳排放量。这一点是毋庸置疑的。这不仅是作为中国对世界的责任或是贡献，更重要的是给她自己的公民一个更加黄明的未来。

王韬
Tyndall气候变化研究中心及Sussex能源研究小组

"Each part per million of CO2 corresponds to about 2.1 billion tonnes of atmospheric carbon. The current level – about 380ppm – is 800 billion tonnes"
This is a very suspicious estimation. In page 196 of the Stern Review, which you sited later, it says "Over the past two centuries, around 2000 Gt CO2 (Gt = billion tonnes) have been released into the atmosphere through human activities (mainly from burning fossil fuels and land-use changes). The Earth’s soils, vegetation and oceans have absorbed an estimated 60% of these emissions, leaving 800 Gt CO2 to accumulate in the atmosphere. This corresponds to an increase in the concentration of carbon dioxide in the atmosphere of 100 parts per million (ppm), thus an accumulation of around 8 Gt CO2 corresponds to a 1 ppm rise in concentration." This is big difference between these two results, yours 2.1 GT=1ppm and Stern's 8Gt CO2=1ppm. This is because the 800 billion tonnes is the human emission over the last two century, without including the carbon emission from natural process and the ambient CO2 concentration. The 800 Gt CO2 emission are responsible to the 100ppm increase in CO2 consternation, not the 380ppm CO2 concentration as a whole.

This leads to different estimation of carbon emission in the future. In the same page of the Stern Review, it suggested 550 ppm CO2 would allow roughly 3700 GtCO2. After 60% absorption by vegetation, soils and the oceans, this means about 1500 Gt CO2 increase in atmosphere, which is 1500 billion tonnes, way above your 400 billion tonnes estimate. It is right to say the increased temperature will reduce the absorption of vegetation, soils and the oceans, but they will continue to absorb CO2 unless catastrophic change happens. The value given in the Stern's Review is a bit optimistic compared with the others. IPCC (1996) published the CO2 emission to different CO2 concentration. The CO2 emission value to stabilise the CO2 concentration at 550ppm was between 870 to 990 Gt CO2 and for 450ppm it was around 640 Gt CO2.

The share to China according to the population is an equal and fair argument. But when we think about how much good produced in China is actually consumed by Chinese themselves, you can estimate how much carbon emission has been transferred to China's own fair share, but for others consumption and welfare.

Nevertheless, China has to act more actively in controlling carbon emission now, this point is definitely right. It is not just China's duty or contribution to the world, but more importantly to give its own people a brighter future.

Tao Wang
Tyndall Centre for Climate Change Research and Sussex Energy Group

谢谢王韬的批评指正。关于“每一个ppm的二氧化碳含量对应21亿吨大气层中的碳（非二氧化碳）”的数据来自索科洛（2004年到2006年）的研究，其中他使用的是英吨（1英吨=1016.047吨）。了解更多关于引起两者差异的原因还是很有趣的。我发现你所提到的IPCC的数据是1996年发布的。这个数据在2001年的报告中仍是一样的吗？在其第四版的评估报告中这个数字还会是一样的吗（2007）？我们都认为关键问题是中国必须要立即采取行动来实施减排。据《纽约时报》（11月25日）报道，中国在2009年将会超过美国成为世界上最大的二氧化碳排放国，这比预计的要早了十年。“很大原因是因为汽车数量的急剧增加，但主要原因还是因为中国不断扩大的对煤的需求量，煤是所有用于发电的燃料中污染最严重的。中国的用煤量已经超过美国、欧盟和日本的总量。每7到10天中国就会建起一座燃煤发电厂，提供的电力足以供应整个达拉斯或圣迭戈的家庭用电。”强调一下，西方人们也应该立即采取行动。一家叫TXU的公司已经宣布将在德克萨斯州建11座新电站，电站将使用旧的煤粉技术。这是美国现有的燃煤发电技术中污染最严重的，也是利润最高的。一名美国著名科学家评论说，TXU的执行主管应该立即因置上千万人口的生命于不顾而被送上法庭。但是中国也要行动起来。我们现在可以说同舟共济，而且这条船还岌岌可危，前方风惊浪险。我们是应该认识到是谁过去的所作所为使人类陷入目前的困境，但这并不是某个国家或某些人有意要陷人类于这种境地或是使情况更糟糕。这只是满足了一小部分富人的利益。卡斯帕•亨德森

Thanks to Tao Wang for the critique.

The figure of one part per million of CO2 corresponding to about 2.1 billion tonnes of atmospheric carbon (not carbon dioxide) is taken from Socolow (2004 and 2006) who actually uses long tons (multiply by 1016.047 for a more precise conversion to metric tonnes). It would be interesting to learn more about the reason for this apparent discrepancy.

I note that the IPCC figure to which you refer is referenced as 1996. Did this hold in their 2001 report, and will it still hold in their fourth assessment report (2007)?

We agree that a key issue is that China needs to act now to control carbon emissions.

It has been reported (New York Times, 25 Nov) that China will surpass the United States as the world’s largest emitter of carbon dioxide by 2009, a decade ahead of previous predictions. "A big reason is the
explosion in the number of automobiles, but the
main reason is China’s ravenous appetite for
coal, the dirtiest of all the fuels used to
produce electricity. Already, China uses more
coal than the United States, the European Union
and Japan combined. Every week to 10 days, another coal-fired power plant opens somewhere in China, with enough capacity to serve all the households in Dallas or San Diego".

To emphasize again, those of us in Western countries need to act. A company called TXU has said it will build 11 new power plants in Texas use the old pulverized coal model technology, which is much dirtier than alternatives easily available in the United States, but is much more profitable.

One famous and distinguished American scientist has commented that executives at TXU should immediately face trial for threatening the lives of hundreds of millions of people.

But China needs to act too. We are all in a small boat together, and the boat is lying low in the water in rough seas. We should recognise who contributed most to getting us into this situation in the past, but it is in no country or people's true interest to make the situation even more dangerous than it already is, merely to benefit a few very rich people.

Caspar Henderson